The Origin of Humankind

by Richard Leakey

  Unfortunately, there is no state of harmony among these three lines of evidence. There are points of connection but no consensus. The difficulty anthropologists face even with such an abundance of evidence is a salutary reminder of how very difficult it often is to reconstruct evolutionary history.

  The discovery of the Turkana boy’s skeleton gives us an excellent idea of the anatomy of early man some 1.6 million years ago. We can see that early Homo erectus individuals were tall (the Turkana boy stood at close to 6 feet), athletic, and powerfully muscled. Even the strongest modern professional wrestler would have been a poor match for the average Homo erectus. Although the brain of early Homo erectus was larger than that of its australopithecine forebears, it was still smaller than that of modern humans—some 900 cubic centimeters compared with the average 1350 cubic centimeters of Homo today. The cranium of Homo erectus is long and low, with little forehead and a thick skull; the jaws protrude somewhat, and above the eyes are prominent ridges. This basic anatomical pattern persisted until about half a million years ago, although there was an expansion of the brain during this time to more than 1100 cubic centimeters. By this time, Homo erectus populations had spread out from Africa and were occupying large regions of Asia and Europe. (While no unequivocally identified Homo erectus fossils have been found in Europe, evidence of technology associated with the species betrays its presence there.)

  More recently than about 34,000 years ago, the fossil human remains we find are all those of fully modern Homo sapiens. The body is less rugged and muscular, the face flatter, the cranium higher, and the skull wall thinner. The brow ridges are not prominent, and the brain (for the most part) is larger. We can see, therefore, that the evolutionary activity giving rise to modern humans took place in the interval between half a million years ago and 34,000 years ago. From what we find in Africa and Eurasia in the fossil and archeological record of that interval, we can conclude that evolution was indeed active but in confusing ways.

  The Neanderthals lived from about 135,000 to 34,000 years ago, and occupied a region stretching from Western Europe through the Near East and into Asia. They constitute by far the most abundant component of the fossil record for the period we are interested in here. There is no question that ripples of evolution were going on in many different populations throughout the Old World during this period of half a million to 34,000 years ago. Aside from the Neanderthals, there are individual fossils—usually crania or parts of crania, but sometimes other parts of the skeleton—with romantic-sounding names: Petralona Man, from Greece; Arago Man, from southwestern France; Steinheim Man, from Germany; Broken Hill Man, from Zambia; and so on. Despite many differences among these individual specimens, they all have two things in common: they are more advanced than Homo erectus—having larger brains, for instance—and more primitive than Homo sapiens, being thick-skulled and robustly built (see figure 5.1). Because of the varying anatomy of the specimens from this period, anthropologists have taken to labeling these fossils collectively as “archaic sapiens.”

  The challenge we face, given this potpourri of anatomical form, is to construct an evolutionary pattern that describes the emergence of modern human anatomy and behavior. In recent years, two very different models have been proposed.

  The first of them, known as the multiregional-evolution hypothesis, sees the origin of modern humans as a phenomenon encompassing the entire Old World, with Homo sapiens emerging wherever populations of Homo erectus had become established. In this view, the Neanderthals are part of that three-continent-wide trend, intermediate in anatomy between Homo erectus and modern Homo sapiens in Europe, the Middle East, and western Asia, and today’s populations in those parts of the Old World had Neanderthals as direct ancestors. Milford Wolpoff, an anthropologist at the University of Michigan, argues that the ubiquitous evolutionary trend toward the biological status of Homo sapiens was driven by the new cultural milieu of our ancestors.

  FIGURE 5.1

  Neanderthal relations. Neanderthals share some features with Homo sapiens, such as a large brain, and some with Homo erectus, such as a long, low skull and prominent brow ridges. They have many unique features, however, the most obvious of which is extreme protrusion of the midfacial region.

  Culture represents a novelty in the world of nature, and it could have added an effective, unifying edge to the forces of natural selection. Moreover, Christopher Wills, a biologist at the University of California, Santa Cruz, identifies the possibility here of an accelerating pace of evolution. In his 1993 book The Runaway Brain, he notes: “The force that seems to have accelerated our brain’s growth is a new kind of stimulant: language, signs, collective memories—all elements of culture. As our cultures evolved in complexities, so did our brains, which then drove our cultures to still greater complexity. Big and clever brains led to more complex cultures, which in turn led to yet bigger and cleverer brains.” If such an autocatalytic, or positive feedback, process did occur, it could help promulgate genetic change through large populations more rapidly.

  I have some sympathy with the multiregional evolution view, and once offered the following analogy: If you take a handful of pebbles and fling them into a pool of water, each pebble will generate a series of spreading ripples that sooner or later meet the oncoming ripples set in motion by other pebbles. The pool represents the Old World, with its basic sapiens population. Those points on the pool’s surface where the pebbles land are points of transition to Homo sapiens, and the ripples are the migrations of Homo sapiens. This illustration has been used by several participants in the current debate; however, I now think it might not be correct. One reason for my caution is the existence of some important fossil specimens from a series of caves in Israel.

  Excavation at these sites has been going on sporadically for more than six decades, with Neanderthal fossils being found in some caves and modern human fossils in others. Until recently, the picture looked clear and supported the multiregional-evolution hypothesis. All the Neanderthal specimens—which came from the caves of Kebarra, Tabun, and Amud—were relatively old, perhaps some 60,000 years old. All the modern humans—which came from Skhul and Qafzeh—were younger, perhaps 40,000 to 50,000 years old. Given these dates, an evolutionary transformation in this region from the Neanderthal populations to the populations of modern humans looked plausible. Indeed, this sequence of fossils was one of the strongest pillars of support for the multiregional-evolution hypothesis.

  In the late 1980s, however, this neat sequence was overturned. Researchers from Britain and France employed new methods of dating, known as electron spin resonance and thermoluminescence, on some of these fossils; both techniques depend on the decay of certain radioisotopes common in many rocks—a process that acts as an atomic clock for minerals in the rocks. The researchers found that the modern human fossils from Skhul and Qafzeh were older than most of the Neanderthal fossils, by as much as 40,000 years. If these results are correct, Neanderthals cannot be ancestors of modern humans, as the multiregional-evolution model demands. What, then, is the alternative?

  Instead of being the product of an evolutionary trend throughout the Old World, modern humans are seen in the alternative model as having arisen in a single geographical location (see figure 5.2). Bands of modern Homo sapiens would have migrated from this location and expanded into the rest of the Old World, replacing existing premodern populations. This model has had several labels, such as the “Noah’s Ark” hypothesis and the “Garden of Eden” hypothesis. Most recently, it has been called the “Out of Africa” hypothesis, because sub-Saharan Africa has been identified as the most likely place where the first modern humans evolved. Several anthropologists have contributed to this view, and Christopher Stringer, of the Natural History Museum, London, is its most vigorous proponent.

  FIGURE 5.2

  Two views of modern human origins. The multiregional model, left, states that Homo erectus populations expanded out of Africa close to 2 million years ago and became established througho
ut the Old World. Genetic continuity was maintained throughout the Old World by gene flow between local populations, so that the evolutionary trend toward modern Homo sapiens occurred in concert wherever populations of Homo erectus existed. The “Out of Africa” model, right, states that modern Homo sapiens evolved in Africa recently and quickly expanded into the rest of the Old World, replacing existing populations of Homo erectus and archaic Homo sapiens.

  The two models could hardly be more different: the multiregional-evolution model describes an evolutionary trend throughout the Old World toward modern Homo sapiens, with little population migration and no population replacement, whereas the “Out of Africa” model calls for the evolution of Homo sapiens in one location only, followed by extensive population migration across the Old World, resulting in the replacement of existing premodern populations. Moreover, in the first model, modern geographical populations (what are known as “races”) would have deep genetic roots, having been essentially separate for as much as 2 million years; in the second model, these populations would have shallow genetic roots, all having derived from the single, recently evolved population in Africa.

  The two models are also very different in their predictions of what we should see in the fossil record. According to the multiregional-evolution model, anatomical characteristics that we see in modern geographical populations should be visible in fossils in the same region, going back almost 2 million years, when Homo erectus first expanded its range beyond Africa. In the “Out of Africa” model, no such regional continuity over time is expected; indeed, modern populations should share African characteristics, if anything.

  Milford Wolpoff, the strongest proponent of the multi-regional-evolution hypothesis, told an audience at the 1990 gathering of the American Association for the Advancement of Science that “the case for anatomical continuity is clearly demonstrated.” In northern Asia, for instance, certain features, such as the shape of the face, the configuration of the cheekbones, and the shovel shape of the incisor teeth, can be seen in fossils 750,000 years old; in the famous Peking Man fossils, which are a quarter of a million years old; and in modern Chinese populations. Stringer acknowledges this, but he notes that these features are not confined to northern Asia and therefore cannot be taken as evidence of regional continuity.

  Wolpoff and his colleagues make a similar argument for Southeast Asia and Australia. But, as Stringer points out, the supposed sequence of continuity is built on fossils dated at only three time points: 1.8 million, 100,000, and 30,000 years ago. This paucity of reference points, says Stringer, weakens the case in the extreme.

  These examples illustrate the problems anthropologists face. Not only are there differences of opinion over the significance of important anatomical features, but, Neanderthals aside, the fossil record is much slimmer than most anthropologists would like (and than most nonan-thropologists believe). Until these impediments are overcome, a consensus on the larger question may remain out of reach.

  We can assess fossil anatomy from a different perspective, however. Neanderthals appear to have been stocky individuals with short limbs. This stature is an appropriate physical adaptation to the cold climatic conditions that prevailed throughout much of their range. The anatomy of the first modern humans in the same part of the world, however, is very different. These people were tall, slightly built, and long-limbed. A lithe body stature is much more suited to a tropical or temperate climate, not the frozen steppes of Ice Age Europe. This puzzle would be explicable if the first modern Europeans were descendants of migrants from Africa rather than having evolved in Europe, and the “Out of Africa” model therefore derives some support from this observation.

  It receives further support from another direct observation of the fossil record. If the multiregional-evolution hypothesis is correct, then we would expect to find early examples of modern humans appearing more or less simultaneously throughout the Old World. That’s not what we see. The earliest-known modern human fossils probably come from southern Africa. I say “probably” because not only are these fossils fragmentary parts of jaws but there is a degree of uncertainty about their true age. For instance, the fossils from Border Cave and Klasies River Mouth Cave, both in South Africa, are thought to be a little more than 100,000 years old, and are adduced as support by proponents of the “Out of Africa” hypothesis. However, the modern human fossils from the caves of Qafzeh and Skhul are also close to 100,000 years old. It is possible, therefore, that modern humans first arose in northern Africa or the Middle East, and then migrated from there. Most anthropologists favor a sub-Saharan origin, however, based on the overall weight of the evidence (see figure 5.3).

  No fossils of modern humans of this age have been found anywhere else in the rest of Asia or Europe. If this reflects evolutionary reality and is not simply the perennial problem of a lamentably incomplete fossil record, then the “Out of Africa” hypothesis does look reasonable.

  The majority of population geneticists support this hypothesis as being the most biologically plausible. These scientists study the genetic profile within species, and how it might change through time. If populations of a species remain in geographical contact with each other, genetic changes that arise through mutation may spread throughout the entire region, by means of interbreeding. The genetic profile of the species will alter as a result, but overall the species will remain genetically unified. There is a different outcome if populations of a species have become geographically isolated from each other, perhaps because of a change in the course of a river or the opening of a desert. In this case, a genetic change that arises in one population will not be transferred to other populations. The isolated populations may therefore steadily become genetically different from one another, perhaps eventually becoming different subspecies, or even different species altogether. Population geneticists use mathematical models to calculate the rate at which genetic change may occur in populations of various sizes, and can therefore offer suggestions about what might have occurred in ancient times. Most population geneticists, including Luigi Luca Cavalli-Sforza, at Stanford, and Shahin Rouhani, of University College, London, who have commented extensively on the debate, are skeptical of the feasibility of the multiregional-evolution model. They note that the multiregional model requires extensive gene flow across large populations, linking them genetically while allowing evolutionary change to turn them into modern humans. And if new dates for Java Man fossils, announced early in 1994, are correct, Homo erectus expanded its range beyond Africa almost 2 million years ago. Therefore, not only would gene flow have to be maintained over a large geographical area, according to the multiregional-evolution model, it would also have to be maintained over a very long period of time. This, conclude most population geneticists, is simply unrealistic. With premodern populations spread across Europe, Asia, and Africa, there is a greater likelihood of producing geographical variants (such as we indeed see among archaic sapiens) than a cohesive whole.

  FIGURE 5.3

  A map of fossils. The map shows the location (and age in thousands of years) of fossils that relate to the origin of modern humans. The Neanderthals were restricted to the shaded area. The earliest specimens of modern humans have been found in sub-Saharan Africa and the Middle East.

  We’ll leave fossils aside for a while, and turn to behavior, by which I mean its tangible products, tools and art objects. We have to remember that the vast preponderance of human behavior in technologically primitive human groups is archeologically invisible. For instance, an initiation ritual led by a shaman would involve the telling of myths, chanting, dancing, and body decoration—and none of these activities would enter the archeological record. Therefore we need to keep reminding ourselves, when we find stone tools and carved or painted objects, that they give us only the narrowest of windows onto the ancient world.

  What we would like to identify in the archeological record is some kind of signal of the modern human mind at work. And we would like that signal to throw some light
on the competing hypotheses. For example, if the signal appeared in all regions of the Old World more or less simultaneously, we could say that the multiregional-evolution model describes the most likely manner in which modern humans evolved. If, instead, the signal appeared first in an isolated location and then gradually spread to the rest of the world, this would give weight to the alternative model. We would hope, of course, that the archeological signal would coincide with the pattern from the fossil record.

  We saw in chapter 2 that the appearance of the genus Homo coincides roughly with the beginning of the archeological record, some 2.5 million years ago. We saw, too, that the increased complexity of stone-tool assemblages 1.4 million years ago, moving from the Oldowan industry to the Acheulean, followed soon upon the evolution of Homo erectus. The link between biology and behavior is therefore very close: simple tools were made by the earliest Homo; a jump in complexity occurred with the evolution of Homo erectus. That link is seen again with the appearance of archaic sapiens, some time after half a million years ago.

  After more than a million years of relative stasis, the simple handaxe industry of Homo erectus gave way to a more complex technology fashioned on large flakes. And where the Acheulean industry had perhaps a dozen identifiable implements, the new technologies comprised as many as sixty. The biological novelty we see in the anatomy of the archaic sapiens, including the Neanderthals, is clearly accompanied by a new level of technological competence. Once the new technology had become established, however, it changed little. Stasis, not innovation, characterized the new era.