Today the fossil record of human evolution can be divided roughly into three parts. Each has its own cast of characters, some species uncovered by paleoanthropologists, some populations revealed by DNA discoveries. The first and oldest of these parts is the most shadowy.
The lineage of humans and our hominin relatives divided from the ancestors of chimpanzees sometime around seven million years ago, a date estimated by comparing the full DNA of humans and other primates. A handful of fossil discoveries represent this earliest time period of our evolution, from 7 million up to around 4.3 million years ago, the most important discovered during the 1990s by the field team of Tim White, the same young researcher Mary Leakey had worked with in Tanzania 20 years before.
The best of these fossils is a partial skeleton found in the Middle Awash region of Ethiopia and given the name Ardipithecus ramidus. Its anatomy reveals an ape the size of a male chimpanzee with a brain around the size of a chimpanzee’s as well. With opposable first toes, short thumbs, and extremely long fingers, Ardipithecus seems to have been as good in the trees as the great apes of the present day. But it had some features more like those of later members of the human lineage: smaller canine teeth, a more compact pelvis, and a skull that seems to have rested atop a vertical spine. Ardipithecus did not walk on two legs like humans, but it may have held itself more upright than apes do today.
Beyond that important discovery, fossil fragments have been found from more ancient sites, representing species with names like Sahelanthropus tchadensis and Orrorin tugenensis. But they preserve only bits of the anatomy, making it hard to know much about them. These species have been found in eastern or central Africa—but so far we have not found any hominin fossils from this early time period in southern Africa.
The second great part of human evolution, between 4.2 million and 1.5 million years ago, is represented by the range of species known as australopiths, including Raymond Dart’s Australopithecus africanus and Robert Broom’s Paranthropus robustus. These species all walked upright, as indicated by a pelvis, legs, feet, and spine that share features with the bones of modern humans. They would have been as awkward on all fours as people are today, but they also seem to have retained the ability to climb. Australopith bodies were small compared with humans today, and their brains averaged a third the size of ours. All had big premolar and molar teeth, and some had even larger molars and massive jaw muscles, allowing them to rely on a wide range of foods. They lived across sub-Saharan Africa, but no trace of them has been found on any other continent.
The third part of our evolutionary history began around 1.8 million years ago with the species known as Homo erectus, the first hominin to emerge from Africa and spread into other parts of the world. It had a body that was human in size, and a brain about 50 percent larger than the brain of any of the australopiths. Anthropologists believe that this larger body and brain enabled Homo erectus to navigate larger territories, use stone tools, and eat a higher-quality diet, including meat obtained by hunting. For many anthropologists, these early populations of erectus were truly the first humans.
Over hundreds of thousands of years, erectus and its descendants branched into varieties and species living in different parts of the world, including the Neanderthals, all grouped together by paleoarchaeologists in the category of “archaic humans.” Africa remained the center of our evolution, home to the greatest diversity of these archaic humans and ultimately the place where modern humans arose.
Great discoveries in genetics during the past 10 years have shown that the later part of our evolutionary process was very complex. Humans today owe the vast majority of our heritage to the first modern human populations in Africa, which lived only 200,000 years ago.
We do not know why, but the African population grew and began to spread again across the rest of the world. When it did, those people came into contact with Neanderthals and other populations. Some members of those different varieties of humans mixed their DNA into the growing modern human gene pool. Eventually these modern humans, still mostly African in heritage, spread throughout the world, migrating to places including the Americas and Australia, where no people before them had ever lived. Some 10,000 years ago, people in the Near East, and later in other parts of the world, developed agriculture, an innovation that contributed to the massive recent growth of the human population.
Science has achieved rapid progress in the third major part of our evolution, making the story at once more complicated and more interesting. But who were the ancestors of these first humans, and what set them on a different path than the australopiths? The australopiths were successful and diverse. They existed for more than two million years before the first species that we describe as humans arose. So far, our tremendous progress in genetics can tell us nothing about these extinct branches of our family tree. We rely upon fossil discoveries for that information.
Those fossil discoveries have been frustratingly fragmentary. At one time almost everyone assumed that habilis led to erectus, which then led to sapiens. This was the textbook version of human evolution, a story of direct descent that is probably familiar to many readers of this book. But as new fragments have been added to the fossil and archaeological records, the picture has become more complicated. Over the years, scientists have revisited the original habilis discoveries and now point out that we know almost nothing about their bodies. What little we do know suggests that they were very much like australopiths and not very humanlike at all.
In Louis Leakey’s time, it had seemed that stone tools had been intimately connected with the evolution of habilis and erectus—so much that habilis was imagined as the original toolmaker. But during the 1990s and 2000s, the record of stone tools was pushed back earlier and earlier. In 2015, evidence of early stone tools was found near Lake Turkana in an ancient layer more than 3.2 million years old, nearly double the age of the Olduvai Gorge discoveries. These tools existed long before any of the fossils we have found of hominins with a large brain, even the brain of habilis. A few fossil specimens between three and two million years old have been attributed to Homo, the earliest being part of a jaw from Ethiopia. But none of these fossils gives us anything indicating the size of the brain.
In other words, the transition from australopiths to Homo still raises some of the most compelling questions in our evolution. We cannot answer those questions without better fossil evidence. Maybe there was something worth discovering in South Africa.