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Pre-modern and modern Homo, and tools

Pre-modern Homo

Homo habilis fossils have been found in East and South Africa and dated to 2.4-1.4 Mya. His brain was slightly larger (550-680 cm3) than those of his predecessors. It has been claimed that the cranium shows evidence for a developed Broca’s area1, suggesting that he may have used spoken language to communicate, although Broca’s area is hardly a sufficient condition for speech. His face and teeth were smaller than those of australopiths, so maybe he was more carnivorous, but his body was more ape-like. He may have used stone tools, but it is difficult to know if these were made by him or by other species in the same region. Whatever the case, the supposition that he had done so resulted in his being considered the first homo. Nevertheless some researchers find little to distinguish between H. habilis and australopithecus. It also is not clear that H. habilis was an ancestor of H. erectus, as the time periods of the two overlapped by a half million years.

Skull of Homo habilis, photographed by author at Olduvai Gorge Museum, Oct 2012.

Skull of Homo habilis, photographed by author at Olduvai Gorge Museum, Oct 2012.

A recently found jawbone dating from 2.75-2.8 Mya, pushes back the date of the oldest fossil of genus Homo.1)“Oldest human fossil found, redrawing family tree:, http://news.nationalgeographic.com/news/2015/03/150304-homo-habilis-evolution-fossil-jaw-ethiopia-olduvai-gorge/.

Homo rudolfensis lived in East Africa around 1.9-1.8 Mya. His brain was larger and his face longer than those of H. habilis, but his chewing teeth were larger, more like those of Paranthropus. Some paleontologists think he was an Australopithecus; some others, the same species as H. habilis. No good example of his skeletal structure has been discovered. A recent find of a cranium and jawbones.2)New fossils confirm diverse species at the root of our lineage”. Smithsonian Human Origins Program,http://humanorigins.si.edu/research/whats-hot/australopithecus-sediba-%E2%80%93-new-analyses-and-surprises. http://humanorigins.si.edu/research/whats-hot/new-fossils-confirm-diverse-species-root-our-lineage supports the idea of H. rudolfensis as a Homo.

Homo ergaster (considered by many paleontologists to be early African H. erectus) lived 1.9-1.5 Mya in East Africa. He was the first hominin to have a body silhouette similar to that of modern humans and his proportions indicate that he lived on the ground. He was 1.7 m tall and capable of running and of walking long distances. His brain, at 850 cm3, was bigger than that of Paranthropus, but not yet that of a modern human, one more piece of evidence that larger brain size followed bipedalism on the evolutionary time scale, rather than preceding it. It is easy to imagine H. ergaster as the first “naked ape”. H. ergaster used primitive tools of the “Olduwan” culture.3)Tools will be discussed in more detail in a later paragraph.

Homo erectus lived 1.9-0.14 Mya (or even later), making him the longest lived human species to date. Modern humans are far from having achieved such a long species lifetime. Like H. ergaster, he had a habitually upright posture and his or, rather, her wide pelvis would have permitted birth of larger-brained babies.

H. ergaster/erectus (hereafter referred to simply as H. erectus) needed good nutrition in order to provide energy to that enlarging brain, so he may have mastered the use of fire and cooked his food, thus making food digestible even with his relatively smaller teeth. He was generally carnivorous, and the richer nutritive value of meat meant he could have a shorter digestive tract, which in turn made energy available faster. Hunting and butchering were enhanced by the use of double-sided stone cutting tools, which the archaeological record shows started around 1.76 Mya – the Acheulean technology. Note that 1.9 Mya, H. erectus coexisted with H. rudolfensis, H. habilis and P. boisei, and by .143 Mya, with H. sapiens.

H. erectus was the first species to expand beyond Africa, starting as early as 1.8 Mya. This was the first of many surges of expansion of hominins from Africa. Remains have been found in Asia (“Java man” in Indonesia, “Peking man” in China, Georgia), Africa and maybe Europe, although it is not impossible that some of these were different species. Migration will be discussed more later.

Several recent discoveries witness the work-in-progress character of paleoanthropology.

A skull discovered in 2001 from Dmanisi, Georgia, dated at 1.8-1.7 Mya possesses certain features of Homos habilis, erectus and rudolfensis, suggesting that they may not be distinct species.4)“Complete skull from Dmanisi”. Smithsonian Human Origins Program, http://humanorigins.si.edu/research/whats-hot/complete-skull-dmanisi. This also is the oldest good fossil evidence for hominins outside Africa. Oldowan tools were found on the site.

Two more recent discoveries either clear up or confuse questions concerning H. erectus, depending on one’s point of view. One is the discovery in Buia (Eritrea) in 1998 of a nearly complete cranium, some pelvic bones and two incisors dated to 1.4-0.6 Mya. The long, ovoid brain case, wide cheekbones, massive brow ridges and medium-sized brain (~750-800 cm3, a preliminary result) make him look like H. erectus. However, the parietal bones of the cranium are claimed to represent a more modern trait. The discoverers see him as a link between H. erectus and H. sapiens and claim that the date of first H. sapiens morphology has been pushed back to around 1 Mya.

The second discovery comes from the Afar region of Ethiopia, where in 1997 a crushed skull was found in the Dakanihylo sedimentary layer, also dated to around 1 Mya. The reconstructed skull of this “Daka man” has a long, sloping forehead, massive brow ridges and brain case shaped rather like that of Buia man, giving him a resemblance to H. erectus specimens found at that time in far-away Asia. Since H. erectus first appeared around 1.8 Mya in Africa, the discoverers of Daka man claim this to be evidence that by 1 Mya, he had become one single world-wide species.

From 1.8 to 0.7 Mya, during the Pleistocene, glacial cycles increased in intensity. It was during this period that H. habilis and H. rudolfensis died out. The paranthropi soon followed suit, leaving only the genus Homo to spread to three continents.

Modern Homo

Homo heidelbergensis is considered by many paleontologists to be the first modern human. Fossils attributed to this species have been found in Germany, Greece, Ethiopia, Gambia, the U.K. and Spain, although he certainly originated in Africa. Some paleontologists think some specimens represent different species, such as H. antecessor (from Spain), H. cerpanensi (Italy) or H. rhodesiensis (Zambia). His large brain capacity of 1000-1300 cm3 confirms him as a Homo, although he had very large brow ridges and a flat face. He used fire and wooden spears, hunted large animals and built shelters of wood or rock. Some paleontologists think he was our ancestor, but wonder who was his. Many consider him to be a transition species between H. erectus and H. neanderthalensis. They lived between about 0.6 and 0.1 Mya.

Neanderthals

Homo neanderthalensis lived about 400-40 Mya over a large region extending from Western Europe into Asia, but concentrated mostly in Europe and the Near East. His body was shorter and more robust than that of modern humans, well adapted for cold, mountainous environments. His brain was even larger than ours, 1500-1750 cm3, but he weighed more, so his encephalization (ratio of brain mass to body mass) was similar. Neanderthals hunted large animals but also ate plants. They controlled fire, used sophisticated tools, lived in shelters and wore clothes they made themselves. Indeed, it is now thought that Neanderthals participated in about the same activities as H. sapiens. Discovery first of a Neanderthal hyoid bone (a throat bone necessary for enunciation) and then of the his possessing the FOXP2 gene required for speech and language indicate that he may well have used language. Rock paintings found in Spain and dating from 43.5-52.5 Kya show that Neanderthals were artistic.

Much has been written about Homo neanderthalensis – entire books, including at least one novel sympathetic to him (William Golding, The Inheritors). Neanderthals are usually portrayed as ugly, ignorant, primitive and lacking in culture. They coexisted with H. sapiens for over 100,000 years and, in comparison with them, they were primitive and ignorant. But “ugly” is is in the eye of the beholder. And culture they had, though it is debated as to how much. There is evidence that they buried their dead and scratched designs into shells, an early example of art. It has recently been discovered that intercourse did take place between the species, as modern Europeans and their descendants have about 1-4% of their genes from Neanderthals. They therefore might be considered the same species as H. sapiens in spite of morphological differences. Which idea raises the question of what is a species.

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

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

The Earth was now subject to Milankovich-cycle glacial periods which came all the way down into northern continental Europe. During interglacial periods, such as those 500 and 400 Kya, Neanderthals penetrated northwards. During those of 320 and 220 Kya, they made it as far north as England and Wales. Between 120 and 70 Kya, they advanced as far as Siberia. But as the Earth became cooler after 100 Kya, they were forced to move south again. Their last holdouts were in Croatia, Russia and Gibraltar, not later than 28 Kya.

Neanderthals evolved during their long stay on Earth, as did their contemporary African Homos. Morphological traits distinguishing Neanderthals from modern man become more and more accented, the farther west they are found. Genetic studies of Neanderthals from across Eurasia suggest three different groups according to  where they lived — western Europe, the Mediterranean and the East. So as modern man moved west from the Middle East, he met populations showing more distinct, because more developed, Neanderthal morphology. Neanderthal extinction followed the same East-West gradient, those in the east disappearing before those farther west.

Evolution and variation within a species complicates distinguishing the species from others. So stating that such a fossil is such a species is rather like taking a snapshot of a moving object.

Theories abound as to what brought about the demise of the H. neanderthalensis species. The principle accused are the rapidly fluctuating climate, competition for resources and physiological differences from their fellow man, H. sapiens. Opinions on this question are often influenced by species identification or political correctness. The jury is still out.

Denisovans

Less well known is the recently discovered Denisovan species, discovered only in 2008. First, a young girl’s finger bone was found in the Denisova cave in Southern Siberia. Since then, a toe and some teeth have also been found. While this may not sound like much, the important thing is that scientists have been able to make analyses of both nuclear and mitochondrial DNA of the species. Denisovans were found to be genetically closer to Neanderthals than to H. sapiens, but distinct enough to deserve being considered a separate species. More recent genetic studies suggest that Denisovans and Neanderthals had a common origin about 1 Mya which may have been H. heidelbergensis. Denisovans and Neanderthals then diverged around 640 Kya, after leaving Africa.

Denisovan genes have been found in the Melanesian people of Papua, New Guinea. Apparently, the people who migrated there first shared genes with the Denisovans before moving on to New Guinea 45 Kya. Denisovans may also have given present-day Tibetans a gene which facilitates living at high altitudes. And they contributed around 1% of the genes of modern Europeans.

Homo sapiens

Bipedalism, temperature and a larger brain

To simplify only a little, all the characteristics of modern humans are due to two main traits: bipedalism and bigger brains. That is the correct chronological order, as small-brained Australopiths were already bipedal to some extent and evenly completely bipedal H. ergaster had a brain of only about 850 cm3. In some ways, bipedalism provided conditions necessary for an enlarged brain.

Bipedalism led to a non-grasping foot, simplified ankle and knee joints, a narrow, vertical and bowl-shaped pelvis (to support innards), related modifications to the hip joint and femur (the thing old folks break so easily), and a vertical, S-shaped spine (which pains many of us). It also freed up the hand, which could then develop other skills, such as making tools. Upright walking is thought to have brought modifications in the spatial relations of throat components (pharynx, larynx) necessary for speech, but keeping us from breathing and swallowing simultaneously, once we are past puberty.

Most important, bipedalism led to temperature regulation in the body and to improved brain nourishment. Under the hot African sun, being upright meant that less light hit the body during the hottest time of day. So fur could disappear (except on top of the head). Improved temperature control came from sweat glands in the skin coupled with blood circulation, together constituting a natural heat pump capable of cooling or warming the body depending on external conditions. This allowed the maintenance of the strict temperature range needed by enzymes responsible for metabolic processes. Since a significant part of this takes place in the brain, bipedalism indirectly allowed a bigger brain. Also, upright walking requires less energy than moving about on all fours, leaving more energy for the brain. And since the upright posture allowed faster movement, men became better hunters and were able to obtain more meat protein, which provided more energy, from which – again – the brain gained. All these features reinforced each other.

In addition to an improved nervous system and cognitive ability, a larger brain contributed to changes in the face and skull structure. Food not only nourished the larger brain, it also played a roll in the structure of the jaw and teeth needed for mastication. The chin, unique to H. sapiens, attaches muscles used for fine lip movements necessary for speech. The brain developed a basic language function, at least for a default form of grammar found the world over.

Bipedalism also is important because before man could move “out of Africa”, he had first to move out of the trees! Otherwise, he could never have crossed the very different landscapes which he encountered – forests, grasslands and sometimes even seas.

So changes in posture, internal organs, brain size and interaction with the environment followed one upon another in a continual evolution towards our present (temporary) state.

Where, when and how?

To answer the first two questions, where and when, the oldest fossils of H. sapiens are from Ethiopia and date from 200 Kya. Fossils, archeology (tools and art), genetics and language studies confirm (with some reservations) the “Out of Africa” model, which holds that hominoids all developed in Africa, mainly East Africa, and then expanded to the west of the world. The alternative model, the “multiregional” hypothesis, which posits that hominoids migrated long before and then developed into local variants on site, is generally considered erroneous. Still, it is true that a number of migrations did take place, such as the one by H. erectus around 2 Mya and the more recent ones around 60-45 Kya by modern humans. It seems that when modern humans, or archaic H. sapiens, wandered out to the rest of the world, they met and to some extent mixed genes with locally variant species evolved from the earlier migrants. This would explain the small percentages of Neanderthal and Denisovan genes in those of modern humans.

As usual in paleontology, there is dispute about which was the first modern human fossil to be found. Deserving or not, Cro-magnon man generally wins the prize, having been found in a cro (shelter) on the farm of Mr Magnon in south-central France in 1868.5)Modern Homo bones known as the “Red Lady”, because they were smeared with ochre, had been discovered in Wales in 1822-23. These early H. sapiens were more robust than modern humans, but otherwise resembled them closely. Actually, some of them had bigger brains than we do.

Fossil remains of H. sapiens have been found in Romania from 35 Kya; southeast Asia, maybe 40 Kya; and in the New World in Alaska, c. 12 Kya, and the Clovis Culture in North America, 11 Kya. Evidence for the existence of pre-Clovis cultures in Pennsylvania dating from 14 and possibly up to 20 Kya is controversial.

By 40 Kya, Homo’s skill at tool-making had increased to the point where he began to make works of art – cave paintings and engravings, and carved bones.

Painting in the Grotte de Lascaux, by Prof saxx via Wikimedia Commons

Painting in the Grotte de Lascaux, by Prof saxx via Wikimedia Commons

More about tools – the Paleolithic

Establishing a chronology for tool fabrication and use is difficult for at least two reasons. First, the only tools which have remained over time are the hard ones – made of stone or fossilized bone. Tools made of softer materials such as wood or bone have not survived. Second, finding tools near a fossilized hominin remain does not necessarily prove that the tools were made or used by that particular hominin. Still, we must do our best with the facts available.

The Paleolithic, or Old Stone Age, is taken as extending from the first known appearance of stone tools about 3.3 Mya in Kenya6)Before 2014, the oldest was 2.6 Mya in Ethiopia. and extending to the end of the last Ice Age about 10 Kya. It is divided into three periods.

  • The first is the Early Paleolithic (also called the Lower Paleolithic, because the corresponding geological layer is located below the others), which is divided into two overlapping periods according to prevalent technologies.
    • The first period, the Oldowan (after Olduvai Gorge), extended from at least 2.6 Mya to about 1 Mya. Hammerstones were used to knock sharp-edged chips off core rocks to make choppers. They were made and used by late australopithecines and maybe by paranthropus and H. ergaster/habilis. It was during this time that the first expansions of Homo from Africa took place.
    • The second period, from 1.7 Mya to about 250 Kya, was that of the Acheulean technology, which spread from Africa into the Middle East and on to India, south of the Movius Line. This technique made double-sided chips to use for such items as hand axes. Such technology required planning by the toolmaker and therefore augmented brain power. Hominins of the period were H. erectus and, later, H. heidelbergensis.

Oldowan chopper from Ethiopia, by Didier Descouens via Wikimedia Commons

Oldowan chopper from Ethiopia, by Didier Descouens via Wikimedia Commons

Biface from Saint Acheul, France, from Wikimedia Common

Biface from Saint Acheul, France, from Wikimedia Commons

  • The Middle Paleolithic, about 250-30 Kya, introduced the technique of making fine flakes of stone which could be attached to sticks to make spears. Fire came into general use. Principal hominins were Neanderthals and earliest modern humans.
  • The Late Paleolithic, about 40-10 Kya, saw the use of bone, antlers and ivory to make still finer tools such as needles or harpoons. Hunting and fishing thus improved. Symbolic art, musical instruments and throwing devices dating from this period were made and used by anatomically modern humans. The oldest known musical instruments are bone flutes from 35 or more Mya, but most likely there had been previous instruments of less survivable material.
Paleolithic flute from about 43 Kya, by José-Manuel Benito Álvarez via WIkimedia Commons

Paleolithic flute from about 43 Kya, by José-Manuel Benito Álvarez via WIkimedia Commons

As mentioned, examples of H. erectus have been dated in Asia up to around 2 Mya. The tools found with them were those of the Oldowan technique. Acheulean tools date back to to 1.76 Mya and have been found only south of a line running from present-day Denmark to the Gulf of Bengal, the so-called Movius Line, named after the paleontologist who first noted this distribution. The explanation for this geographical distribution is generally agreed to be that H. erectus first took his Oldowan tools with him when he migrated east before around 2 Mya. Later, the Acheulean developed in East Africa (even though it is named after a site in France where it was first discovered) and subsequent migrations carried it to most of the rest of Africa and the Near East (Georgia), from where it moved east into India and northwest into Europe around 600 Kya. Asiatic H. erectus eventually died off and were replaced by H. sapiens.

Similar considerations hold for the arrival of modern man in Europe. H. erectus seems to have arrived in southern Europe over 1 Mya; chipped stone tools have been found from 1.2 Mya. Although the Acheulean technology originated in Africa around 1.4 Mya, there are no examples of it in Europe from this time, This fact is explained if we accept that early hominins in Europe descended from those in Georgia 1.7 Mya, as this was a logical station between eastern Africa and Europe. Because of glacial periods which affected Europe over this period, there were most likely numerous attempts at settling during warm periods, many of which failed due to climate extremes: The settlement of Europe was not a one-time event. Only since 600 Kya has northern Europe (north of the Alps) been permanently settled and these settlers brought the Acheulean technology with them.

So for the cases of both Asia and Europe, absence of Acheulean tools for the earlier peoples leads to the recognition of different waves of migration across hundreds of thousands of years.

Summary of expansion of Homo sapiens

Evidence from paleontology, archeology and genetics all concur that modern man originated in Africa around 150 Kya. But even before that time, pre-modern Homo had begun what became the expansion of man from Africa to the rest of the world. The first step took place about 2 Mya when H. erectus, or whatever preceded him, spread out into Eurasia. His earliest known descendants lived in Georgia about 1.8 Mya. A second migration may have taken place about 200 Kya into China and India. Another around 130 Ky spread into the Middle East. Humans may have reached Australia 60 Kya.

But the big one, “ours”, so to speak, was the expansion around 60-50 Kya into the Middle East and thence into Europe and Asia. These Homo sapiens replaced the Neanderthals and H. erectus and, about 11 Kya, they made it to the Americas via Siberia. Man became the dominant species on Earth, for better and for worse.

Overall summary

After dinosaurs disappeared from the surface of the Earth about 65 Mya, the number and size of mammal species took off, eventually ranging in size from tiny mouse-like creatures up to elephants and whales. During a particularly warm period of the mid-Eocene, primates came into existence, at first small squirrel-like creatures. About 23 Mya, the primate line split into Old-World Monkeys (catarrhines) and hominoids and the latter split into hylobatids (gibbons and the like) and hominids. From hominids sprang pongines (orangutans) and hominines and the latter begat panins (chimps and bonobos) and hominins. The first hominins were the precursors of man, but not all of them were his ancestors. No direct line from the LCA7)Last Common Ancestor of chimps and hominins can be distinguished; the tree of life is rather a bush, with the branches hidden and many twigs representing dead ends. However, there is clear evidence for over twenty species intermediate between the time of the LCA and modern Homo.

First, there were some fairly difficult-to-classify species found in East and Central Africa, dating from 7 to 4.5 Mya. Though the consensus seems to be that these represented steps in a generally man-like direction, they are all subject to some controversy as to whether they are hominins or on another line. Some even may have lived before the LCA.

Then, from 4.5 to 2.5 Mya, there evolved a fairly heterogeneous group called australopithecines, one genus of which was australopithecus. Although they possessed varying degrees of bipedalism and stronger chewing teeth, their brains were still about the size of a chimp’s. Up to at least four of these species lived at the same time. A second group, which followed up to about 1 Mya was paranthropus, sometimes classed as a genus of australopithecines. These were more robust versions of australopithecus, with a chewing apparatus capable of masticating tough roots and nuts.

From around 2.5 Mya, true Homo, the same genus as modern humans, appeared on the scene. Members of these species tamed fire and invented cooking, made and used stone tools and hunted large animals. The first one to really look more or less like a modern human, Homo ergaster, had long legs, an upright body and could walk long distances and even run. Another (or maybe they were the same), Homo erectus, migrated out of East Africa as far as Asia around 2 Mya, taking along Oldowan tool technology. More migrational surges took place until finally, around 60,000 years ago, another, more modern species made their way into the Middle East and from there to Europe, Asia and, eventually, the Americas, this time with the Acheulean technology. Along the way, they mixed their genes with those of the local populations evolved from earlier H. erectus – Neanderthals and Denisovans and perhaps others. Nevertheless, most of our genes originated in Africa over ~60 Kya.

The best-known of the Homo which became extinct is Homo neanderthalensis. Neanderthals have been the victims of much bad press. Compared to the H. sapiens, with whom they shared the environment during their last 100,000 years or more, they were primitive. But they made and used tools and their own clothing, they probably buried their dead and they made decorations which may be considered an early form of art. And they left some of their genes in us.

The demise of the Neanderthals left, for the first time, only one species of hominin on Earth – H. sapiens sapiens. We are now up to the geological present.

 

Notes   [ + ]

1. “Oldest human fossil found, redrawing family tree:, http://news.nationalgeographic.com/news/2015/03/150304-homo-habilis-evolution-fossil-jaw-ethiopia-olduvai-gorge/.
2. New fossils confirm diverse species at the root of our lineage”. Smithsonian Human Origins Program,http://humanorigins.si.edu/research/whats-hot/australopithecus-sediba-%E2%80%93-new-analyses-and-surprises. http://humanorigins.si.edu/research/whats-hot/new-fossils-confirm-diverse-species-root-our-lineage
3. Tools will be discussed in more detail in a later paragraph.
4. “Complete skull from Dmanisi”. Smithsonian Human Origins Program, http://humanorigins.si.edu/research/whats-hot/complete-skull-dmanisi.
5. Modern Homo bones known as the “Red Lady”, because they were smeared with ochre, had been discovered in Wales in 1822-23.
6. Before 2014, the oldest was 2.6 Mya in Ethiopia.
7. Last Common Ancestor

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, http://humanorigins.si.edu/research., 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!”. http://www.hominides.com/html/references/bipede-homme-primate-deloison.php. 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”, http://news.nationalgeographic.com/2015/05/150520-oldest-stone-tools-discovery-harmand-archaeology/ 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, http://humanorigins.si.edu/evidence/human-fossils/species/australopithecus-sediba 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, http://humanorigins.si.edu/research.
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!”. http://www.hominides.com/html/references/bipede-homme-primate-deloison.php.
5. “Wrong Turn Leads to Discovery of Oldest Stone Tools”, http://news.nationalgeographic.com/2015/05/150520-oldest-stone-tools-discovery-harmand-archaeology/
6. ”Australopithecus sediba – new analyses and surprise”, Smithsonian Human Origins Program, http://humanorigins.si.edu/evidence/human-fossils/species/australopithecus-sediba

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, http://www.ucmp.berkeley.edu/paleo/paleowhat.html. 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 www.hominides.com/html/dossiers/hominoide.php (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, http://www.ucmp.berkeley.edu/paleo/paleowhat.html.
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 www.hominides.com/html/dossiers/hominoide.php (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.

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

next…