2.1.1 Human Evolution

The concept of evolution over time is one we are all likely familiar with. Consider, for example, how technology has evolved. The first true smartphones appeared on the market at the beginning of this century, but these complicated devices didn’t spring all at once from the minds of ambitious engineers. Rather, these engineers built on technology that had evolved and improved over many decades. In the mid-1800s, telegraph technology first demonstrated that electricity could be used for long-distance communication. That technology paved the way for the first telephones, which were basic and expensive but over many decades became more sophisticated, more common, and cheaper. By the early 1980s, electronics companies had begun selling telephones that used radio technology to communicate wirelessly. Over time these devices were made faster and smaller, and companies added features like cameras, microprocessors, and eventually internet access. With these evolutionary transformations, the smartphone was born.

Modern humans are not smartphones, and in human history, chance biological adaptations to a changing environment drove the evolutionary process rather than the minds of inventors conceiving of technical innovations. But the evolutionary process that eventually gave birth to our species resembles the technological evolution of smartphones and other devices in some important ways. Just like we can trace the evolution of the smartphone back to the telegraph, so can we trace our own evolution back to a very distant ancestor called Australopithecus, who lived in eastern and southern Africa between 2.5 and 4 million years ago. Lucy, previously mentioned, was of the genus Australopithecus (Figure 2.3). A genus is a taxonomic rank that includes several similar and related species within it. Like us, members of Lucy’s species afarensis (named for the Afar region of East Africa where she was found) were capable of walking upright and likely used tools. Beyond that, however, they were very different from us. They had plenty of hair like chimpanzees, fingers and arms well suited for climbing trees, and brains about one-third the size of ours. Despite these differences, scholars have concluded that the genus Homo (“human”) evolved from Australopithecus somewhere around two to three million years ago.

Possibly the earliest member of the genus Homo was Homo habilis (Figure 2.4). This species appears in the archaeological record about two to three million years ago. Habilis means “handy”; it was thought at one time that this was the first species to have created stone tools. We now know that stone tools predate Homo habilis, but the name has stuck. Homo habilis resembles us in many ways, with a large brain similar to ours as well as small teeth and a face we might recognize as human. But members of the species also had many ape-like characteristics, such as long arms, hairy bodies, and adult heights of only three or four feet.

Around two million years ago, a species even more similar to us, Homo erectus, emerged in East Africa. It likely evolved from Homo habilis. As the name erectus for “upright” suggests, this species lived entirely on the ground and walked—even ran—exclusively in an upright position. The consequences of this evolutionary shift were huge. Being upright meant that the body’s digestive organs were pulled down lower and into a smaller space. That in turn necessitated a change in diet, away from tough plants and toward easier-to-digest items like nuts, fruits, tubers, honey, and even meat. Living on the ground was also more dangerous because it made eluding predators more difficult. However, the upright position left Homo erectus’s hands free to use tools. This advantage likely led to further evolutions that made human hands more dexterous over time, prompting the wider adoption of ever more sophisticated tools.

Members of Homo erectus shared other close similarities with modern humans. They were about the same height as we are, sometimes reaching just over six feet. They made and used relatively sophisticated stone tools and relied on fire for both warmth and cooking food. They likely constructed huts of wood and fur in which to live and worked cooperatively with each other to hunt and forage. The position of the larynx in the throat also suggests that Homo erectus may have had some capacity to communicate vocally, which would have aided in cooperative endeavors. Finally, archaeologists have uncovered strong evidence that Homo erectus cared for the sick and elderly. This evidence includes the discovery of the remains of individuals who suffered from debilitating diseases yet lived a long time, indicating that assistance was both necessary and provided by others. Together these characteristics made the species highly adaptable to changing climates and environments, helping explain why its members survived for so long. Homo erectus populations lived until as recently as about 100,000 years ago.

Sometime between 1.8 and 1.5 million years ago, Homo erectus started migrating into other parts of Africa and beyond, reaching North Africa, the Near East, Europe, and East and South Asia over hundreds of thousands of years. The reasons for this extensive migration are still debated, but they likely included climate change and the desire to follow certain types of prey. Homo erectus appears to have stayed close to rivers and lakes during migration, hunting and eating animals like rhinoceroses, bears, pigs, and crocodiles, as discoveries in the Near East have confirmed. Populations evolved to adapt to the different environmental conditions into which they moved. Over time this led to a diversity of human species, including Homo heidelbergensis; Homo neanderthalensis, or Neanderthals; Denisovans; and modern humans, or Homo sapiens (sapiens means “wise”). Some of these species, like Denisovans and Neanderthals, emerged outside Africa. Others, like Homo heidelbergensis and Homo sapiens, emerged in Africa first and later migrated to other areas.

The extent to which these different human species interacted with each other remains unclear. DNA evidence from a bone found in Siberia has shown that a girl (who died at age thirteen) was born there of a Denisovan father and a Neanderthal mother. Another recent study demonstrated that modern European and Middle Eastern populations have between 1 and 4 percent Neanderthal DNA. This appears to suggest that mating between Homo sapiens and Neanderthals was quite common. The careful work of archaeologists and other scholars has also made clear that some species evolved from others. For example, both Neanderthals and Denisovans appear to have evolved from populations of Homo heidelbergensis.

At some point between forty thousand and fifteen thousand years ago, the diversity of human species declined and only Homo sapiens remained. Two models attempt to explain why. The first and most commonly accepted is the “out of Africa” model. This theory suggests that modern humans emerged first in Africa approximately 200,000 years ago and then, approximately 100,000 years ago, expanded out of Africa and replaced all other human species. The second model is often called the “multiregional evolution model” and proposes that Homo sapiens evolved from Homo erectus in several places around the same time. This model emerged as an explanation for the great diversity of modern human traits in different populations around the world. But it relies primarily on the study of fossils and archaeological records rather than on genetic data.

These theories about human evolution are not necessarily mutually exclusive, and the real answer to the puzzle may be a combination of the two. For example, it’s entirely possible that modern humans or a common ancestor did emerge first in Africa. Then, as this species expanded around the world, it mixed its genetic information with that of other human species. The DNA evidence collected in recent years certainly suggests a more complicated picture, and the debate has not yet been settled. The tools of both archaeology and molecular genetics continue to reveal new insights into the puzzle of human evolution and the rise of Homo sapiens. And the conclusions we can draw about our distant past will continue to change as we learn more.

Dueling Voices

What Happened to Neanderthals in Europe?

For tens of thousands of years before Homo sapiens arrived in Europe, the continent was home to Neanderthals (Figure 2.5). Then, about forty thousand years ago, right around the time modern humans entered Europe, the species neanderthalensis began to rapidly die out. For more than a century and a half, scholars have been trying to understand why.

Figure 2.5 This modern reconstruction of what a Neanderthal woman may have looked like demonstrates a striking resemblance to modern humans. (credit: “Reconstruction of Neanderthal woman (makeup by Morten Jacobsen)” by Public Library of Science/Wikimedia Commons, CC BY 2.5)

One theory is that modern humans replaced the Neanderthals in Europe through violent competition, including a type of warfare between the two groups. Another model argues that the competition was less about violence and more about resources. This theory posits that modern humans were simply better tool makers, had better survival strategies, and possibly experienced lower mortality rates and higher birth rates. Neanderthals simply couldn’t keep up, and their small population dwindled and then disappeared entirely.

Modern DNA analysis has opened the door for a new theory, that mating occurred between the two species and that the population of Neanderthals was simply absorbed by modern humans. The presence of small amounts of Neanderthal DNA in modern human populations lends some credibility to this idea, but it seems unlikely to explain the total disappearance of Neanderthals.

Finally, analysis of climate change in Europe has revealed some variations that could have weakened Neanderthal populations and led to their disappearance in some areas. As of now, no one theory can account for everything. It seems possible that several factors were at play rather than a single primary cause, so the debate goes on.

• How might more than one of these reasons, or all of them, have contributed to the decline of Neanderthals in Europe?
• Can you think of any other explanations for the extinction of Neanderthals?