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.

CLASSIFYING FOSSILS

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.

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.

PRIMITIVE AND ARCHAIC HOMININS

Primitive 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.

Archaic Hominins

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.

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.

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.

ARCHAEOLOGICAL RECORD

Overlapping with Archaic Homo is the LOWER PALEOLITHIC (2.5mya-200kya) archaeological period.

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.

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.