Rise of mammals and early hominins

Life appeared on Earth about 3.5 Gya. At first, bacteria dominated. In numbers they still do, but they are not alone. We will now take up where we left off after the K-T extinction and the beginning of the first epoch of the Cenozoic Era.

Paleocene Epoch, 65 Mya

Although the earliest true mammals evolved during the late Triassic, they remained small and relatively inconspicuous until around 65 Mya, the time of the K-T extinction and the disappearance of the dinosaurs. It Is not clear how mammals managed to survive. One theory, based on the hypothesis that dinosaurs were killed by extreme heat after a huge asteroid struck the Earth, would have it that mammals stayed safe either in their burrows or under the sea until the worst heat had passed. In this case, the heat would have had to pass quickly. Perhaps a more likely suggestion is that particles in the air, whether from asteroid impact or volcanic activity, reduced incident sunlight so that photosynthesis was diminished. Animals which depended on plants for food (or on animals dependent on plants) died out, whereas those which ate organisms like insects or worms, which in turn fed on detritus, survived. Possibly both processes – and others – contributed. Be that as it may, after the dinosaurs died out, surviving mammals could creep out of their holes to occupy the old econiches as well as new ones. The number of mammals and mammal species underwent an extraordinary increase.

Mammals are characterized by:

  • having differentiated teeth, i.e., teeth differently shaped to fulfill different functions in different parts of the mouth (e.g., incisors, canines and molars);
  • being endothermic, or warm-blooded, which allows them to adjust their body temperatures according to external conditions, an advantage for adaptation to different climates;
  • bearing their young alive (in most cases);
  • producing milk to feed their young (mammary glands);
  • having fur or hair on their bodies.

Three orders of mammals which survived are still around today. Monotremes are rather rare egg-laying mammals, such as the platypus. Remote ancestors of mammals all laid eggs and these still do. Marsupials, such as kangaroos and koalas, do not lay eggs, but their young are born underdeveloped and must be protected in the mother’s pouch as they grow and develop. Placental mammals, such as humans, protect their young within the mother’s body. They are the most divers and wide-ranging of contemporary mammals.

It is useful to show again the climate change graph from the last article.

65 million years of climate change, from Wikimedia Commons

65 million years of climate change, from Wikimedia Commons

At the beginning of the Tertiary Period, climates were warm and forests spread across all the continents. Angiosperms, plants fertilized by insects, grew everywhere, so plants, insects and animals evolved together. Among animals, one group, the archonta, were the ancestors of today’s bats, flying squirrels, tree shrews and primates.

Eocene Epoch, 55 Mya

During the warmth of the early Eocene Epoch, there was no ice and plant life abounded. Placental mammals and the first primates appeared. As forests developed, mammals evolved to inhabit them, ranging over what are now Europe, North America, Asia and Africa. As greenery spread, photosynthesis increased and therefore so did the amount of oxygen in the atmosphere.

There are numerous candidate fossils for the first primates. They include Purgatorius (Montana, North America, c. 65 Mya, at the end of the Paleocene), Altiatlasius (Morocco, c. 55-56 Mya), Teilhardina (North America and Asia, c. 56 Mya), Notharctus (North America, c. 50 Mya) and Eosimias (China, c. 45 Mya). Altiatlasius is generally considered to be the oldest known primate, even though only a few molars and a piece of jaw have been found. Early primates were small, squirrel-like creatures, but their paws could grasp and their eyes looked forwards, thereby giving them improved stereoscopic vision, so they were well able to walk or run or climb in trees.

Oligocene Epoch, 33.9 Mya

As the Eocene warmth gave way to cooling, the Antarctic ice cap formed; glaciations accelerated the cooling of temperatures and absorption of water, causing increased aridity. Forests regressed, leaving grasslands behind. The necessity of adaptation to new conditions brought about the disappearance of fauna in higher latitudes and the appearance of new ones. Mammals became larger, sometimes huge. It was at this time that the first modern monkeys evolved in Africa or in Asia, exactly where being a matter of some controversy. One such creature, the Aegyptopithecus, or Egyptian Monkey, lived in the forests of what is now Egypt around 35-33 Mya and possessed the gross anatomical traits of later monkeys. He weighed around 6 kg, lived mostly in trees and had opposable thumbs on all four feet, ideal for holding onto tree limbs.

In Western Europe, a sudden change in fauna known as the Grande Coupure involved the extinction of many species. The fossil trace of hominoids is then lost until the Miocene.

Miocene Epoch, 23 Mya

Around 23 Mya, an increase in temperatures was followed by a subsequent division of monkeys into two lines: One, the Cercopithecoidea, now taxonomically considered a super-family, gave rise to the Old World monkeys (catarrhines) remaining in Africa and Asia today. The other was Hominoidea, which gave rise to tail-less gibbons, orangutans, gorillas, chimpanzees and humans. Hominoids flourished, beginning what has been called the “Golden age of hominoids”. One early hominoid, Proconsul, 25-23 Mya, rather resembled a monkey but was definitely not one, being tailless and having a larger brain and ape-like teeth. Later, 17 Mya, Morotopithecus could move in a vertical stance, suspended from tree branches (brachiation). This technique may later have been transferred to the ground to give upright walking.

With the closing of the Tethys Sea around 16 Mya, African simians could migrate to Europe and Asia. Except for orangutans in Asia, all but those remaining in Africa eventually became extinct.

Several hominoid fossils have been found dating from around 14 Mya. One, Kenyapithecus sometimes called Afropithecus), had thickly enameled teeth and may have been an ancestor of modern hominoids.

Subsequent, faster glacial fluctuations reduced the size of northern forests and brought about a rapid diminution of the relative numbers of tailless hominoids compared to cercopithecoids. In fact, hominoids barely survived; 8 Mya, only a few species remained. In Asia, only the ancestors of todays orangutans and gibbons made it. The only other hominoid survivors were in Africa and are dealt with in the next section. Today there remain only five hominoid species for 80 species of Old World Monkeys. Even in Africa, they were greatly outnumbered by cercopithecoids.

After Kenyapithecus, few fossil hominoids occur before the arrival of the australopiths, leaving a 7‑My fossil-less period known as the “African ape gap”. Nevertheless, more fossils are turning up all the time. Samburapithecus, from 9.5 Mya, probably lived before the LCA,1)The last common ancestor of chimps and humans, if you have forgotten. which the molecular-clock method dates to between 6 an 8 Mya.

Possible and probable hominins

Detailed descriptions of the various hominin species and their characteristics do not make for an exciting literary experience for many people, but this is unavoidable. There is much lack of certainty in this domain, but a general evolutionary trend is perfectly clear.

As the above figure shows, the climate since c. 10 Mya has been one of cooling and therefore drying. Some forests have given way to grasslands, like present-day African savannas. At the time when today’s hominin fossils were alive and thriving, the area was what paleontologists call a “mosaic” – a mixed regions of woods, savannas and riverside forests. So creatures living there would have benefited from living both in the trees and on the ground. That is what they did.

Those readers wishing to ignore the details can skip from here to the paragraph “Homo sapiens”.

To date, four species have been discovered of which one may be the first hominin.

The oldest, Sahelanthropus tchadensis (also referred to as “Toumai”), dated about 7-6 Mya2)Some authors cite slightly different dates. In general, we use those of the Smithsonian Human Origins Program,, lived in what is now Chad, which was then a mosaic environment and not yet a desert.3)It goes without saying that the statement that a species “lived” in a place at some time means that its fossils have been found at that place and dated to that time.  Only a cranium and several mandibles have been found. The cranium is no bigger than a present-day chimp’s, with a small brain and large brow ridges, but its sloping face, its small canines and enlarged cheek teeth arranged in the shape of a C, and the position of the foramen magnum (the relative importance of which is disputed in this case) suggest that it was a hominin, not an ape, and capable of walking upright or living in trees. In this case, it came (probably just) after the LCA, so the fact of its having lived west of the Great African Rift negates the thesis (“East Side Story”) that hominins evolved in eastern savannas where their upright posture, enabling them to see over the grass, could have been an adaptive trait. All hominoids, chimps and all, are capable of walking upright, but can not do so as fast or as far as humans.

Of Orrorin tugenensis, who lived in Kenya about 6.2-5.8 Mya, only some teeth, a femur and some phalanges have been found. These fragments indicate that he could live well in trees but was capable of frequently walking upright. His teeth were thickly enameled like those of later australopiths, but smaller. The thick enamel may not be unique to hominin-like creatures and some paleontologists question even whether Orrorin was a hominin.

Two forms of Ardipithecus have been found. One, Ardipithecus kaddaba dates from 5.8-5.2 Mya and rather resembles a chimpanzee, but was capable of walking upright.

The more recent one, Ardipithecus ramidus, (“Ardi”) dates from around 4.5 Mya in what is now Ethiopia. Although his foramen magnum was somewhat farther forward than that of chimps, he still was more like a chimpanzee than a hominin, although he had a U-shaped jaw, unlike apes. A reconstruction of a crushed pelvis indicates Ardi was at home in trees or on the ground, probably more the former, as his hands were those of a tree dweller. Nor could he run long distances upright. He may represent an additional line of hominoids to pangins and hominins, or he might lie along the line to Australopithecines. Whether or not he was a direct ancestor of man is debated. We will shortly have more to say about Ardi.

Archaic and transitional Homo

This rather large group includes the sub-tribe Australopithecina and its two genera, the earlier Australopithecus and the later Paranthropecus. Although the skulls and overall size of australopiths were chimp-like, their front teeth were relatively smaller and their dentitions starting to approach the parabolic shape of those of humans. Also, australopiths could walk upright. They therefore show both ape-like and hominin characteristics, putting them on the road from the former to the latter.

Paranthrops had huge teeth and massive jaws.

Australopithecus hominins

Australopithecus lived during a warm climatic period in a mosaic environment of forest and savanna in east and south Africa between about 4.2 and 2.5 Mya, a period of well over a million years. Members of this genus generally had well developed mandibles and teeth for eating tough roots and plants and they were used to walking upright as well as moving about in trees. Their brains were not significantly bigger than chimps’ brains, although they may have been organized differently. There are at least five species considered to be members of the genus Australopithecus; they are a rather heterogeneous lot.

Australopithecus anamensis lived in current-day Kenya about 3.85-2.95 Mya. He could walk upright or move about in trees. His teeth resembled more those of humans than of chimps. Opinions about him differ:

  • Some paleontologists think Au. anamensis may be the ancestor of Au. garhi who in turn would be ancestor of Homo.
  • Fossils bones of Au. anamensis also have been found in Ethiopia, where he lived at the same time as Ardipithecus ramidus. This suggests that he may have been a descendant of Ardi and ancestor of Au. afarensis, as is also suggested by his parabolic jaw structure.
  • Others wonder if he is not simply the same species as Au. Afarensis.

The status of Au. anamensis is a good example of the current state of uncertainty concerning hominid fossils.

Australopithecus afarensis lived in Ethiopia and Kenya roughly 3.7-3.0 Mya and so lived over a period of about a million years. Many bones of this species have been found, including the so-called “First Family”. With an ape-like face and cranium and long arms for climbing in trees, but small canines and definite bipedal capabilities in the knee and hip bones, she possessed a mixture of ape-like and hominin characteristics which enabled her to survive important environmental changes. Fossil remains of over 300 individuals have been found. Not only was she one of  the longest-lived hominids, she is also the best known, because of Lucy. If there is a common “spokesperson” for australopiths in general and afarensis in particular, it is certainly Lucy, the 40% complete skeleton of a young afarensis woman found in Ethiopia. Her fame as a hominin fossil has made the tour of the world.


Lucy's skeleton. Cast from Museum national d'histoire naturel, Paris. Photo from Wikipdedia Commons

Lucy’s skeleton. Cast from Museum national d’histoire naturel, Paris. Photo from Wikipdedia Commons

Although Lucy could and did walk upright, she certainly had an awkward, swaying gait. Her brain was somewhat larger than a chimp’s and she probably used natural objects which she found, like sticks and stones, for tools.

Partial copy of Laetoli footprints. Replica in National Museum of Nature and Science, Tokyo. Photo from Wikipedia Commons

Partial copy of Laetoli footprints. Replica in National Museum of Nature and Science, Tokyo. Photo from Wikipedia Commons

Footprints of two hominins from the same period, about 3.6 Mya, have been found at Laetoli, in Kenya. The big toe of the walkers is in line with the foot and the walk is a heel-first-toes-last walk which suggests either modern humans or Australopithecus to different studies. Since the observed short stride corresponds to the short legs of Au. afarensis, some of whose bones have been found nearby in the same sediment layer, it is generally accepted that the prints are of that species. Even so, some scientists insist the footprints are those of Au. anamensis.

On the basis of these footprints, it has been suggested that bipedalism did not evolve from quadripedalism, but had always been possible and was instead lost by monkeys and others who became uniquely quadripedal.4)Yvette Levoisin, “L’homme de descend pas d’un primate arboricole!”. The least one can say is that this idea does not seem to have caught on very much.

Another skull from about 3.5 Mya, which has been named Kenyanthropus platyops, is controversial. One paleontologist thinks it is just a deformed skull of Au. afarensis; another, an ancestor of Homo rudolfensis. However, a recent discovery of what would be the oldest known stone tool dates back to 3.3 Mya.5)“Wrong Turn Leads to Discovery of Oldest Stone Tools”, It was found near the site of Kenyanthropus, so it may have been used by him.

Australopithecus africanus is similar to Au. afarensis in possessing both ape-like and human-like features, so much so that some paleontologists consider them to be the same species. It lived in east and south Africa 3.3-2.1 Mya. It had teeth more like those of humans than of australopiths, with smaller canines arranged in a semicircle. It had a flat face and the foramen magnum placed as for an upright posture. Its brain was bigger than that of Au. afarensis and it was both bipedal and arboreal. So it is considered by some to be an ancestor of Homo.

Australopithecus bahrelghazali (called “Abel” by his discoverers) lived about 3.5-3 Mya in Chad, west of the Rift. Many paleontologists think he is just a geographical variant of Au. afarensis. Its discoverer, of course, does not agree.

Of Australopithecus garhi, who lived about 2.5 Mya in Ethiopia, only pieces of a skull have been found. Although a nearby partial skeleton may go with the skull, this is not yet proven. Although it might just be an Au. afarensis or a female Paranthropus aethiopicus, it was considered to represent a separate species because of the previously unknown combination of a small brain and large molars. Bones found nearby indicate that a sharp-edged tool had been used to remove meat.

The most recent australopith, both for its life period and its discovery, is Australopithecus sediba, who lived in south Africa between 1.977 and 1.98 Mya.6)”Australopithecus sediba – new analyses and surprise”, Smithsonian Human Origins Program, A. sediba has certain details of its teeth, arm and leg length and upper chest like earlier australopiths, but other tooth traits and lower chest like humans. Its discoverers think it is descended from Au. africanus, and that it shows features more like Homo than like any other australopith. For them, it could help understand the transition from late australopiths to direct ancestors of humans, in which case it could link the origin of humans to South rather than East Africa. As usual, not everyone agrees. A study of its teeth finds it to be distinct from east African australopiths but close to south African Au. africanus. On the other hand, a study of its jaw finds it to be distinct from Au. africanus. Although it was bipedal, it had what is described as a hyper-pronating gait, meaning that its feet rolled inward at the end of each step.

So there are two lines suggested — and debated:

  • one in east Africa (Ardipithecus ramidus → A. anamensis → A. → afarensis → Homo), and
  • one in south Africa (A. africanus → A. sediba → Homo).

Paranthropus hominids

Though this group was originally thought to be australopiths, they are now considered to be separate species because of their more robust frame, in particular, their chewing apparatus – jaw bone and muscle, and teeth. They are a more homogeneous group than australopiths.

Paranthropus aethiopicus lived 2.7-2.3 Mya in East Africa. A jawbone and a skull have been found, the latter with a protruding face, strong jaw and well developed sagittal crest. But its most striking feature is a set of huge, thickly-enameled megadont teeth in a powerful jaw attached via large zygomatic arches to a sagittal crest in order to permit the chewing of tough, fibrous foods. This guy could eat really tough things like roots. Some paleontologists think he was a robust form of Australopithecus, maybe intermediate between Au. afaransis and P. robustus.

Paranthropus boisei (originally called Zinjanthropus boisei, “Zinj” for short) lived 2.3-1.2 Mya in East Africa. His skull has a massive jaw, megadont teeth – even bigger than those of P. robustus – and flaring cheekbones to hold his strong chewing muscles. He is often referred to as “Nutcracker man” and one study indeed finds that he ate nuts. His brain was bigger than that of his predecessors and increased gradually in size over time, as he flourished for about 1 million years. One hypothesis is that he evolved from P. aethiopicus. He is generally considered to be a side branch of our evolutionary tree because he lived in east Africa at the same time as the first Homo species.

Skulls of Au. Boisei, photographed by author at Olduvai Gorge Museum, Oct 2012.

Skull of Au. Boisei, photographed by author at Olduvai Gorge Museum, Oct 2012.

Paranthropus robustus, lived 1.8-1.5 Mya in South Africa. He had an imposing, wide face, with large zygomatic arches, a sagittal crest and robust, almost megadont jaws for chewing tough fibrous foods. He may have been the user of bone tools found nearby.

General appearance of Australopithecus and Paranthropecus

If you met an Australopith in the street, you would wonder why he was loose. Even if he were wearing a suit, you would probably call the nearest zoo or circus to inform them that one of their stars had escaped. They rarely reached 1.4m in height. Their brains were small and they were certainly covered in fur and had chimp-like faces with protruding muzzles. Even if the one you saw walked on two legs, you would have thought he was walking on his hind legs, because his “arms” more resembled front legs, with his hands pretty much like his feet. And his way of walking would probably make you wonder how long he had been down out of his tree – or whether you were out of yours.

Paranthropus specimens were slightly taller than early Australopiths. Still, if you met one in the street you would take one look at his massive jaw and choose a different street.

Next, pre-modern and modern Homo and tools.

Notes   [ + ]

1. The last common ancestor of chimps and humans, if you have forgotten.
2. Some authors cite slightly different dates. In general, we use those of the Smithsonian Human Origins Program,
3. It goes without saying that the statement that a species “lived” in a place at some time means that its fossils have been found at that place and dated to that time.
4. Yvette Levoisin, “L’homme de descend pas d’un primate arboricole!”.
5. “Wrong Turn Leads to Discovery of Oldest Stone Tools”,
6. ”Australopithecus sediba – new analyses and surprise”, Smithsonian Human Origins Program,

Hominins, geology and climate

“In short, paleontology is the study of what fossils tell us about the ecologies of the past, about evolution, and about our place, as humans, in the world.”1)University of California Museum of Paleontology, Paleontology, the study of the evolution of ancient life, draws information not only from the discovery and study of old bones, but also from archeology, genetics, linguistics, climatology and other fields. Interpretations of existing data differ and can change with each new discover of fossils. Since new bones are discovered quite often, paleontology is constantly a Work in Progress.

That explains why this article may well be the one with the most occurrences of words like “maybe”, “perhaps” or “thought” (as in “thought to be…”), indicating uncertainty in the understanding of some findings. This situation casts no doubt on the overall results showing the evolution of our species, Homo.

The evolution of man and his family bush

It would be nice to be able to draw a family tree for mankind. There exists much evidence for numerous intermediate species between man and his last common ancestor (LCA) with chimpanzees. But it is currently impossible to distinguish a linear sequence of species on such a tree. To continue the metaphor, the tree really looks more like a bush, with twigs sticking out in all directions, masking the underlying branches. Nevertheless, it is convenient to group together some twigs whose similar characteristics indicate they may sprout from a common branch.

Before going further, some vocabulary is necessary. A primate is a mammal of the order Primates (logically enough), mostly arboreal, ranging in size from lemurs to gorillas, and including, among others, monkeys, chimpanzees, gibbons and man. Hominins are species on the main human twig of the bush of evolution, members of the family Hominidae.2)Wikipedia lists six classifications for humans beneath the family Hominidae: subfamily Homininae, tribe Homini, subtribe Hominina, genus Homo, species, H. Sapiens, subspecies H. s. sapiens. Who can possibly remember and distinguish those three different endings for homini – ai,i and a? The chimpanzee twig is composed of panins.

There are some points of which we are quite certain:

  1. Man has evolved from some creature which was the common ancestor of both man and the chimpanzee, which genetic analysis shows to be the current species closest to us.
  2. Among all the forms of primates which have preceded modern man, it is difficult to distinguish a unique, linear sequence of forms, each one evolved from the one before. Nevertheless, overall changes show clearly that evolution has taken place.
  3. The genetic notion of “molecular clocks” indicates that hominin evolution has taken place for up to 7 million years, often during periods of extreme climate change (shown in a later figure) which some species survived better than others.3)The so-called genetic clock calculates duration based on the number of genetic changes taken place multiplied by an approximate time per change.
  4. Astounding as it may appear to us now, at most times in our evolutionary history, different forms of man existed at the same time. The best known example is that of Neanderthals and Cro-Magnons. They lived near each other in western Europe and even shared some genes, so it is clear that “social” interaction took place between the species. Imagine living near a group of animals of another species, another kind of animal, a sort of ape with which you could communicate (and even copulate). Would we try to enslave or annihilate them (or use them for experiments), as is our wont?
  5. “It” (the evolution of primates from earlier forms into man) all started in Africa.

Characteristics of hominins

The following criteria are generally taken to show that a given fossil is more like a hominin than a panin. Hominins are all creatures attached to the human branch since the LCA; panins, to the chimp branch.

  • More perfected bipedalism, a greater ability to walk upright on the two hind legs. A number of factors are associated with this ability:
    • a more vertical trunk, wider hips, straighter and lockable knees, lower limbs longer than upper, feet suited for walking rather than for climbing in trees;
    • relatively forward placement of the foramen magnum, the hole where the spinal column enters the skull, due to the erect posture;
    • greater height;
  • less prognacious (flatter) face;
  • greater cranial volume and brain size (larger for hominins), which requires changes in pelvic size in order that such large-brained babies can be born;
  • skull, jaw and dental structure (related to diet):
    • teeth in a parabolic row;
    • smaller and less protruding canines, relatively larger incisors and larger chewing teeth (molars);
    • more robust mandibles (lower jaws).

The two most important developments are bipedalism and increased brain size, as all the others depend on these two. Bipedalism seems to have prepared the way for bigger brains (as we shall see shortly).

Either such taxonomic4)Taxonomy is the practice and science of classification. characteristics or genetic analysis may be used to classify different families and species of primates as shown in the figure.  Results from the two methods are not necessarily the same.

Hominoid families with dates, diagram by author.

Hominoid families with dates, diagram by author.

Another way to see this is in the following table. The difference between the table and the diagram in the placement of the Gorillini may indicate the method of analysis used (taxonomic or genetic)5)There is disagreement about placing gorillas under hominoids or hominids. See (in French).

Hominoidea super-family

Hominoidea super-family

Groups of hominins

Species may be grouped together according to some common characteristics. The next figure indicates the time period of most currently known fossil hominins. Different colors indicate different groups.

Timeline and grouping of principal fossil hominid species

Timeline and grouping of principal fossil hominid species, diagram by author

In this figure, a significant number of hominin species are grouped in two different ways. One grouping6)Based on Wood, 2005, is indicated by the background colors:

  • beige – possible and probable hominins
  • blue – archaic and transitional Homo
  • green – pre-modern Homo
  • pink – Homo group

The color of the vertical bars representing the time when the species lived represents the grouping of the Smithsonian Museum of Natural History:

  • brown – Ardipithecus group
  • green – Australopithecus group
  • magenta – Paranthropus group
  • blue – Homo grouping
  • pale green – not grouped by Smithsonian (considered controversial)

While general characteristics of different species among the Australopiths and others evolve across the ages, different parameters do not always evolve together. For instance, Au. anamensis has chimp-like canines but fairly evolved bipedalism, whereas Pa. aethiopicus has smaller canines but its foramen magnum is near the back. Nevertheless, from the bottom of the bush to the top, overall evolution does occur and near the top we find our modern species panins and hominins – in particular, us.

Many paleontologists think that H. erectus is a later and Asian version of H. ergaster; others think they are different. In either case, there were at least four species of hominins living around 2 Mya,

Before considering these groups in detail, it is necessary to take a look at the preceding rise of mammals and at the role of climate in evolution.

Geology, climate and evolution

Global temperature is a function of many variables, but there are two main ones:

  1. how much energy is received from the sun and
  2. how much of it is trapped by the oceans and the atmosphere, rather than being reflected back out into space.

Considerations of energy received must take into account solar activity.

The energy falling onto the Earth’s surface depends on its orbit – the angle of its rotational axis relative to the plane of the orbit, the precession of the orbit7)The elliptic orbit depends on two foci, one of which is at the sun. The other rotates slowly around the sun and the changing shape of the orbit, which modifies the distance of the Earth from the Sun. Taking all these into account leads to the calculation of so-called Milankovitch climate cycles. These agree largely with the results from geology.

How much energy is retained by the Earth depends on the distribution of land and sea, the properties of the land’s surface (reflective or absorbing) and the composition of the atmosphere (the much-discussed greenhouse effect and the ozone barrier).

The period when primates developed, the beginning of the Eocene epoch (55 Mya), was the warmest moment in the Tertiary and the warmth spurred growth and evolution. Since the Eocene peak, global temperatures have been gradually decreasing, with short-term fluctuations superimposed on the general background. The next figure shows the general behavior that has been observed.

At the beginning of the Oligocene (33.9 Mya), a period of rapid cooling brought to an end the warmth of northern forests, with disastrous effects which almost wiped out our ancestral line.

65 million years of climate change, from Wikimedia Commons

65 million years of climate change, from Wikimedia Commons

As we have seen, these changes in temperature are to a great extent due to geology – the movement of tectonic plates. As plates have moved, oceans have opened (such as the separation between Antarctica and Australia or South America) or closed (Tethys Sea, Isthmus of Panama). This opening and closing of channels changed sea currents (e.g., the Gulf Stream) and led to formation of the antarctic and arctic ice caps8)This paragraph is only a summary, ignoring chronology. Formation of the antarctic ice cap coincided with the drop in temperatures at the beginning of the Oligocene, c. 35 Mya, whereas the Isthmus of Panama was closed c. 4-3 Mya and the arctic ice cap formed around 2.5 Mya., which in turn brought about lowering of global sea levels. The ice caps themselves reflect solar energy back into space, causing further cooling. Coming together of continents has created mountain chains (Africa pushed up the Alps; India, the Himalayas) which have altered meteorological conditions, especially rain patterns (such as the Asian monsoon). During the latter part of the ice age, melting continental ice sheets have caused sea levels to rise. Geology, climate and evolution all go together.

The next figure shows the general lowering of temperatures over the last 5 My, as well as the cyclic character of temperatures. The relative increase over the last 10,000 years began at the end of the last great Ice Age, which started some 130 Kya and only ended about 10 Kya. We are currently in a warm, interglacial period. There is no reason to expect this warmth to continue very long (on a geological time scale).

Global temperature over 6 My, from NASA Goddard Institute for Space Studies

Global temperature over 6 My, from NASA Goddard Institute for Space Studies

Now continue to the rise of mammals and early hominins.

Notes   [ + ]

1. University of California Museum of Paleontology,
2. Wikipedia lists six classifications for humans beneath the family Hominidae: subfamily Homininae, tribe Homini, subtribe Hominina, genus Homo, species, H. Sapiens, subspecies H. s. sapiens. Who can possibly remember and distinguish those three different endings for homini – ai,i and a?
3. The so-called genetic clock calculates duration based on the number of genetic changes taken place multiplied by an approximate time per change.
4. Taxonomy is the practice and science of classification.
5. There is disagreement about placing gorillas under hominoids or hominids. See (in French)
6. Based on Wood, 2005,
7. The elliptic orbit depends on two foci, one of which is at the sun. The other rotates slowly around the sun
8. This paragraph is only a summary, ignoring chronology. Formation of the antarctic ice cap coincided with the drop in temperatures at the beginning of the Oligocene, c. 35 Mya, whereas the Isthmus of Panama was closed c. 4-3 Mya and the arctic ice cap formed around 2.5 Mya.

Archean, proterozoic and paleozoic — the rise of life

The Archean Eon – appearance of life

Geology and atmosphere

Geologists refer to the period from about 3.8 to 2.5 Gya as the Archean Eon. During this time rocks on the surface came together to form cratons, which would become the central cores of continental plates. The oldest still-existing continental rocks date from the beginning of the Archean. Since that time,average surface temperatures have been slowly cooling. There is evidence that the rock cycle (volcanism-sedimentation-metamorphism) has been in action since about then.

The Earth’s atmosphere developed slowly. Volcanoes coughed up gases and water vapor which was subsequently held down by Earth’s gravity. Asteroids which crashed onto the Earth’s surface probably brought along some gases. But the early atmosphere was mostly carbon dioxide and water, with small amounts of other substances including only very little oxygen. This composition changed only early in the Proterozoic Eon.

Solar energy received at the surface of the Earth was about 20 to 25 % lower than present, which might have made the planet too cold for life to be established1)“Climate puzzle over origins of life on Earth”,, but the CO2 retained heat beneath the atmospheric layer, causing a greenhouse effect which slowly raised atmospheric temperatures. Sunlight striking the water vapor caused photochemical dissociation, the breaking up of the water molecules and the bonding of the resulting oxygen atoms together to create ozone, or O3. In time, the ozone came to protect the surface of the Earth from ultraviolet radiation from the sun, but it also prevented further chemical dissociation, which therefore does not play in important role in the oxygenation of the atmosphere.


Life first began and developed on Earth during the Archean Eon, somewhere about 3.8 or 3.5 Gya, where evidence for the older date is disputed. At that time, surface conditions were still difficult, with high temperatures and an atmosphere lacking in free oxygen but rich in CO2.

Life may be considered as that which can regenerate itself viably. There are different hypotheses about its origin on Earth, especially

  • the “primordial soup” hypothesis,
  • the hydrothermal vent hypothesis
  • the volcanic pool hypothesis.

The first type considers life to have been brought about using energy from electricity (lightning) in a mixture of gases including water and methane. Such production of organic molecules has been demonstrated in the laboratory, but fails to be convincing because it depends greatly on the composition of the atmosphere at the time. More recent experiments with more realistic mixtures have produced similar organic compounds. The discovery of amino acids on meteorites adds weight to the hypothesis that varying atmospheric conditions could lead to production of  organic molecules.2)Prothero (2007), 147-52.

The second type exists in two varieties. The first supposes that life came into being in “black smokers”, hydrothermal vents formed along undersea ridges such as the Mid-Atlantic Ridge. Sea water leaks down through fissures in the rock and is super-heated by nearby magma. The super-heated water may attain a temperature of 400°C, but the immense pressure keeps it from boiling. It dissolves minerals which are then liberated when the mineral-laden water comes back up and hits the relatively colder sea water, emitting sulfur-bearing black molecules which look like, but are not, smoke and which pile up to produce “chimneys”. White smokers, on the other hand, carry barium, calcium and silicon. It was thought that the reaction of hydrogen sulfide from the vent with water would provide the energy necessary for the formation of life. Although life does abound in these vents, it is not at all like ours. One finds, for instance, extremophile organisms which live in darkness and get their nourishment from hydrogen sulfide.

The second hydrothermal-vent model proposes that life originated in alkaline hydrothermal vents. Here is how it goes.

"Nature Tower”, an alkaline “chimney” in the Lost City group. From NOAA.

“Nature Tower”, an alkaline “chimney” in the Lost City group. From NOAA.

Whorls and pores in a thin section of a Lost City chimney, from NOAA.

Whorls and pores in a thin section of a Lost City chimney, from NOAA.

In places on the ocean floor, peridotite rock, which is normally found deep in the Earth’s mantle, has been pushed up to the surface by faulting. The rock contains olivine, which reacts with sea water to form the minerals serpentine and magnetite; the process is called serpentinization, The reactions are exothermic and increase the volume of the reactants. The heat is generated by chemistry and does not come from hot magma, as is the case with “black smokers”. The result is an alkaline solution (pH = 9-11) rich in calcium and H2. The rocks produced have lower density and so expand and push up. They crack and more sea water moves in to react with remaining olivine. On contact with colder sea water, the calcium precipitates out, forming white structures like chimneys.3)These results are for the Lost City Hydrothermal vents. Results differ some for other vents. Eventually, small cracks and “cells” form within the rock. Rising fluids are very alkaline (basic) and thereby precipitate calcium carbonate and other alkaline substances out when they hit the cold sea water. These then build up on the pile of rock already started and soon “reverse stalactites” are produced by the carbonate left behind by the thermally rising water. Iron in the olivine is oxidized, leading to production of reducing gases hydrogen, methane and hydrogen sulfide.4)“The Lost City 2005 expedition”, NOAA, These gases in turn are a source of energy. So the rising “chimneys”, which may reach many meters in height, are associated with a source of energy, gases like those in the “primordial soup” and small cell-sized alveoli or compartments. Such an environment may well be suited to abiotic hydrocarbon production.5) The compartments contain and protect their contents as well as ensuring their concentration, making excellent conditions for the production of inorganic precursors to organic life. From these, prokaryotes and archea would have evolved independently around 3.8 Gya and eukaryotes later, around 2 Gya.

The third hypothesis is more recent.6)Van Kranendonk, Martin J., Deamer, David,  and Djokic, Tara, “Life springs”, Scientific American, August 2017, 22. It posits the combination of simple molecular building blocks, perhaps from space,  using thermal energy from volcanic pools, like those at Yellowstone or in Iceland. As external conditions change, they could evolve in a Darwinian manner as they survive through cycles of wet, dry and moist in land-based hot springs. Such organisms have been called progenotes.7)There is some disagreement as to whether progenote simply means LUCA (last universal common ancestor). Others, more specifically, call it ‘”a theoretical construct, an entity that, by definition, has a rudimentary, imprecise linkage between its genotype and phenotype (Woese, 1987)”—a creature still experiencing progressive Darwinian evolution, in other words.’ From

Be that as it may, the appearance of cell membranes allowed the formation of simple cells, called prokaryotic cells. As far back as 3.5 Gya. these composed cyanobacteria (“pond scum”). which are the ancestors of the blue-green algae still alive all over the globe today. As such, they are the oldest currently-living beings. But they are extremely important for another reason.

Some of these bacteria formed mats which mixed with sand. As the sandy mixture became muddy, the cyanobacteria migrated upwards and the process repeated, resulting in lumpy layers of colonies called stromatolites, descendants of which survive in isolated parts of the world such as the coastal waters of western Australia.

Stromatolites in limestone near Saratoga Springs, NY, by M. C. Ryget via Wikimedia Commons

Stromatolites in limestone near Saratoga Springs, NY, by M. C. Ryget via Wikimedia Commons

Living stromatolites in Shark Bay, Australia, by Paul Harrison via Wikimedia Commons

Living stromatolites in Shark Bay, Australia, by Paul Harrison via Wikimedia Commons

Stromatolites existed from about 3.5 Gya to 0.5 Gya, but are still found in a few places such as Shark Bay, Australia, or the Pacific Coast of Baja California. They survive only in especially salty water (twice the sea’s normal saltiness) or in places with especially strong currents, as both conditions limit predators such as snails which otherwise would devour them.

Cyanobacteria have been called the “working-class heroes of the Precambrian Earth”8)Knoll (2003), 42 and were fundamental to the development of life. The importance of these organisms cannot be stressed too much, as they were the first organisms to carry out photosynthesis, the use of energy from the sun to convert carbon dioxide into nutrients and free oxygen, which is returned to the atmosphere. Over hundreds of millions of years during the Archean and Proterozoic Eons, as cyanobacteria used photosynthesis to get the energy necessary for their own metabolism, they brought about the gradual transformation of atmospheric CO2 into oxygen necessary for other forms of life9)The capability of stromatolites to accomplish this task alone has been questioned and other mechanisms suggested., such as ourselves. At the same time, the greenhouse effect was reduced and, thereby, global temperatures. Much CO2 was also dissolved in the seas, where it combined with calcium to form calcium carbonate, which in turn solidified to form limestone. Limestone, ocean water and corals are huge stores of carbon dioxide (carbon sequestration).

Interestingly, thousands of the minerals found on Earth today are due to oxidation by oxygen dissolved in water. So oxygen has not only allowed life to begin, but has also thoroughly changed our mineral environment. In other words, the biosphere and the geosphere have evolved together.

Photosynthesis took place in the top layer of stromatolites and each layer lived off the layer above. As such, they represented an early symbiosis or way of living together – an example of what we now call ecology.

The Proterozoic Eon — the dance of the continents

Geology and atmosphere

The Proterozoic Eon ran from about 2500 to 542 Mya. It has been so designated because of the appearance of more complex organisms during this period, in spite of widespread glaciation early on. At this time, plate tectonics came into its own, with cratons moving about on the surface of the Earth in what has been called a “stately dance”, i.e., a slow one. It was like some kind of round, with one continent dancing for a while with another, then separately, then with a third. At least three times, they all came together to form a single supercontinent. As they smashed into each other, they brought about the rise of mountains, a process geologists call orogeny. As they rifted and came apart, seas formed between them.

There is rather weak evidence for a perhaps continent-sized landmass dubbed Vaalbara 3.3 Gya. It is much more certain that there existed a continent-sized landmass called Ur about 3.1 Gya, made up of cratons from what now are South Africa, Australia, India and Madagascar. Ur lasted for about 300 Gy, undergoing various combinations with other continents, as will now be seen.

It should be remembered that these reconstructions of ancient cratons or continents from geological and other data are to varying extents uncertain as to the details.

About 2.7 Gya, the first supercontinent came into being – Kenorland (also called Superia). Since the atmosphere was devoid of oxygen at the time, only acid rain fell, and this eroded and dissolved the land, leading to the deposit of sediments along the continent’s coasts. About 2.4 Gya, as oxygen started accumulating in the atmosphere, Ur broke away and Kenorland started to fragment.

By 2 Gya, there were at least five continents:

  • the Laurentian supercraton, the geological core of North America;
  • Ur, composing current India, western Australia and South Africa;
  • Baltica and Ukrainian cratons;
  • a continent comprising most of what are now South America, China and Africa.

About 1.9 Gya, the supercontinent Columbia10)Also called Nena, Nuna or Hudsonland. had been formed. Since it was situated on the equator, its interior was hot and dry. There were therefore no or little ice caps and ocean levels were relatively high.

By around 1.6 Gya, Columbia was split between east and west to form a new sea between the pieces. Since they were still at the equator, ice remained low and ocean levels high.

By 1.2 Gya, a new supercontinent now called Rodinia was forming, again near the equator, so its interior was hot, dry and lifeless and no sediments were formed. The supercontinent was surrounded by a superocean now called Mirovia.

Reconstruction of the supercontinent Rodinia, by John Goodge [Public domain], via Wikimedia Commons

Reconstruction of the supercontinent Rodinia, by John Goodge [Public domain], via Wikimedia Commons

The next supercontinent, Pangea, would form in the Phanerozoic Eon.

It was around the end of the Proterozoic and beginning of the Phanerozoic Eons that the oxygen content of the Earth’s atmosphere began to increase significantly. Alternating layers of red, iron-containing minerals and silica minerals called banded iron formations (or BIFs) indicate fluctuations in the oxygen levels of oceans about 2.5-2.0 Gya, at least not before 1.8 Gya. Iron(II), or Fe2+, is soluble in water, but oxidizes to iron(III), or Fe3+, which precipitates. Since BIFs exist in sedimentary rocks, it is thought than fluctuating levels of oxygen in the sea water led to the alternating bands of minerals.

Later formations called red beds, which are sedimentary sandstone or shale, exist from 1,8 Gya. Their red color is due to the mineral hematite, Fe2O3, formed by the oxidation of iron, but this time on land. So by this time, the air must have contained enough oxygen to oxidize iron. Red beds are also common in rocks from the Phanerozoic Eon.

The Lal Qila, or Red Fort, in Delhi is built of red-bed sandstone. Photo by author's wife.

The Lal Qila, or Red Fort, in Delhi is built of red-bed sandstone. Photo by author’s wife.

In summary:

  • From 2.5-2.0 Gya, fluctuating oxygen content in seawater formed BIFs.
  • Since 1.8 Gya, increasing atmospheric oxygen has oxidized Fe to hematite.


Life now went through a long, slow period, the main events being oxidation of the atmosphere and evolution of eukaryotes.

Evolution and the atmosphere

As shown by fossil evidence, stromatolites thrived in the Proterozoic and continued their conversion of atmospheric CO2 into O2. The first oxygen produced had been gobbled up by chemical reactions with elements like iron. Several types of indirect evidence, based on the presence of certain molecules in rocks, indicate that around 2000 Mya, the content of free oxygen in the atmosphere increased significantly. In spite of evidence for important fluctuations in oxygen levels over the millenia since then, the oxygen content of the atmosphere  has been generally increasing for the last two billion years.

Estimated evolution of atmospheric O2 percentage, by Heinrich D. Holland via Wikimedia Commons. The red and green lines are ranges of estimates.

With the atmosphere richer in oxygen, other forms of life evolved. More complex cells called eukaryotes appeared about 1400 Mya. Such cells incorporate smaller components called organelles. Examples are the cell nucleus and the mitochondria essential to the generation of energy for the cell. It is now widely accepted that organelles within eukaryotes were bacteria which entered the original cell, be it prokaryote or some sort of proto-eukaryote, and stayed – a process referred to as endosymbiosis.

Prokaryotes reproduce by a process of mitosis, duplication and division, after which each “child” organism is essentially a clone of the “parent”.  Eukaryotes also duplicate themselves by mitosis, but they reproduce by meiosis, a process in which a selection of genes from each parent is combined with a selection from the other.11)The subject of reproduction through mitosis and meiosis will be discussed in more detail in the chapter on biochemistry and cellular biology. It is widely accepted that some of the organelles within eukaryotes were originally bacteria which entered the original cell, be it prokaryote or some sort of proto-eukaryote, and stayed — another example of ecological coexistence.

Prokaryotes divide by a process of mitosis, after which each “child” organism is essentially a clone of the “parent”. Eukaryotes also duplicate themselves by mitosis, but they reproduce by meiosis, a process in which a selection of genes from each parent is combined with a selection from the other.12)This subject will be discussed in more detail in the chapter on biochemistry and cellular biology. This method of reproduction leads more rapidly to greater diversity of genes and, so, to the formation of new species. Only eukaryotes form multicellular organisms, a necessity for more advanced forms of life.

Taking into account biochemistry and evolutionary history, biologists now divide life into three domains: bacteria and archaea, (both prokaryotes), and eukarya, the last two being descended from the first in a yet-to-be-agreed-on order. Current eukarya include plants and animals – such as us.

Tree of Life. Eukaryotes are colored red, archaea green and bacteria blue. From Wikimedia Commons

Tree of Life. Eukaryotes are colored red, archaea green and bacteria blue. From Wikimedia Commons

Climate instability and glaciations

The Earth’s climate now entered a period of great instability. The initial cause may have been imbalances in the geosphere and biosphere. The period of existence of a single continent, Rodinia, surrounded by a single ocean under an atmosphere still low in oxygen came to an end as Rodinia split in two around 750 Mya. New coasts brought more and shallow coastal seas and bays which allowed more algal blooms. These may have gobbled up CO2 just as rock weathering did too, leading to global cooling.

Whatever may have been the cause, Earth now embarked on an instable period of glaciations referred to as “Snowball Earth”m although “slushball” might be a better term. Evidence for glaciers between 740 and 580 Mya ago comes from all around the Earth. Life survived, probably in warm, underwater hydrothermal vents. Eventually, CO2 pumped into the atmosphere and methane, CH4, perhaps manufactured by methanogen bacteria, conspired with other feedback effects to bring about global warming and end the glaciations. Then it started all over again. Over 150 million years, at least three cycles of ice age followed by global warming occurred.

  • The Stutian glaciation peaked about 720 Mya;
  • the Marinoan glaciation, about 650 Mya; and
  • the Gaskiers glaciation, about 580 Mya.

Characteristic rocks left behind by retreating glaciers attest to these cycles of cold and hot. Between the second and third cycles, oxygen levels reached levels near those of today and animal life took off.

Ediacaran fossils

Fossils usually only show the harder body parts of the fossilized organisms. But from the end of the Proterozoic, around 575-542 Mya13)The Vendian Period of the Proterozoic Era ran between 600 and 542 Mya., fossils were discovered which also showed the softer body parts of strange and complex organisms. Named after the Ediacaran Valley in Australia where they were first discovered, they have since been found around the world in places such as Charnwood Forest, England, or Mistaken Point, Newfoundland.

Charnia, from Charnwood Forest, by Verisimilus via Wikemedia Commons

Charnia, from Charnwood Forest, by Verisimilus via Wikimedia Commons

Dickinsonia costata, by Verisimilus via Wikemedia Commons

Dickinsonia costata, by Verisimilus via Wikimedia Commons

The Ediacaran fossils are difficult to interpret. They seem to be generally flat, multi-sectioned organisms, often described as “quilted”, without any internal structure.  Charnia, for instance, seems to be a flat, fractal construction without any central stalk. They do not resemble any modern organisms and are generally considered to represent an evolutionary dead end in spite of their being complex, multi-celled organisms. In any case, since they date from as much as 575 Mya, they do show that multi-cellular life existed before the Cambrian. After the Ediacarans had lived alone for up to 90 million years, they disappeared forever as small shelled organisms and trilobites took over.

The Phanerozoic Eon – rise of complex organisms

The Phanerozoic Eon is divided into three eras:

  • the Paleozoic (542-251 Mya),
  • the Mesozoic (251-65.5 Mya) and
  • the Cenozoic (65.5 Mya to today… about).

The Paleozoic Era

During the Paleozoic, the buildup of cratons and mountains continued; glaciers and shallow seas were formed. Life spread from the sea to occupy the land; and fishes, reptiles and primitive mammals evolved.

Geologists have found a huge increase in the number, variety and, especially, the complexity of fossils dating from around 542 Mya in western Canada and in China. This date has therefore been adopted as the beginning of the Paleozoic Era, which is considered to run from 542 to 250 Mya. It is itself broken down into six subdivisions called periods, named as follows:

  • Cambrian (542-500 Mya),
  • Ordovician (500-440 Mya),
  • Silurian (440-410 Mya),
  • Devonian (410-360 Mya),
  • Carboniferous (360-290 Mya) and
  • Permian (290-250 Mya).


Tectonic plates continued moving during the Paleozoic. Around the beginning of the Paleozoic, 542 Gya, Rodinia broke up into Gondwana and Laurentia. But about 300 Gya, the sea between them shrank and tthey collided to form the supercontinent, Pangea14)Also spelled Pangaea. (Figure). The superocean surrounding it is called Panthalassa. For once, a supercontinent was not located right at the equator; about ¾ of Pangea was in the southern hemisphere.

About 175 Mya, Pangea fragmented into seven major continents. The rift between Gondwana and Laurentia formed what would become the North Atlantic Ocean. A huge mountain chain which had stretched diagonally across the great continent left its remains in the Appalachians of North America and the mountains of Ireland, northern Scotland and western Scandinavia. South America and Africa separated to form the South Atlantic. Antarctica and Australia broke loose from Gondwana and India, from Africa. (See Figure.)

Life in the sea

The extraordinary increase in the number of multi-cellular animal phyla which took place at the beginning of the Paleozoic has been referred to as the Cambrian Explosion. It is seen today especially as an explosion of fossils. In fact, the word “explosion” is an exaggeration which has led at least one scientist to react and call it the Cambrian “slow fuse”.

For 2 billion years after the appearance of life on Earth before or around 3.5 Gya, only single-celled prokaryotes existed, cyanobacteria diligently working to increase the oxygen content of the atmosphere. Then the enigmatic fossils of the Ediacaran fauna show that multi-celled, invertebrate organisms came and, it seems, went between about 600 and 545 Mya.

The next logical step, the development of some sort of skeleton or carapace, came about in the early Cambrian, about 545-520 Mya, in the form of “small shelly fossils” (SSFs), or just “little shellies”. These tiny creatures had shells of calcium phosphate, presumably because atmospheric conditions did not yet favor the calcium carbonate shells of today. So for about 25 My, the so-called Cambrian Explosion was represented simply by small shelled creatures – not much of an explosion!

Somewhat later, extraordinary fossils including soft parts of the animals were deposited in two remarkable sites. The first one was Chengjiang, China, with fossils dating around 515 Mya. Among the Chengjiang finds is the oldest fish, which is also the oldest vertebrate, dating from about 500 Mya.15)The last few paragraphs are based on Prothero (2007), 161-70.

Haikouella lanceolata, from the Chengjian fossils, by Didier Descouens via Wikimedia Commons

Haikouella lanceolata, from the Chengjian fossils, by Didier Descouens via Wikimedia Commons

Probably the most famous of the Cambrian fossils are those of the Burgess Shale field of about 505 Mya (Middle Cambrian), now in Canada.16)Burgess shale fossils and their importance. Some of them were pretty strange and are still the subject of study and hypotheses.

Opabinia, a Burgess Sha Nobu Tamura via Wikimedia Commons

Opabinia, a Burgess Shale fossil, by Nobu Tamura via Wikimedia Commons

Hallucigenia, Burgess Shale fossil by Apokryltaros via Wikimedia Commons

Hallucigenia, Burgess Shale fossil by Apokryltaros via Wikimedia Commons

Sponges, considered to be the most primitive animals alive today, had appeared in the late Vendian17)Or Ediacaran, about 650-450 Mya. (end of the Protozoic).

In the early Cambrian, radially symmetry echinoderms were the ancestors of today’s starfish and sea urchins. From about 530 Mya, other invertebrates like brachiopods and worms started to leave fossil traces. Brachiopods, which were shellfish with hard upper and lower valves (as opposed to the left and right valves of modern oysters and scallops, to mention the most edible of them), grew wild on the sea floors.

About 520 Mya, trilobites appeared and invertebrate, multi-celled life was off and running.

Although there do exist fossil tracks of mostly worm-like creatures from 555 Mya, the organisms represented by the Cambrian-period fossils were of a new kind. Cambrian organisms grew to be larger and more complex because of their support structure. During the early Paleozoic, continents were under shallow seas for periods of several million years at a time, so life was dominated by creatures of the seas, including reef builders. These organisms had no internal skeletons, meaning they were invertebrates, but they did have a hard exoskeleton or carapace. The support this gave was advantageous in several ways: It shielded them from the sun, allowed them to retain moisture, gave support for a muscle system and protected them to some extent from predators. Later, skeletons would provide a mineral store, since bones store minerals like calcium and phosphorus from the blood and are able to pass them back to body cells when they are needed. Many types of these creatures existed in the Paleozoic seas. From tiny creatures, larger ones evolved.

Trilobites, a type of arthropod, became a dominant form of marine life. They existed in thousands of different species on every continent for some 270 million years, so long that they have been referred to as the “mascots” of the Paleozoic. They ranged in size from several millimeters to over 50 centimeters. Some had eyes with many crystalline lenses, like fly eyes. Over time thousands of species of trilobites existed – on every continent. Near the end of the Cambrian, there were three trilobite mass extinctions due to climate change and other factors (continental movements, evolution of predators). But trilobites survived.

Small trilobite, 5cm (Ohio), photo by author.

Small trilobite, 5cm (Ohio), photo by author.

Larger trilobite, ~40 cm (Lourinho, Portugal), photo by author

Larger trilobite, ~40 cm (Lourinha, Portugal), photo by author

Diversification took off. Radially symmetry echinoderms were the ancestors of today’s starfish and sea urchins. Brachiopods, shellfish with hard upper and lower valves (as opposed to the left and right valves of modern oysters and scallops, to mention the most edible of them), grew wild on the sea floors next to many other creatures.

At the end of the Ordovician and the beginning of the Silurian, two mass extinctions took place, separated by around 4 million years. They are referred to as the Ordovician-Silurian extinction events. Since most life was in the sea, it was this sea life which suffered, It is estimated that 60% of marine invertebrates were destroyed. The extinctions were probably largely caused by climate change due to movement of the continents.

Mass extinctions, from Openstax College

Mass extinctions, from Openstax College

In the Silurian period, eurypterids (looking like scorpions or crayfish) developed which were capable of living in salt or fresh water, an important step in animal evolution. The ammonoids and nautiloids whose fossils we find so beautiful appeared toward the end of the Paleozoic.

Evidence for the second stage of the Cambrian Explosion, 525-520 Mya, comes from Chengjiang, China, a site where some soft parts of the organisms have been conserved in the fossil record. Among the Chengjiang finds is the oldest fish, which is also the oldest vertebrate, dating from about 500 Mya.

The first fossil evidence of fishes show species which had spinal cords (making them chordates) but no internal skeletons or jaws. The latter evolved from gills only later. Fish became numerous in the Devonian Period, which is often referred to as the “Age of fishes”. Although many types later became extinct, some of their ancestors survive even today: cartilaginous fish, like sharks or rays; fish with bones, like today’s trout or bass; and lobe-finned fish, like today’s lungfish.

At the end of the Devonian, another series of extinctions referred to collectively as the Late Devonian mass extinction took place. Individual events may have been separated by over millions of years. Mostly marine life was affected and trilobites were almost finished off.

Life on land

Plants first developed in water. The date of their migration onto land is still debated but seems to have taken place at least by around 480 Mya and perhaps as early as 600 Mya, in the late Precambrian. Low mossy plants appeared on land during the Ordovician. The migration of plants to land was facilitated by the development of a cellulose-based support structure and the ability to transport water in their stems. The oldest known such vascular plant dates from the mid-Silurian, about 430 Mya, and represents an important advance, as such plants had internal tubes by which water and nutrients could mount from the soil to replace moisture that was eliminated from the plant’s upper parts.

With the advent of woody stems, plants developed to the point where the Carboniferous Period was one of dense areas of vegetation, tree-like plants and swamps. Carboniferous plants were all seedless and so had no flowers. This plant material decayed and was eventually transformed by heat and pressure into the fossil fuels we are busily burning up in a tiny fraction of the time it took to make them.

Such carbon sequestration lead to higher oxygen levels in the atmosphere. The oxygen content of the Carboniferous atmosphere was 50-100% greater than now (Figure) and this had an effect on evolution. Giant insects evolved, including a dragonfly with a 65cm wingspan. Later, when oxygen levels came back down, the giant insects disappeared.18)In my opinion, fortunately.

During the Silurian, tiny arthropods appeared on land. They did not have a digestive system capable of making them herbivores, but lived off decayed matter. During the Devonian, skeletal changes which permitted animals to support themselves on land facilitated the transition from fishes to tetrapods (four-footed animals, including birds). The first land-based tetrapods were still aquatic or amphibious animals and probably lived mainly in ponds, but were capable of breathing air, so they could move to another pond in times of drought. They also laid their eggs in water, which furnished nutrients for the young, which were essentially fish (like tadpoles).

So first plants moved onto the land. They were followed by small arthropods, which ate decayed matter from the plants. And then tetrapods followed and ate plants and arthropods. It is all about getting enough to eat.

A very important evolutionary step was the development of the amniotic egg. This protected the young inside a protective cover and provided the nutrients that young amphibians were forced to get from water. This development contributed greatly to the evolution of amniotes (the first of which resembled small lizards), which now could leave the water completely. These animals split into two groups, synapsids (early mammals) and sauropsids (early reptiles).19)There is some disagreement here. Some authors refer to the first amniotes as reptiles and later speak of “mammal-like” reptiles. or “stem mammals”. It seems easier to speak of amniotes which were the ancestors of both mammals and reptiles. The first reptiles date from the mid-Carboniferous, during which life on land and sea reached a new peak of development and diversity.

Tectonically, what today would be Europe and North America were then situated in tropical climates near the equator. Indeed, because no land mass was over either pole, polar ice caps were limited and the Earth’s temperature gradient was less pronounced. On land, huge tree-like plants grew in swamps and life reached all the continents. Insects and tetrapods swarmed through the undergrowth. But no bird sang and no flower lent color to the scene.

Near the end of the Carboniferous, as Gondwana (the southern continent comprising today’s South Africa, South America, Antarctica, Australia and India) approached the poles, as seen in the following figure, there was a period of glaciation which lasted into the Permian. Remaining glacial features on these continents provide evidence for plate tectonics, as some of these continents now occupy much warmer latitudes20)Benton, 90.

The Permian Period was dominated by the existence of the supercontinent Pangea. Around the equator, the Carboniferous swamps had given way to deserts and these arid conditions were well suited to the development of reptiles.

To the east, projecting into the continental land mass, was the Tethys Sea, which was swarming with life. This was also true of the Zechstein Sea in the north, the area of current northern Europe. Parts of the Zechstein evaporated, leaving behind minerals (evaporites) which helped furnish raw materials for the Industrial Revolution – plaster of Paris, gypsum and substances used for the production of acids and ammonia.

The distribution and variety of organisms today is a result of the existence and subsequent breakup of Pangea. During its existence, no waterways blocked migration roots, so animals, at least those who could support the aridity of the interior, were free to move about to new habitats. The later breakup of Pangea was an equal boon to evolution as organisms isolated from one another tend to evolve in different ways from similar beginnings. Simply put, “isolation begets diversity.”

The time of Pangea was one of much development in the forms of life. By its end, dinosaurs and early mammals had developed. Many insects existed, including cockroaches, which are still with us, alas.

The end-Permian extinction

The Paleozoic Era ended with the greatest of all the mass extinctions, the end-Permian extinction. It is estimated that 96% of sea and 70% of land species disappeared21)McDougall, 321.. The date of the extinction marks the end of the Paleozoic and the beginning of the Mesozoic Era, largely accepted as 251 Mya. The causes of the extinction are still uncertain. It may have been initiated by volcanic eruptions in Siberia which increased the amount of methane and CO2 in the atmosphere, bringing about a “runaway greenhouse phenomenon”.22)Benton, 118. This in turn could have caused ocean currents to change in such a way as to lower oceanic oxygen content, bringing about extinction of the bottom end of the aquatic food chain. It could have caused acid rain which killed land plants vital to survival of animals. The CO2 would have been absorbed by the oceans and led to their acidification, wiping out many marine organisms. What is clear is that it took around 20 million years for life to recover, far longer than after the other known mass extinctions. When life did attain its previous diversity, its forms had changed. The few remaining trilobites had been completely eliminated.

A recent study23)“Ancient whodunit may be solved: The microbes did it!|” March 2014: MIT News, indicates that eruptions in the Siberian Traps increased the amount of nickel in the Earth’s crust and this was a nutrient for a microbe, a methane-producing archaea called Methanosarcina, which had undergone a genetic change at about that time. It is suggested that the microbe emitted vast amounts of methane into the atmosphere and that changed the climate. Who knows…

Don’t stop now. Continue global history with the ages of reptiles and mammals.

Notes   [ + ]

1. “Climate puzzle over origins of life on Earth”,
2. Prothero (2007), 147-52.
3. These results are for the Lost City Hydrothermal vents. Results differ some for other vents.
4. “The Lost City 2005 expedition”, NOAA,
6. Van Kranendonk, Martin J., Deamer, David,  and Djokic, Tara, “Life springs”, Scientific American, August 2017, 22.
7. There is some disagreement as to whether progenote simply means LUCA (last universal common ancestor). Others, more specifically, call it ‘”a theoretical construct, an entity that, by definition, has a rudimentary, imprecise linkage between its genotype and phenotype (Woese, 1987)”—a creature still experiencing progressive Darwinian evolution, in other words.’ From
8. Knoll (2003), 42
9. The capability of stromatolites to accomplish this task alone has been questioned and other mechanisms suggested.
10. Also called Nena, Nuna or Hudsonland.
11. The subject of reproduction through mitosis and meiosis will be discussed in more detail in the chapter on biochemistry and cellular biology.
12. This subject will be discussed in more detail in the chapter on biochemistry and cellular biology.
13. The Vendian Period of the Proterozoic Era ran between 600 and 542 Mya.
14. Also spelled Pangaea.
15. The last few paragraphs are based on Prothero (2007), 161-70.
16. Burgess shale fossils and their importance.
17. Or Ediacaran, about 650-450 Mya.
18. In my opinion, fortunately.
19. There is some disagreement here. Some authors refer to the first amniotes as reptiles and later speak of “mammal-like” reptiles. or “stem mammals”. It seems easier to speak of amniotes which were the ancestors of both mammals and reptiles.
20. Benton, 90
21. McDougall, 321.
22. Benton, 118.
23. “Ancient whodunit may be solved: The microbes did it!|” March 2014: MIT News,

Geophysics and plate tectonics

The discoveries made by geologists and paleontologists about earth’s history make one of the most interesting and essential stories ever. That is why so many folk traditions have imagined their own versions of the story in the days before the empirical sciences. Even though the details of some of the mechanisms behind the dynamics of earth science are not yet completely understood, the knowledge gained explains very satisfyingly – and in great detail – how and why the earth and its inhabitants, including humans, have come to be what they are today. It also gives hints as to what may happen tomorrow. So read on…

Geological time scale, by author

Geological time scale, by author

There is still discussion as to the standard to be used for “million years ago”: “Mya” or “MA”. We will use the more intuitive Mya = million years ago, Gya = billion (thousand million, or giga) years ago. “My” then defines a duration of a million years.

Some geophysics (Earth physics)

First, let us look at the composition of the Earth.

Minerals and rocks

Almost all rocks are made up of minerals, which are naturally occurring, inorganic, crystalline solids. Geologists distinguish them by their color, luster (how they reflect light), transparency (compare diamond and coal), streak (powdered form), hardness, tenacity, and cleavage and fracture (how they chip or break).

The mass of the Earth is constituted of the following proportions of elements by mass1)Marshak, 142.:

  • 32.1% iron
  • 30.1% oxygen
  • 15.1% silicon
  • 13.9% magnesium
  • 8.8% other elements, especially aluminum and calcium.

The most common elements in rocks at the Earth’s surface are silicon and oxygen. These form silicate, a mineral having a tetrahedral structure rather like that of methane, as we saw in the carbon chapter. This image shows two of them bound together by sharing an oxygen between two tetrahedra.

Silicate double tetrahedra, « Silicate-double-tetrahedra-2D » par Benjah-bmm27 — Travail personnel. Sous licence Domaine public via <a href=

Silicate double tetrahedra, by Ben Mills via Wikimedia Commons

This silicate binding can be extended to give the many different crystalline structures, such as rings and chains, which make up the rocks around us. For instance, quartz crystals have the following structure.

Ball-and-stick model of part of the crystal structure β-quartz, a form of silicon dioxide, SiO2, by Ben Mills via Wikiemedia Commons

Ball-and-stick model of part of the crystal structure β-quartz, a form of silicon dioxide, SiO2, by Ben Mills via Wikimedia Commons

Beryl has a ring structure, shown here both as atoms and in a tetrahedron representation.

Unbranched 6er single ring of beryl from Brown and Mills via WIkimedia Commons

Unbranched 6er single ring of beryl from Brown and Mills via Wikimedia Commons

Non-silicate minerals (e.g., carbonates, sulfates and sulfides, oxides, etc.) make up only 5-8% of the crust.

Rock cycle and types

As almost everyone knows, there are three types of rocks:

  • igneous — formed when molten rock (magma or lava) cools;
  • sedimentary — formed when mineral or organic particles, such as weathered or eroded rocks, settle on the Earth’s surface or at the bottom of a body of water;
  • metamorphic — formed by the transformation of other types by intense heat or pressure, without melting.

Rocks may be intrusive (formed beneath the surface of the Earth) or extrusive (formed above the surface). Among igneous rocks, there are several types based on the amount of silica in them:

  • felsic rocks contain > 65% silica and are generally light-colored;
  • mafic rocks contain 45-55 % silica and are generally dark-colored;
  • ultramafic rocks contain <45% silica.

Some common examples:

  • granite — intrusive, igneous and felsic
  • rhyolite — extrusive, igneous and and felsic
  • basalt — extrusive, igneous and mafic
  • sandstone — extrusive and sedimentary
  • quartzite– intrusive and metamorphic

Rocks and minerals are formed in a series of cycles. Volcanic activity causes hot magma to pierce the Earth’s surface, where it cools and hardens into various sorts of igneous rocks. Other material may be coughed up from below ground at the same time as igneous or metamorphic rock. As these rocks are weathered by ice, water and wind, they are broken down into smaller grains which are then carried to the sea or lake floors, where they accumulate, eventually to form sedimentary rocks. These rocks may then be pushed by subduction down to a hotter, denser level where they are converted into igneous or metamorphic rocks and the cycle begins again2)This is only one of the cycles of nature. It may be compared to, e.g., the water cycle (clouds – rain – bodies of water – evaporation – clouds) or the nitrogen cycle (fixation – assimilation – nitrification – denitrification).. Such cycles have been active for 4 billion years.

Interior structure of the Earth

The Earth consists of several layers. From the center:

1. The core, around 2500 km in radius, is composed of heavy metals such as iron and nickel and has two layers:

  • the inner core, believed to be solid;
  • the outer core, shown by seismic waves to be a very viscous liquid.

2. The layer outside the core is called the mantle and constitutes about 2/3 of the Earth’s mass. It is around 2800 km in radius and is composed mostly of light elements with some heavy ones. It has three layers:

  • The mesosphere: Although temperatures here are high enough to melt the rock, the intense pressure keeps it solid.
  • The asthenosphere: This layer is also solid, but still can flow, in a very slow plastic flow.
  • The lithosphere: This is the outermost layer of the mantle and comprises both the outermost mantle layer and the crust. The lithosphere is rigid and relatively brittle. The mantle of the lithosphere is distinguished from the crust by its mineral composition.

3. Outside the so-called Moho discontinuity in the lithosphere lies the Earth’s crust, attached to the outer mantle. Above the Moho, the rocks are lighter and felsic, constituted primarily of silica, the principal constituent of surface rocks. There two kinds of crust:

  • The thicker continental crust is about 20 km in depth (higher for mountains) and is composed mainly of granites, relatively light felsic rocks. The existence of dry land today depends on the existence of this relatively light rock.
  • The thinner oceanic crust is typically less than 10 km thick, and is composed mainly of basalt, which is denser than continental crust.
Earth's core , from "This dynamic earth" ia USGS

Earth’s core , from “This dynamic Earth” via USGS. Distances are depths, measured from the surface.

The relative densities of the two crust types are important in plate tectonics.

The Earth’s magnetic field

Because of its iron core, the Earth is like a bar magnet and has a dipole magnetic field with north and south poles. This is thought to be due to a dynamo effect, wherein charged particles in the hot core move about due to convection or the Earth’s rotation.

Actually, the polarity changes randomly in geologic time: The poles actually reverse. If this were to happen today, Antarctica would be found in the south! Fortunately for human navigation, this does not happen frequently, the last time having been about 780 thousand years ago. Geologists find a record of this reversing polarity in the phenomenon known as magnetic striping – variations in direction of the magnetic field of the rock on the ocean floor.

As it “burns”, the sun continually spews out charged particles, mostly electrons and protons, creating a solar wind. Fortunately for us, the magnetic field due to the Earth’s metal core deflects these particles so that most of them pass around the Earth without doing any harm. Otherwise, they could actually blow away the Earth’s atmosphere. Mars is much smaller than Earth and has lost its original molten core and magnetic field, so the solar wind has stripped away its atmosphere, leaving the red desert that space probes are exploring today.

As the solar wind passes over the poles, where the magnetic field is most intense, they sometimes ionize the air and produce the beautiful wavy colors of the aurora.

The Earth’s magnetic field can deflect only a certain amount of the solar wind. In 1859, an enormous solar storm called the Carrington storm released so much electromagnetic energy and solar matter that aurora were seen as far south as the Caribbean and as far north as Santiago, Chile. Telegraph communication was often interrupted and operators reported sparks jumping from their equipment. If it happened again now, things like GPS satellites and telecommunications equipment could suffer.

Plate tectonics

Plate tectonics (from the Greek tecton, builder or carpenter) is the theory which explains how plates of continental crust move about on the surface of the Earth. It answers the question of how and why they move. It explains the existence of volcanoes and mountains and furnishes the background for all Earth science. It has been said to be the theory of “how the Earth works”. It has transformed and integrated modern geology just as quantum mechanics and relativity have modern physics or evolution, biology.

The idea of plates, continent-sized hunks of lithosphere moving on the surface of the earth, was first suggested by the comparison of the eastern border of South America with the western border of Africa. Since then, understanding of the Earth’s interior has explained how this works, although some details of the driving mechanism (especially, mantle plumes and hot spots) are still subjects of discussion .

According to the theory, the Earth’s lithosphere is composed of plates, seven main plates and several smaller ones. The plates are essentially huge hunks of rock centered around very ancient cores called cratons, most of which were formed during the Precambrian eons, perhaps as early as 4.3 Gya. They were certainly in existence by around 3800 Mya, according to the age of sediments in western Greenland. Recent studies suggest that plate tectonics began about 3 Gya.3)“New study zeros in on plate tectonics’ start date’, The oceanic plates are thinner (less than 15km), but are composed mostly of very heavy basalt. The continental plates are thicker (up to 200km) but are composed mainly of granitic rocks, which are lighter. The plates essentially float on the asthenosphere, the next layer down, which is not a liquid, but nevertheless can flow very slowly due to its very high temperature.

It is currently thought that thermal convection due to the Earth’s hot core causes the plates to move very slowly about. This movement has caused the configuration of land – the continents – to change over geological history.

The Earth's tectonic plates, from "This dynamic earth" at USGS

The Earth’s tectonic plates, from “This dynamic Earth” at USGS

When a plate of dense, oceanic rock collides edge-on with a plate of less dense continental rock, it is pushed down under the edge of the continental plate in a process called subduction. The descending ocean plate may create a deep trench at the boundary. Because of the heat from the rocks’ rubbing together and because water vapor released by the oceanic crust lowers the melting point of the rocks in contact with it, much of the rock melts, creating magma. Some of this rises through faults in the crust and a line of volcanoes may be formed along the edge of the continental plate above the subduction zone. The western coast of South America and much of that of North America are regions where the Pacific plate is being subducted under the American plates, forming the Andes and other mountains.

The Indian plate, which was once attached to east Africa, has moved northward and swung around to plow slowly into the southern edge of the Eurasian plate. The impact between the two equally dense continental plates brought about no subduction, but instead pushed up the string of mountains known as the Himalayas. The process has not ended and the Himalayas are still growing.

Subduction, from "This dynamic earth” at USGS

Subduction, from “This dynamic Earth” at USGS


Ocean-floor magnetic striping, from "This dynamic earth” at USGS

Ocean-floor magnetic striping, from “This dynamic Earth” at USGS

Volcanic processes deep in the Earth below the ocean beds may break through the thinner oceanic crust, letting magma escape. This magma usually forms basalt on cooling, so the Earth’s ocean floors are paved with dense basalt.

Right down the middle of the Atlantic Ocean, there is a ridge formed by magma pushed up by volcanic processes. As the magma flows out in east and west directions, perpendicularly to the ridge, the plates on either side grow at a rate of about 2.5 cm per year, so that their outer parts are pushed further east and west. In this way, the plates on either side of the Atlantic grow outwards from the mid-ocean ridge. In plate tectonics, this movement of two plates away from each other is called a divergent boundary or rift.4)There is still some disagreement as to whether the rifting is pushing the plates apart or whether the moving plates are pulling away at the rift.

Not all magma moves to the side immediately. It tends to pile up, so that the central line of activity is higher than the surrounding area, forming an underwater mountain chain, such as the one down the center of the Atlantic. Running along the middle of the sub-oceanic chain, there is a trench where the magma comes out.

All the world’s oceans have come about in such a rifting process. In the case of the Atlantic Ocean, the oceanic and continental crusts on each side form single lithographic plates, so the above-water parts of the continental plates of the Americas and of Africa and Eurasia seem to move apart.5)The mechanism explaining this phenomenon is still a subject of  discussion. Since the surface area of the Earth remains the same, plates which are added to in one place must be destroyed elsewhere. As the Atlantic gets wider, so must the Pacific get smaller. This illustrates the widening or disappearance of oceans.

Magma also may break through the ocean floor at a single spot, know as a hot spot, bringing about the formation of a volcanic island above it. As the oceanic plate slides across the hot spot, a series of volcanoes may be formed, as is the case for the Hawaiian Islands or Icelandic volcanoes.

Molten magnetic rock solidifies in such a way that magnetic dipoles in it conserve the orientation of the Earth’s magnetic field. As this changes periodically, so does the magnetization recorded in the rocks. The symmetric distribution about the trench of the magnetized rocks points clearly to spreading of the sea floor.

Plates may converge on each other, as in subduction along the Pacific shore of South America, or diverge from each other, as is probably happening in the Great African Rift, or slide alongside each other, as in the San Andreas Fault in California. Sometimes, two zones of continental crust may converge, piling up mighty mountain chains, as has been the case with the Pyrenees, the Alps or the Appalachians and is still going on in the Himalayas.

Iceland is a fascinating geological showplace where the mid-ocean mountain range has been pushed up above the water level by a local hot spot. Over time, the country has moved relative to the hot spot, leaving volcanic craters along the path. The whole island country sits astride the rift between the North American and European plates.

I celand sits astride the joint of two plates, from "This dynamic earth” at USGS

Iceland sits astride the joint of two plates, from “This dynamic Earth” at USGS

Looking out over the mid-Atlantic Ridge towards the European plate at Þingvellir, Iceland. Photo by author.

Looking out over the mid-Atlantic Ridge, the the rift between the American and European plates, at Þingvellir, Iceland. Photo by author.


So according to the theory of plate tectonics, the continents we know have been in different configurations over the eons, sometimes separate, sometimes joined together into different large continents. The last time they have were joined together into a single supercontinent, it was called Pangea, about which, more later.

In our solar system, plate tectonics is unique to Earth; no continental plates are known on other planets. Plate tectonics is of great importance for the evolution of living organisms.

The Earth’s atmosphere

Life on Earth depends on its atmosphere, a thin layer of gases, mostly nitrogen and oxygen, as well as some hydrogen and carbon and minute quantities of other elements. The atmosphere protects us and provides an environment suitable for life. It in turn depends on several other factors: the Earth’s temperature, its size (and, therefore, gravity) and its distance from the sun. In fact, in the beginning of Earth’s existence, the sun may was not so bright, so temperatures would have been lower than one might expect.

What the atmosphere brings to the Earth

The atmosphere is important not only for its breathable gases but also for the protection it affords. It is essential in acting as a greenhouse gas to conserve the warmth from the sun. This effect can be overdone, as has been the case on Venus, which is much too hot to sustain life. The atmosphere also protects us from dangerous ultraviolet rays.  And it shields us from countless meteorites, which are burned up from friction in the atmosphere before they can strike the ground and make huge craters, as has occurred on the atmosphere-less planet Mercury or on the Moon.

What the Earth brings to the atmosphere

The atmosphere is held onto the Earth by the gravitational force of the Earth itself. Earth’s mass and therefore its gravity is just right to hold onto the heavier gases nitrogen and oxygen that we need to breathe in order to survive. In contrast, Mercury’s gravity is too weak to hold on to much of anything, so it has lost all its atmosphere. Jupiter, which is much larger than Earth, retains light gases as well, but packs them all down into concentrations such that its atmosphere is enormously dense. Tremendous storms rage across its surface, the most obvious one being the famous Red Spot.

The Earth’s size and its distance from the sun are in a delicate balance. If the Earth were closer to the sun and therefore hotter, or if it were smaller and had less gravity, the movement of the atmosphere’s molecules would be great enough to allow them to escape the Earth’s gravitational attraction. Similarly, the relation between the production of heat by Earth’s core and the size of the Earth is crucial to maintenance of a temperature conducive to life. The temperature of the Earth’s surface also depends on its distance from the sun (closer would mean hotter) and its rotation (toasting each side evenly). The planet Venus is much closer to the Sun and its surface is burned up.

The Earth’s temperature and gravity guarantee conditions necessary to the existence of water in three phases – gas, liquid and solid – which are the source of all Earth’s weather and the rain cycles which irrigate the land. On Titan, one of Jupiter’s moons, analogous conditions hold for different reasons, namely that its atmospheric molecules are much cooler than Earth’s, so its lesser gravity is adequate to retain them. But the greatly reduced temperature relative to Earth’s means that the phases there are of methane, not oxygen. All the water on Titan is frozen hard as steel.

All these things, the hot magnetic core, the atmosphere, the distance from the sun, the size and the orientation of the Earth work together to make life possible on our planet. Without any one of them, things would be a lot different. The Earth – and we, ourselves – must be considered privileged to be in a set of conditions propitious for life.

How do we know all this?

Geological clues come from different sources.

  • Volcanic and other magma activity, along with measurement of seismic waves (which may be artificially induced) can show the configuration of the Earth’s mantle.
  • Various rock formations, folds and magnetic properties give evidence of the action of plate tectonics. Evidence for plate tectonics also comes from fossils in the plates, especially along edges of plates which were formerly contiguous. The mid-oceanic trenches have been measured by traditional means and by satellite.
  • Quantities as diverse as the distribution of elements (e.g., iridium) and isotopes (e.g, oxygen) can give evidence about oxygen concentration or water temperatures.
  • Measurement of remaining quantities of radioactive isotopes of certain elements with known  decay rates can be used to determine the time which has passed since the element was incorporated into an object, rock or fossil. This method has been used in combination with fossil dating to determine the sequence of geological events described in this document.
  • Fossils show the evolution of living beings and can be used to establish relative dates of the rocks in which they are found.

Now on to the first eon, the Hadean.

Notes   [ + ]

1. Marshak, 142.
2. This is only one of the cycles of nature. It may be compared to, e.g., the water cycle (clouds – rain – bodies of water – evaporation – clouds) or the nitrogen cycle (fixation – assimilation – nitrification – denitrification).
3. “New study zeros in on plate tectonics’ start date’,
4. There is still some disagreement as to whether the rifting is pushing the plates apart or whether the moving plates are pulling away at the rift.
5. The mechanism explaining this phenomenon is still a subject of  discussion.

The Mesozoic and Cenozoic Eras — reptiles and mammals

The Mesozoic Era – age of reptiles

The Mesozoic began after the End-Permian mass extinction, 251 Mya, and ended in the less catastrophic but better-known Cretaceous-Tertiary (“K-T”) mass extinction, 65 Mya. Like the Paleozoic, it is subdivided into periods:

  • the Triassic (251-200 Mya),
  • the Jurassic (200-145 Mya) and
  • the Cretaceous (145-65 Mya).

As life in the Mesozoic was dominated by reptiles, including dinosaurs, it is often called the “Age of Reptiles”.


As the Mesozoic opened, there was only one continent, Pangea. Pangea did not just form and then sit stationary for millions of years; tectonic activity continued throughout the era. Evidence from fossils (types of organisms) and sedimentary rocks (formation from sand dunes) indicate that the interior of Pangea was quite arid.

As plates shifted, rifts developed and sea water periodically spilled into the rifts. In between periods of flooding, evaporating water left behind salts (evaporites). Measurement of the ages of these salts gives geologists a calendar of the opening of the rifts and therefore the development of the inter-continental oceans.


Movement of the continents, from “This dynamic earth” via USGS

The main geological event of the Mesozoic was the breakup of Pangea. The Tethys Sea penetrated into the flank of Pangea from the east and around the end of the Triassic, 200 Mya, it split the supercontinent into two parts, Laurasia in the north and Gondwanaland in the south. Around the end of the Jurassic, 50 million years later, Gondwanaland rifted and South America and Africa began to separate. The Atlantic Ocean opened up. The resulting changes in continental and oceanic configurations brought about changes in the climate. It is thought that the climate during this era was generally warm, with a reduced temperature gradient between the equator and the poles. This spurred evolution, for instance, through the creation of new econiches.

Life in the sea

The ancient “mascot” of the Paleozoic, the trilobite, did not survive the end-Permian extinction. This diverse group of arthropods had proliferated in the seas for over 250 million years. In terms of longevity, they beat out the soon-to-become-dominant dinosaurs. Nor were there any reefs during the early or middle Triassic. It is amazing anything did survive. The extinction was like pushing the restart button for life on Earth. But survive, it did.

An expansion in the numbers of plankton brought about abundance at the bottom of the food chain. Skeletons of these tiny beings, which precipitate to the sea bottom after death, contain calcium carbonate and silica. They pile up on the sea bed to form chalk, hence the name, cretaceous. The cliffs of Dover have their origin in these minuscule creatures. Elsewhere, along the edges of the Tethys Sea, organic material decayed and metamorphosed to produce the oil found today in places like Russia, the middle East – the source of so much pollution and conflict.

Marine reptiles of new sorts appeared. The ichthyosaur, a giant fish-like marine animal was a land reptile which had returned to the sea but continued bearing its young alive. However, it was not the ancestor of current whales or dolphins.

Life on land

Some land plants survived the extinction. Forests of conifer and ginkgo trees, among others, grew up. Flowering plants (angiosperms) showed up only late in the Mesozoic, around 100 Mya, but quickly spread to all environments. Such plants produce seeds which, like the amniotic egg for animals, are furnished with a protective skin and and plentiful nourishment. Through nourishment and pollination, plants and insects have co-evolved ever since the Cretaceous. Plants developed colors and structures that protected them from all but specific insects. Insects simultaneously developed so as to live off certain flowers. Flowering plants and insects form an inseparable symbiosis. Think ecology.

The acknowledged stars of the Mesozoic were the reptiles, who came to dominate the sea, the land and the air for almost 200 million years. Dinosaurs diversified by occupying econiches left vacant by the animals killed off in the end-Permian extinction. They evolved to eat vegetation, meat, fish and even insects. They lasted so long and went so far that, had they had historians or philosophers, these might well have thought that they were the end product of evolution and, so, eternal. How about that?

So-called stem reptiles1)Or captorhinids; ancestors of archosaurs. already had been around at the end of the Permian, having evolved from amphibians. Although their numbers were greatly diminished by the end-Permian extinction, they made a comeback in the early Mesozoic.

At that time, there were principally two types of reptiles which had survived the extinction. The cynodont (of the therapsid group) was one of the lucky survivors from the late Permian. It was a mammal-like reptile and had skin instead of scales. It probably had hair and whiskers and may have been endothermic (warm-blooded). In spite of being a reptile, details of its skull and jaw were mammal-like. It remained small and furtive throughout the Mesozoic, living in burrows by day and sneaking out at night to scavenge or hunt for insects. Among its descendants is the species Homo.

The other surviving reptiles were the archosaurs2)Again, there is disagreement as to whether the archosaurs are the animals discussed here or others which appeared only in the early Triassic., whose current descendants include turtles, snakes, lizards, crocodilians and birds. They also evolved into dinosaurs.

The oldest dinosaur fossils date from around the early Triassic, 240 Mya. The fossil record of dinosaurs is extremely incomplete, but it is known that they spanned the globe, largely due to their amniotic egg and scaly skin, which enabled them to live far from the sea. As Pangea started breaking up in the Jurassic, the climate grew moister, which reduced aridity, making a much greater area habitable by animals.

Dinosaur eggs from Museu da Lourinhã, Portugal. Photo by author.

Dinosaur eggs from Museu da Lourinhã, Portugal. Photo by author.

The boundary between these two periods was marked by the Triassic-Jurassic mass extinction. This extinction occurred in two or three steps across some 18 million years. Although various causes are ascribed, it is known that at least half the species on Earth disappeared, allowing dinosaurs to take over as the dominant species. So the Jurassic was to be the age when dinosaurs ruled.

It is not certain whether dinosaurs were warm or cold-blooded. The biggest ones would have needed high blood pressure in order to pump blood from the heart to the brain, many meters away, maybe even in an upward directions; this would have implied a mammal-like circulatory system, making them warm-blooded.

The first dinosaurs were quite small, about 1 meter long. Many of them were bipedal. In the course of their long reign of almost 200 million years, they diversified and evolved. Many grew larger or developed defensive armor, like the stegosaurus or the triceratops. Dinosaurs did evolve quite a lot, and not all species of them lived simultaneously. The fight between Tyrannosaurus rex and Stegosauras during the excerpts from Le sacre du printemps in the original movie “Fantasia” could not have taken place, as these animals lived at different times.

Paleontologists identify two types of dinosaurs, based on the bones of their hip and pelvis: ornithischian ( “bird-hipped”) and saurischian (“lizard-hipped”). The bird-hipped dinosaurs could be either bipedal or quadrupedal and were all herbivores. The stegosauraus was one of them. The lizard-hipped dinosaurs are in turn divided into two groups: sauropods and theropods. Sauropods were the commonest and were huge, including some of the largest land animals ever, diplodocus. Theropods were all bipedal and carnivorous and included the infamous tyrannosaurus.

Characteristics of a  very limited number  of dinosaurs are indicated in the following table.

Name Length/height Weight (kg) Diet Pedalism Time
coelophysis 3m/2m 27 carn 2  late Triassic
megalosaurus 9m/- n/a carn 2 mid Jurassic
stegosaurus 9m/- n/a herb 4 late Jurassic
archaeopteryx 0.5m/0.2m 0.4-0.5 carn 2, flying late Jurassic
diplodocus 26m/8m 20000-25000 herb 4 late Jurassic
triceratops 9m/3m 5500 herb 4 late Cretaceous
tyrannosaurus 12m/5.6m 7000 carn 2 late Cretaceous

Some of these dinosaurs are pictured in the following gallery (not to size).

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Birds only evolved in the late Jurassic, 140-150 Mya. One of their reptilian fore-fingers grew long enough to support a web of skin which helped the animal to plane in the air – the first wing. The early pterosaur (“winged lizard”) was a flying reptile, not yet a bird. Birds and pterosaurs evolved from reptiles along separate lines (following figure). The transition between reptiles and birds is thought to be archaeopteryx, which had a reptilian skeleton but possessed feathers formed for flight. This made it a better flyer than the pterosaur. During the Cretaceous, these animals developed hollow bones like modern birds. It is now known that a number of dinosaurs had feathers, which probably served as insulation since they did not have the asymmetric form needed to act as an airfoil.

The world was now enormously different from the Paleozoic, enhanced (from our point of view) by flowers and bird song.

Possible family tree of dinosaursk birds and mammals. from Wikipedia

Possible family tree of dinosaursk birds and mammals. from Wikipedia

K-T extinction

At the end of the Cretaceous and just before the Tertiary, there was another great extinction. Though much less serious than the end-Permian extinction, this one has captured more imaginations. This is partly due to the fact that it brought about the end of the dinosaurs, except for birds, thereby permitting the rise of mammals and the eventual arrival of humans. It is also due to the explanations offered for the extinction.

One explanation depends on volcanic explosions in the Deccan Traps, in what is now central India. The other explanation cites the crash of a mammoth asteroid into the Earth on the coast of what is now Yucatan, forming the Chicxulub Crater. Both events occurred very near the K-T border. The asteroid would have caused a massive tsunami. Both events would have flung debris and gases (sulfur dioxide and carbon dioxide) into the air, darkening the planet and reducing temperatures.3)According to one study, temperatures would have been reduced by at least 26°C, which would have brought about temperatures around -15­C. or Cooling surface waters would have brought up nutrients from below and favored plankton blooms, affecting plant and animal life.

The result was the disappearance of some three quarters of plant and animal species on Earth, including ammonites and dinosaurs. Fossil evidence shows that the extinction was world-wide. But some members of each group of organisms managed to survive into the Cenozoic Era.

Mass extinctions, from Openstax College

Mass extinctions, from Openstax College

Scientists usually count five major mass extinctions, which are summarized in a preceding figure and in the following table. 

Time Approx. date (Mya) Probable cause(s)4)Causes from Wikipedia, Ibid., and BBC Nature. “Big five mass extinctions”, Principal victims5)Figures from Wikipedia, “Extinction event”,
Ordovician-Silurian 450-440 Climate change (ice age) due to continental movement 60-70% of all species (2nd worst), mainly in sea
Late Devonian 375-360 Asteroid impacts,changes in sea level and chemistry At least 70% of all species, worst in shallow seas
Permian-Triassic (End-Permian, “The Great Dying”) 252 Atmospheric change due to volcanic eruptions – and much more 90-96% of all species (worst), including trilobites and other marine creatures, and insects
Triassic-Jurassic 200 Massive lava floods 70-75% of all species  
Cretacious-Tertiary (“K-T”) 65  Volcanic explosions and basalt floods, asteroid impact 70-75% of all species, including all dinosaurs except birds 

The Cenozoic Era – mountains and mammals

Geology and atmosphere

The Cenozoic Era is considered to be divided into several periods:

  • the “Tertiary” (65-1.8 Mya), commonly broken down into
    • the Paleogene (65-23 Mya) and
    • the Neogene (23-1.8 Mya);
  • the Quaternary (from 1.8 Mya).

Each period is broken down into two or three epochs. Some scientists consider that we now are living in a new epoch, the Anthropocene, dating vaguely from the beginning of the Industrial Revolution (or perhaps the beginning of agriculture), but the term has not yet been accepted by any official geo-chronological body.


The Cenozoic Era has been called the age of mammals, but it could equally well be called the age of orogeny, or mountain-building. The band of mountains running from North Africa to Indonesia, including the Atlas, Pyrenees, Alps, Taurus and Himalayas, was formed during the Cenozoic.

During the Cretaceous, Eurasia and Africa were separated by the Tethys Ocean. When the Atlantic started to spread out and Africa broke loose from Antarctica, it floated northwards towards Eurasia. Initially, small bits of continents within the Tethys (now Spain and Italy) were pushed up and joined to southern Europe, forming the first Alps and Pyrenees. As Africa continued its journey north, the Tethys was closed and the Mediterranean formed. Subduction due to Africa’s continued northward push is responsible for the volcanoes around southern Italy, Sicily and Greek islands like Thira (Santorini).

In a similar way, the Arabian plate moved toward Turkey and pushed up the Taurus Mountains. This plate too is still moving and is responsible for earthquakes in the Middle East.

With the breakup of Gondwanaland, what is now India started moving north. Around 55 Mya, it entered into collision with Asia, administering the coup de grace to the Tethys. The crust of India was too light to be subducted, but some of it slid in under Asia, making the continental crust there almost twice its maximum elsewhere. The result was the chain of the mighty Himalayas and the high Tibetan plateau. In both places, there are sedimentary rocks from the floor of the Tethys, with accompanying fossils, such as the ammonites called shaligrams, which are worshiped by Hindus as an incarnation of the god Vishnu (him again!). India’s encroachment on Asia is still going on at the rate of around 2 cms per year. This process leads to buildup of tension in the rock which, when released, causes enormous earthquakes in China.

Shaligram (ammonite) from the Himalayas, photo by author

Shaligram (ammonite) from the Himalayas, photo by author

As summer heat warms up the high Tibetan plateau, moist tropical air is drawn in. When this precipitates, it brings about the monsoon. This weather phenomenon is both good (irrigation) and bad (flooding and mudslides) for India and Bangladesh.

Since the Earth’s surface maintains a fixed area, the expansion of the Atlantic Ocean causes the squeezing up of the Pacific. As the Pacific plate has subducted almost all the way around its perimeter, volcanic activity has arisen there – the so-called Pacific Ring of Fire.


The climate continued to grow warmer during the Cenozoic, reaching a peak about 55 Mya during the Eocene Epoch. At the beginning of the Oligocene Epoch, about 34 Mya, temperatures started falling steeply, leading to the ice ages of the Quaternary. Temperatures can be deduced from the shape of fossil leaves and from the isotopes of oxygen in limestone formed in the sea. The two methods agree well on the high temperatures of the Eocene epoch and the sharp drop at the beginning of the Oligocene. In the next chapter (Paleontology), we will see the important influence of these climate variations on primate development.

The origins of these temperature shifts are found in plate tectonics. The existence of a large antarctic continent meant that sea waters had to flow around the whole continent. But when Australia and South America separated from Antarctica in the Cenozoic, it became isolated in a circuit of cold waters and a permanent ice cap gradually formed, around 10 Mya. In a similar way, the closing of the Atlantic-Pacific connection by the formation of the Isthmus of Panama 3-4 Mya brought about reinforcement of the warming Gulf Stream, good news for Europe. But this led moisture up to the north, where it precipitated and soon formed a polar ice cap there. Ice caps bring about global cooling. Plate tectonics influences climate which, in turn, influences evolution.

The still-controversial uptake hypothesis suggests that the chemical weathering of recently-exposed rock removes CO2 from the atmosphere. If so, then formation of the Himalayas might in this way have contributed to global cooling.

Since the beginning of the Quaternary, there have been a series of glaciations, or ice ages, with periods of roughly 100,000 years. These cycles, called Milankovitch cycles after the mathematician who first calculated them (without a computer!), have been shown to depend on the Earth’s orbit, its rotation and its distance from the sun. This repeated glaciation has shaped the Earth through erosion and displacement of rocks. The last great ice age started about 130 Kya and left northern Europe and America only around 12-15,000 years ago. We are now living near the peak of a relatively warm interglacial period, but there is no reason to expect that to last indefinitely. Computer models incorporating Milankovitch cycles, continental configurations, atmospheric composition (CO2) and many other factors are currently used to predict the future of global climate.


As the Mesozoic was the age of reptiles, so was the Cenozoic the age of mammals.

During the Tertiary, mammals took over the econiches left empty by the then-extinct dinosaurs, just as these had occupied econiches left vacant after the end-Permian extinction. Like the dinosaurs, mammals rapidly grew in numbers and varieties. They currently range in size from mice to elephants, and of course whales. During the ice ages of the early Quaternary, some grew to be enormously big: a huge rhinoceros-like animal; elephants and mammoths; whales, which include the largest animal ever, bigger even than the largest dinosaurs; various large animals like camels, elks, sloths, bears and so forth. Although their size made them well adapted to cold climates, most of these megafauna died out about 13 Kya, for reasons which are still not understood.

The Earth has been around for 4700 million years and mammals since something like 250 million years but only in great numbers for 65 million years. Man has only been around for at most 4 million years – and even then he did not bear much resemblance to us.

But that is for the next section (What paleontology tells us).

And now…

Plate movements are not finished. At this moment, Africa is on the move. The entire continent is moving slowly north, causing volcanic activity on mainland Italy (Vesuvius) and Sicily (Etna). One day, Africa will smash (slowly) into southern Europe, raising new mountain ranges and obliterating the Mediterranean Sea. Meanwhile, within Africa, the Great Rift Valley is threatening to break off all of East Africa into a new (small) continent, with a new ocean in between it and the rest of Africa. And who knows what will become of poor little Iceland or the eastern edge of North America? The dance continues.

Temperature and sea-level variations will certainly continue. In the long run, whatever that may turn out to be, there is no reason to expect the climate to remain friendly to the human species. Bacteria, not prokaryotes or eukaryotes are the basis and the necessary ingredient for life on Earth. We depend on them, not the other way around. One day we will go the way of the trilobites and the dinosaurs – and probably much sooner than they.

For more, go find out what paleontology tells us.

Notes   [ + ]

1. Or captorhinids; ancestors of archosaurs.
2. Again, there is disagreement as to whether the archosaurs are the animals discussed here or others which appeared only in the early Triassic.
3. According to one study, temperatures would have been reduced by at least 26°C, which would have brought about temperatures around -15­C. or
4. Causes from Wikipedia, Ibid., and BBC Nature. “Big five mass extinctions”,
5. Figures from Wikipedia, “Extinction event”,

What geology tells us

This is the history of the Earth, from about 4.5 billion years ago until now.

The first chapter chronicles the Earth’s hellish beginnings, the Hadean Eon, as well as geophysics, including plate tectonics.

The second chapter follows the rise of life up to the age of reptiles.

The third chapter starts with the age of reptiles, folows their demise and the rise of mammals and up to the present day.

Cheat sheet

Some generally useful information you may want to look up occasionally.

Geological time scale, eons, eras, periods and epochs

Geological time scale and

Geological time scale — red lines are mass extinctions, past or to come…

Types of hominins

Timeline and grouping of principal fossil hominid species

Timeline and grouping of principal fossil hominid species

Biological species classification

Classification of modern humans and house cats, after Wikipedia

Classification of modern humans and house cats, after Wikipedia

The periodic table of the elements

Periodic table of the elements

Periodic table of the elements

Particles of the standard model

Standard Model particle zoo

Standard Model particle zoo

Hominoid clades

Hominoid families with dates.

Hominoid families with dates.

Phylogenetic tree

Phylogenetic tree By MPF [Public domain], via Wikimedia Commons

Phylogenetic tree By MPF [Public domain], via Wikimedia Commons