Outside of Africa, to jump a little ahead in the human story, the melanocortin receptor gene became free once more to collect mutations and become less efficient at triggering the black, radiation-protective form of melanin. That could well have been an advantage for people living in cold northern climates since they require extra exposure to the sunlight that is needed to help synthesize sufficient quantities of vitamin D, the lack of which causes misshapen bones and the disease known as rickets. In every population of the world, women’s skin color is 3 to 4% lighter than men’s, perhaps through sexual selection by men, and perhaps because of mothers’ greater needs for vitamin D.20
To turn to another curious feature of human hair, when did you last see a chimpanzee getting a haircut? Human head hair differs from that of apes in that it never stops growing. If the hair follicles on the human head behaved like those of chimpanzees, they would follow an orderly cycle in which each would grow a hair for several weeks; the hair, after reaching a certain length, would then be shed, and the follicle would grow another hair. With people, this cycle has been lengthened from weeks to years.
The reason that uncontrolled hair growth was favored by natural selection may have been that it offered a means of signaling copious amounts of social information. In every society in the world, people spend an inordinate amount of time in cutting, shaping, braiding, plaiting, curling, straightening, decorating and otherwise gussying up the appearance of their hair. Much the same is true of men’s beards and mustaches. To let one’s hair grow unkempt is a sign of the outcast, or that one is in deep mourning. Trimmed hair sends a variety of important signals about the wearer’s health, wealth and social status. But for all this social signaling activity to occur, humans had first to abandon the self-maintaining hairdos of other apes and acquire hair that required continual attention.21
Geneticists have calculated a date for the birth of the hairdressing industry. Keratin, the protein in hair, comes in a large number of different varieties, each prescribed by a different gene. Humans, chimpanzees and gorillas have much the same set of keratin genes but with a striking difference. One of these genes, the human version of which is known as phi hHaA, produces a working keratin protein in chimpanzees and gorillas but is inactive in people of all ethnic groups. Although the genetic regulation of human hair growth is not yet understood, it seems likely that the inactivation of the phi-hHaA gene is the step by which the hair follicles of the human head have escaped from the orderly cycle imposed on chimp and gorilla hair follicles. By comparing the mutations in various versions of the human and ape gene, researchers have calculated that the human version became inactivated some 200,000 years ago.22 This is long after Homo ergaster had become extinct and about the time that the human line acquired its contemporary physical appearance.
The First Exit from Africa
Because ergaster was adapted to living in dry places, it could survive in many environments. This adaptability made possible a momentous step, the first spread of human lineages to the lands beyond Africa. A close relative and presumably descendant of ergaster, known as Homo erectus, had reached Asia by at least 1 million years ago and maybe much earlier—stone tools recently found in northern China have been dated to 1.66 million years ago.23 By 1 million years ago, ergaster itself had reached both the northern and southern extremities of Africa. And by at least 500,000 years ago a human lineage had reached Europe, perhaps through a second migration from Africa of another ergaster descendant known as Homo heidelbergensis. In Europe, under the glacial conditions that prevailed from 400,000 to 300,000 years ago, these new migrants evolved into Homo neanderthalensis, the Neanderthals, broad-boned, thickset people who were adapted to the cold.
Erectus and the Neanderthals are referred to as archaic humans in distinction to the human lineage that remained in Africa and ultimately became modern. With the departure of the archaics from Africa, the human gene pool was split into three main branches, in Africa, Asia and Europe, and each now followed a separate evolutionary path.
In Africa, it was not until 500,000 years ago, more than a million years after ergaster’s first appearance on the scene, that brain size relative to body size increased significantly, and not until 200,000 years ago that it reached the contemporary standard.
Yet a curious feature of the steadily increasing brain size of the human lineage is that it was not accompanied by any significant change in behavior that is visible in the archaeological record. Just as the Olduwan stone tool kit remained unchanged from 2.5 million to 1.7 million years ago, the Acheulean tool kit that succeeded it was also almost unvaried from its emergence 1.7 million years ago until its disappearance about 250,000 years ago. Erectus in Asia may have varied the formula by using bamboo, which is hard but perishable, in place of stone. This would explain the strange absence of Acheulean hand axes in the Far East. The Neanderthals in Europe used the same tool kit as the human lineage in Africa. “The technologies of these archaic creatures were homogeneous across and even between whole continents. The Acheulean, for example, although varying in minor form, is known from Cape Town to Cardiff,” writes the evolutionary anthropologist Robert Foley.24
FIGURE 2.3. THE THREE HUMAN SPECIES OF 50,000 YEARS AGO.
The world 50,000 years ago was occupied by three human species—Homo erectus in East Asia, the Neanderthals in Europe, and the ancestral human population in northeast Africa. In addition Homo floresiensis, thought at present to be a downsized version of Homo erectus, lived on the island of Flores, in Indonesia.
Because of the ice age conditions that then prevailed, sea level was some 200 feet lower than at present and land area was larger, as shown by the shaded areas round the continents.
The range shown for Homo erectus encompasses sites that range from 1.7 million to 50,000 years ago in age. The species probably did not occupy all of this range throughout the period, and toward the end of it was probably found mostly in southeast Asia. The range and location indicated for the ancestral human population is conjectural.
The Acheulean stone tool kit was followed by one that archaeologists use to define the Middle Stone Age in Africa and the Middle Paleolithic or Mousterian in Europe. The makers of the Middle Stone Age tools were the descendants of ergaster, on the way to becoming large-brained Homo sapiens, while Mousterian artifacts are the handiwork of ergaster’s European cousins, the Neanderthals. The tool kits in both continents are very similar, and both differ very little from the Acheulean. The principal difference is the absence of the characteristic Acheulean hand axes. Perhaps archaic humans learned how to mount smaller stones on handles, and these composite wood-and-stone tools replaced the hand axes.25
What stopped the Middle Stone Age people from leaving Africa as their predecessors had done? It seems the descendants of these predecessors may have been hemming them in. Even by 100,000 years ago, the human lineage in Africa was still using the same tool kit as the Neanderthals in Europe and as Homo erectus in Asia. They evidently enjoyed no competitive advantage over the Neanderthals.
During a warmer period in the ice age that lasted from 125,000 to 90,000 years ago, people came close to escaping from Africa. They extended their range to the region that is now Israel, at the border of Africa and Asia. But during the cold period that prevailed from 80,000 to 70,000 years ago, the Neanderthals expanded their range southward to western Asia and seem to have destroyed the emigrants.26
The humans who lived during the African Middle Stone Age, which lasted from 250,000 to 50,000 years ago, had a way of life that was more sophisticated than their ergaster forebears but only slightly so. They obtained their stone locally, not through trade, suggesting they had small home ranges or very simple social networks. They hardly ever made things of bone, ivory or shell. They were not very good hunters and couldn’t even fish. Their populations were small, as judged by the archaeologists’ standard people-meter, the tortoise test. (People catch large tortoises first, then smaller ones. Tortoises are so slow to replace themselves that the
size of a human settlement can be judged by whether the tortoise bones are large, indicating a sparse human population, or small, meaning rather more mouths to feed.)27
Like the Neanderthals, the Middle Stone Age people seem to have buried their dead, but very simply, and to have collected pigment making minerals, though for an unknown purpose. They left no clear evidence for art or decoration.
This pattern of behavior altered scarcely at all as one millennium followed another. Strangely, the human form was changing much more. In Africa, people began to attain the skull size and skeleton of contemporary humans some 200,000 ago. The oldest known specimens, from a site near the Kibish river in southern Ethiopia, may be about 195,000 years old,28 and fossil remains of people with this new form start to be commonly found about 100,000 years ago.
Anatomically and Behaviorally Modern Humans
Modern human behavior, at least as judged by archaeologists, means behaving like living hunter-gatherers. By this criterion, the humans of 100,000 years ago did not behave like modern humans, even though they looked like them. They are known as anatomically modern humans to denote that they were not so in behavior.
What kept them from attaining a fuller modernity? The question of behavioral modernity is of great significance because it appears to be the last major step in the emergence of the ancestral human population. The components of modern behavior appear most prominently around 45,000 years ago in Europe. At sites throughout Europe, the staid culture of the Neanderthals begins to yield to a set of new and more inventive techniques. There is a new set of stone tools, more carefully crafted to attain specific shapes. There are complex tools made of bone, antler and ivory. The bringers of the new culture made personal ornaments, of materials such as punctured teeth, shells and ivory beads. They played bird-bone flutes. Their missile technology was much improved. They were avid hunters who could take down large and dangerous game. They buried their dead with rituals. They could support denser populations. They developed trade networks through which they obtained distant materials.29
This new modern culture is called the Upper Paleolithic. Some archaeologists have proposed that it was created by Neanderthals or by Neanderthals interbreeding with modern humans. It now seems more likely that the culture was the work of behaviorally modern humans alone, who simply replaced the Neanderthals, over a period of several thousand years, throughout their European domain.
One reason for this interpretation is that several diagnostics of modern behavior can be seen to have appeared first in Africa, in the Later Stone Age, which had begun by at least 46,000 years ago. (The Later Stone Age of Africa and the Upper Paleolithic of Europe are the same archaeological period but for historical reasons have different names in the two continents.) The timing suggests that humans with modern behavior first evolved in Africa and later reached Europe. This hypothesis, made on purely archaeological grounds, has been confirmed by the genetics of modern human populations, all of which point to a diaspora in recent times from an African homeland.
So if behaviorally modern humans arose in Africa, the final stage of human evolution in Africa was that from anatomically modern humans of 100,000 years ago to the behaviorally modern people who appeared some 50,000 years later. What caused that profound transition?
Archaeologists tend to explain changes in terms of culture. But paleoanthropologists, looking at much longer sweeps of time, are more accustomed to seeing evolution and genetic change as the principal shaper of novelty. The paleoanthropologist Richard Klein has proposed that the transition to modern behavior was so profound that it required a genetic change: “Initially, the behavioral capabilities of early modern or near-modern Africans differed little from those of the Neanderthals, but eventually, perhaps because of a neurological change, they developed a capacity for culture that gave them a clear adaptive advantage over the Neanderthals and all other non-modern people.”30
It was obviously a genetic change, not a cultural one, that endowed the australopithecines with upright stature 4.4 million years ago. It was a suite of genetic changes 2.5 million years ago that remodeled the australopithecines into Homo habilis with its larger brain and tool-making ability. A third far-reaching genetic makeover 1.7 million years ago reshaped habilis into the more humanlike erectus and caused a behavioral transition from male and female hierarchies to the pair bond system. And it must have required a fourth genetic revolution, Klein believes, to make possible the emergence of behaviorally modern humans 50,000 years ago.
That genetic revolution was evidently profound enough to affect many different aspects of human social behavior and technical skills, all characterized by a striking new capacity for innovation. The most likely cause of such a transformation, in Klein’s view, would have been the emergence of language.
For a social species, nothing could make a greater difference than the ability to transmit precise thoughts from the mind of one individual to another. Language would have made small groups more cohesive, enabled long range planning and fostered the transmission of local knowledge and learned skills.
It is certain that modern humans could speak before they left Africa, so language must have evolved sometime before 50,000 years ago, and after 5 million years ago when the human line split from that of chimpanzees. Looking in the archaeological record for some sharp increase in behavioral complexity that might signal the evolution of language, there are few likelier moments than the transition from anatomically modern to behaviorally modern humans.
Klein’s argument is not universally accepted by other archaeologists, some of whom have attacked a principal element in his case, the sharp discontinuity he sees between the behaviors present at the end of the Middle Stone Age and the beginning of the Later Stone Age. Two critics, Sally McBrearty of the University of Connecticut and Alison S. Brooks of George Washington University, argue that there was a gradual accumulation of advanced behaviors throughout the Middle Stone Age that eventually added up to modern behavior. “As a whole the African archaeological record shows that the transition to fully modern behavior was not the result of a biological or cultural revolution, but the fitful expansion of a shared body of knowledge, and the application of novel solutions on an ‘as needed’ basis,” the two archaeologists write.31
Klein’s view that there was no modern behavior in Africa prior to 50,000 years ago, once uncontradicted by any evidence, has been challenged by several individual finds. Christopher Henshilwood of the University of Bergen in Norway recently found a set of 41 shells, all perforated in the same way as if meant to serve as beads on a necklace. The shells were excavated from the Blombos cave in southern Africa; the sand in which they were found has been dated to 76,000 years ago by a physical technique.32 Dates of this age are beyond the reach of the reliable radiocarbon method, and the methods used instead are considerably less accurate. If the 76,000 year date is right, however, and if the pierced shells are indeed beads, it would mean that decorative art, a practice associated with behaviorally modern humans, began much earlier than supposed.
Another behavior generally considered modern is fishing. Hence a possible problem for the Klein position is eight barbed points made of bone, which could have been used to harpoon fish. The points come from the Katanda riverside site in Zaire from strata about 100,000 years old. Klein believes that the bone points, if directly dated, would turn out to be less than 12,000 years old.
A difficulty for Klein’s case has long been that of the very early date by which modern humans apparently reached Australia. Since this feat required boat building and some navigation skills, it would certainly count as a modern behavior. The date of human arrival in the continent has long been set at 60,000 years ago, based on a burial at a site near Lake Mungo in southeastern Australia. The finding indicated that modern behavior had been attained in Africa even earlier. But the Lake Mungo date recently turned out to be incorrect: the burial site is only 42,000 years old, with artifacts suggesting an earlier human presence at somewhere between 50,000 and 4
6,000 years ago.33 This date fits quite well with the theory that behaviorally modern humans were able to leave Africa only 50,000 years ago.
As for the barbed points and other artifacts, Klein argues that they are, at least for the moment, anomalies that don’t fit into the established archaeological pattern. If the Katanda points are 100,000 years old, why didn’t such an important technique as fishing spread like wildfire? Yet no other African site shows evidence of fishing until 25,000 years ago. Klein, who does his fieldwork in Africa, has twice found sheep bones in strata belonging to the Middle Stone Age, which ended 50,000 years ago. Since sheep were not domesticated for another 40,000 years, the bones are clearly intrusions from a higher level, introduced by burrowing animals or one of the many other sources of confusion in the archeological record. Archaeologists must expect to find a few later intrusions in any stratum, Klein believes, and should therefore base their conclusions on well established patterns, not on the occasional anomaly. His critics, he believes, are looking at the noise in the record, not its true signal.
Looking at the extraordinary process by which apes were slowly molded into humans, it is easy to think of the end result as some goal that evolution was driving toward. But evolution, of course, is a blind, inanimate process with no goals, let alone any interest in human welfare. It is driven by mutation, natural selection and drift. Mutation—random natural changes in the chemical units of DNA—is the ceaseless generator of novelty in the human genome. That novelty is the raw material on which natural selection acts, rejecting changes for the worse and retaining those that confer reproductive advantage. The mighty tide of genetic drift, through the random selection of genes between generations, makes some genetic variants a permanent fixture in a population and extinguishes many others, reducing the novelties that mutation introduces.
Before the Dawn: Recovering the Lost History of Our Ancestors Page 4
