Chapters 6 & 7: Human Evolution
INTRODUCTION TO HUMAN EVOLUTION
DNA analysis has shown that modern human and nonhuman primates diverged from a common ancestor between 8 and 5 mya. Africa is the birthplace of all hominins (humans or human ancestors). However, because fossil evidence from millions of years ago is sparse, it is hard to assign fossils to species. Distinctive traits, such as bipedalism, make it easier. While scientists have accumulated fossils from thousands of individuals, some dating to 7 mya, most of them are more recent. This means we know a lot about recent ancestors and not as much about older ones. Additionally, fossils don’t come in neat, complete packages. They are often eroded, broken or incomplete. Thus, our dates and theories about their times of existence and even their order in the hominin record are open to reinterpretation whenever a new fossil comes out of the ground.
When new fossils are found, we need to know to which species they belong and we also need to figure out where in our hominin family tree they fit. Is it a new species, is it an evolutionary dead end, does the fossil make us reevaluate our earlier assumptions? But this classification isn’t so simple as there is no universally agreed upon formula for figuring out how much variation can exist before you are looking at fossils from a different species. We just don’t know how much variation to allow for.
HOMININ EVOLUTIONARY TRENDS IN BIOLOGY AND CULTURE
Our earliest ancestors, possibly dating to 7 million years ago (mya), had very small brains, large jaws and large molars. They had arms that were longer than ours and their skeletons look primitive and ape-like. However, they were bipedal, and that distinguished them from the last common ancestor and is one of the hallmarks of a hominin. By 3.6 mya, footprints were fossilized in volcanic ash at Laetoli, indicating we were fully bipedal. There were many bipedal species, but only one of these hominins survived: us. Our hominin history is thus a history of brain enlargement, tooth and jaw reduction, and ever increasing complexity in culture and tool making.
There were a number of different bipedal species that evolved in Africa and went extinct, as bipedalism provided many advantages: the ability to travel long distances without tiring; to carry food, infants and weapons; to see predators; and to reduce the area of the body exposed to sunlight. There were also costs. We are slower than quadrupeds, we are more vulnerable if we injure a leg, and we don’t turn very well at high speeds.
As each hominin evolved, brain size increased, as did the complexity of thought, culture and tools. By 2.5 mya, they had stone tools, which allowed them to be better meat scavengers as well as more quickly deal with daily tasks. This speed allowed more time for thought, expression and leisure. Because eating meat was a more efficient way of feeding themselves high quality calories, which supported brain growth, and because tools made life easier, culture was able to expand and it essentially helped to speed up biological evolution.
The emergence of bipedalism and brain expansion required a number of skeletal changes. The foramen magnum moved downward. Increasing brain size meant changes in head shape. Even the increasing reliance on culture to adapt led to changes in skeletons. As they ate more meat and began to cook, large teeth and jaws decreased in size.
Our earliest ancestors existed from about 7-2 mya. They included Primitive and Archaic hominins. Primitive hominins are ape-like in many ways, but are more closely related to modern humans than to our last common ancestor. Archaic hominins are early humans but they still have many ape-like characteristics. Only once these species become more similar to us do they get included in our genus, Homo, but they are still different enough to get their own species names. All the Homo species besides our own are categorized as Archaic Homo. They existed for nearly 2 million years (2.5mya-200kya). These were the species to first migrate out of Africa. Compared to Archaic hominins, they look more like us in body shape, with smaller teeth and jaws, but the rest of the skeleton is more robust and with smaller brains.
Environment: From 8 to 5 mya, major climate changes occurred; there was a long drying and cooling trend. Forests shrank and woodlands, grasslands and tree stands appeared. The last common ancestor probably lived during the time of dense forests and then, as the woodlands appeared, some of them adapted to life on the ground and in the open. The cooling pattern continued with fluctuations. During cooling cycles, sea levels fell and land was exposed. These cycles caused ice ages that affected the N. Hemisphere for the past million years. Open woodlands of Africa changed into savannahs.
Body: Primitive and archaic hominins were about the size of chimpanzees and smaller than later hominins. They exhibited marked sexual dimorphism. Their arms were a bit longer but their legs were quite short. The common ancestor would have been mainly a knuckle-walker, with short periods of bipedalism. We don’t know the earliest date of bipedalism. Almost all of the archaic homo species are larger than archaic hominins. They weighed around 150 pounds and were at least 5 feet tall. The taller and larger individuals were adapted to tolerate heat, conserve water and protect themselves
Diet: Early hominins were foragers whose diet shifted from the primitive to the archaic as environmental conditions changed. Some of them ate scavenged meat, but overall, their diets were not specialized. As we move from primitive to archaic, the variety of foods they ate increased. The smaller molars and jaws of the archaic homo species indicate a change in diet or that they were cooking. Their diet was higher quality and the shape of their torso (indicates size of guts) tells us they were no longer primarily vegetarians. Their diet probably included more meat and fish, which are high in amino acids, protein and fat. Thus, they could eat less food, but of higher quality, and get the same amount of calories and nutrition than a vegetarian diet provided. The first meat eaters were not hunters but scavengers who went after bones that were rich in marrow.
Intelligence: We don’t know much about the intelligence of early hominins. The average primitive hominin skull measures 350 cc, archaic is 450 cc, and an average chimp, 380 cc. Once tools arise in the record, about 2.5 mya, we can make inferences about cognitive abilities based on the stone tools we find. We know that planning and forethought were required. Also, the tools of each tradition are remarkably similar across wide geographic areas, so knowledge for making tools was a cultural tradition that was transmitted among groups. Some of the more complicated tools would need a verbal language in order to pass on knowledge.
Tools: Some archaic hominins were able to make and use tools at least as well as modern chimps. Stone tools dating to 2.6 mya are direct evidence of tool making ability. But indirect evidence suggests earlier species were also making and using tools between 3 and 4 mya. While the tools were rudimentary and didn’t change much, we need to be careful not to mistake a lack of evolution in tools for a lack of culture change in their makers. We don’t have all the tools ever made. We also know that non-stone tools must have been made that didn’t survive. Also, even though a tool's appearance might have remained stable, its uses might have changed.
Language: There is no way of telling from the shape of the brain whether or not early hominins could speak. But speaking is not the sole means of communicating and they likely had body language and some type of call system, much as modern chimps do. Two things are necessary for a spoken language, a brain that knows what to say and the vocal tract parts that make speech sounds. But we have no direct proof of a spoken language in these ancestors. Complex tools and hunting point towards speech.
Culture: Some of the archaic Homo species lived in caves; that implies they must have controlled fire. These species also lived in, and adapted to, a wide range of climates and habitats. They relied greatly on culture to survive, more so than archaic hominins. In fact, more recent homins have relied on culture to such an extent that despite spreading all over the Old World and having to adapt to a wide range of environments, they all look surprisingly similar, while the earliest hominins, who are mostly confined to Africa, all look quite different. The reason for this is that the earliest hominins didn’t have culture to act as a buffer between the environment and their bodies. Any adaptations that needed to take place had to be accomplished with their biology, whereas later hominins could adapt with culture, through tools or techniques.
PRIMITIVE AND ARCHAIC HOMININS
Researchers have proposed four species as primitive hominins. However, there is such a small amount of fossil evidence that this is still somewhat tentative. These are the oldest purported hominins, dating from 7 mya to around 4.5 mya, and include Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus kadabba, and Ardipithecus ramidus. Recent discoveries have more firmly placed the Ardipithecus species within our hominin ancestral lineage.
Compared to the primitive hominins, the archaic hominins (4.5-2 mya) have larger chewing teeth and wider, larger faces. Their teeth and face size increases are related to a more diverse diet. Almost all the archaic hominin species fall under the genus Australopithecus and are called australopiths. The archaic hominins that have very large chewing teeth fall under the genus of Paranthropus. Australopith species include anamensis, afarensis, bahrelghazali, garhi and africanus. Paranthropus species includes robustus, boisei and aethiopicus.
Australopithecus afarensis is the best-known archaic hominin. It has been found in both Tanzania and Ethiopia, and dates to between 4 and 3 mya. The collection of A. afarensis fossils is big enough to allow for reliable estimates of its weight and size. The most famous A. afarensis is Lucy, who was found in the Afar region of Ethiopia and thus gives the species its name. These were the first fossils we found that indicated hominins were bipeds long before significant brain expansion.
Australopithecus garhi dates to about 2.5 mya and was found in Ethiopia. A. garhi may have made stone tools, as they were found with them and marks on animal bones indicate the use of tools. These animal bones are the oldest evidence of tool use among hominins and their use marks the beginning of the Paleolithic archaeological age (old stone age).
ARCHAIC AND MODERN HOMO
Archaic Homo Species
There are a number of archaic Homo species. Until around 2 mya, the fossil and artifact records are confined to Africa, indicating that hominins hadn't left Africa yet. The oldest fossils outside of Africa date to 1.8 mya in Central Europe.
H. habilis (handy man) lived from around 2.4 to 1.6 mya, overlapping with all three Paranthropus species. It had a brain smaller than 700 cc. It was found with a great many stone tools at Olduvai Gorge in Tanzania. The tools are thus called Oldowan. This is the earliest known stone tool tradition.
H. ergaster (dating to 1.9 mya) was the first hominin whose body shape and size are like ours but with an 800 cc brain. It’s likely they invented the Acheulian (hand axe) tool tradition as well as had the use of fire. They might be the first species to leave Africa.
H. erectus is the first human species known to have left Africa and be widely distributed across the Old World. The average brain size was 1000 cc. They were more muscular than us. It was a successful species, living for almost 2 million years and spreading throughout half of the world. They are also associated with Acheulian tools.
In appearance, H. neanderthalensis is the most distinctive Homo species, likely an adaptation to the very cold weather of Europe. Their brain size is larger than ours. They lived in Europe and W. Asia. If you include early fossils, this species lived from 400 to 24 kya. The oldest sites are in W. European rock shelters and caves. While they are not ancestors of Homo sapiens, they did make very minor genetic contributions to European genomes. They are associated with Mousterian tools.
In 2004 a new kind of Homo species was found in Indonesia and named H. floresiensis. It appeared to be a dwarf form of either ergaster or erectus. The fossils date to 40-18 kya. It was 3 feet tall, walked upright, used stone tools and had a brain size of 380 cc.
Anatomically Modern Humans
Anatomically modern humans (AMH) arise about 300-200 kya. AMH are Homo sapiens, but we use the designation AMH to indicate the time period when there were still some physical changes taking place, and that lasts until about 30-40 kya. There are several differences between modern humans and archaic Homo. AMH are less robust. Our reduction in body size and increasingly complex culture allows us to not only need less food, but to be able to satisfy that need in less time, freeing up chunks of time to help develop culture. AMH culture has more symbols and meanings, vocal communication, and modernization in tools, materials, diets, social organization and an increasing importance in symbolism in art, music, beliefs and language.
Overlapping with Archaic Homo is the LOWER PALEOLITHIC (2.5mya-200kya) archaeological period.
Oldowan: Oldowan stone tools were found with H. habilis fossils. The earliest Oldowan tools were made 2.6 mya, meaning they were made by Australopiths. They were made until about 200 kya. We believe other tools were probably used, but didn’t survive. Oldowan tools consist of scrapers, flakes and hammers. They were effective and razor sharp, allowing users to cut through hides and smash bones, useful for scavenging meat.
Acheulian: Around 1.7 mya, the Acheulian hand axe tradition appears. The oldest of these tools are from Africa, but they are also found in Europe. There were also cleavers and picks. They are different from Oldowan tools because they are worked more on both sides, the edges are more finely worked and they are more symmetrical. They are evidence of more planning, forethought, skill and dexterity. They probably served many uses, such as digging, scraping and dealing with tough plant fibers. H. ergaster might be their inventor, but they are widely found with H. erectus fossils.
Mousterian: The toolkit of later Neanderthals is called Mousterian and lasts from 125-40 kya. Compared to Acheulian tools, they are smaller, lighter and more specialized flake tools like points, scrapers and awls. They are portable and often vary by season or region.
The UPPER PALEOLITHIC lasts from around 40 to 10 kya. It is a time of great expansion in tool types and cultural expression. There is an increase in blade tools, microliths, composite tools and a great many seasonal and regional adaptations. Culture became increasingly complex, symbolic and social. There are many European tool traditions, as well as specialized adaptations of older tool traditions in different regions of the world. While modern humans left Africa by at least 100 kya, the Upper Paleolithic is the time during which they truly spread throughout the world.
MIGRATIONS OF AMH INTO THE OLD WORLD
If we agree that AMH originated in Africa, then their first destination upon leaving would have been the Middle East.
Middle East: The oldest AMH fossils found outside of Africa are found here. These fossils date to around 100 kya. It was from there that AMH went to Asia. Others went to Turkey and from there into either Eastern and Central Europe or Central Asia and Siberia.
Asia: The oldest AMH fossil evidence outside of Africa and the Middle East is in Southeast Asia and Australia, where AMH's arrived by 40 kya. They were skilled seafarers.
Europe: The earliest evidence of AMH's in Europe comes from sites in Central Europe around 40 kya. By 36 kya, they had reached Western Europe. The first AMH fossils discovered here were at a rock shelter which gave us the name of Cro-Magnon for the first modern humans in Europe. When they got to Europe, Neanderthals were already there and they possibly overlapped for a few thousand years. Cro-Magnon culture was rich in tool traditions, cultural expression, art and symbolism.
Central Asia and Siberia: To survive in a cold climate, cultural innovations such as planning, food-storage and clothing are necessary. The AMH's who settled into Central Asia and Siberia had to rely on culture to survive. Evidence for modern humans within the Arctic Circle dates to 27 kya. It was from Siberia that AMH's migrated into the New World on foot across a land or ice bridge in the Bering Strait.
MIGRATIONS INTO THE NEW WORLD
It is probable that there were several migration streams into the New World, possibly occupying different regions. It is likely the first settlers arrived in the New World around 17 kya. The peoples of both Siberia and Alaska had similar toolkits. The majority of evidence we have about the first settlers comes from the Clovis culture in New Mexico which dates to 12 kya. However, recent evidence from Pennsylvania, Virginia, South Carolina, Venezuela, Brazil and Chile is pushing that date back to potentially 15-20 kya.