The first walking apes, woodland primates known as australopithecines, appear in the fossil record 4.4 million years ago. A breathtaking trace of their presence is a line of footprints made nearly a million years later at Laetoli in Tanzania. The tracks of two individuals, perhaps a parent and child, extend for 165 feet across the ash from a nearby volcano, crossed by the tracks of other animals, perhaps fleeing from the eruption. A few frozen seconds of time, with prints that look so human.
Yet, walking and feet aside, the australopithecines seem to have been mostly apelike. With long arms, they retained the ability to move in trees. Their brains were only slightly larger than an ape’s. And as with apes, the sexes were of very different sizes, the males being much larger.
Larger male size, in primate societies, reflects competition between males for females, and is particularly prominent in gorillas, whose harem-keeping males are twice the size of females. Male chimpanzees are 25% larger than females, but in today’s human populations men are only 15% larger than women. Male australopithecines were about 50% larger than females, suggesting that australopithecine society was much like that of chimpanzees, with strong rivalry between males and a separate male and female hierarchy. For two million years of australopithecine existence, there is little sign of human form, apart from the critical upright gait, and no reason to assume that social behavior had changed much from the chimpanzee-like pattern.
Then, from 3 to 2 million years ago, there was another long period of cool, dry climate in which Africa’s forests shrank once more, and many species adapted to living in them fell extinct. The changing climate also put pressure on the australopithecines to develop new sources of food. Their diet, to judge by the nature of the microscopic wear on their teeth, was mostly vegetarian until 2.5 million years ago. At this time the australopithecines, already adapted to living in open woodland, had evolved two quite separate solutions to the problem of survival, according to the evidence of their fossil remains. One of the two new species, known as the robust australopithecines, had developed larger cheek teeth, suitable for eating coarse leaves. The other had emerged with a much more original solution than chewing away at vegetation. It seems to have decided to try its hand at car nivory. Meat-eating allowed for a smaller gut and furnished the extra nutrition that made possible a larger brain.
This second species is known as Homo habilis. The title of Homo is one it does not clearly deserve since, far from being fully human, it retained its apelike body form and still used the trees as a refuge. But it possessed a striking new adaptation. The australopithecines had lived for 2.5 million years with brains scarcely bigger than a chimp’s, but with habilis the brain at last started to expand. Chimpanzees’ brains have a volume of 400 cubic centimeters, compared with the 1,400 ccs of the average modern human brain.8 The australopithecine brain size ranged from 400 to 500 ccs. The brain volume of the known habilis skulls ranges from 600 to nearly 800 ccs.b9
For a species to put resources into growing extra neurons is not as obvious an investment as it may seem. Brawn and teeth count a lot in the strug gle for survival. Brain cells are greedy consumers of glucose and oxygen. The modern human brain is only 3 percent of the body’s weight but uses some 20% of the energy required for metabolic maintenance. “When costs are taken into account, the rarity of the human evolutionary phenomenon is at last understandable,” writes the anthropologist Robert Foley.10
It’s easier to explain how habilis sustained its larger brain than why it got it. Brains require a high quality diet to sustain them, such as meat but not vegetation can provide. Meat-eating requires less tooth power than does chomping through mounds of vegetation and habilis indeed had smaller teeth. And habilis appears on the scene at the same time, 2.5 million years ago, as do the first stone tools. If, as seems likely, habilis was the maker and user of these implements, that would explain its smaller teeth and how it managed to nourish a larger brain; it didn’t need large teeth because it was using tools to hunt or scavenge meat, and the richer diet supplied the energy for its greater cognitive capacity.
Still, that doesn’t explain what specific environmental forces made a larger brain advantageous in the first place. Higher social primates like apes and people probably encounter no problems more challenging than those of dealing with other members of their community. If so, the likeliest reason for habilis’s greater brain size would have been increasing social complexity.
The stone tools associated with habilis are known, rather grandly, as the Olduwan Industrial Complex since they were first found in the Olduvai Gorge in eastern Africa. The tools consist mostly of pebble cores and the rough flakes struck off them. They have a kind of random appearance, as if the maker was not holding any design in mind and was content to accept whatever shape of stone nature might produce. Still, these random pieces of rock would have been useful for a wide variety of purposes, such as cutting through hide and ripping the flesh off of bones.
The technology of the Olduwan Industrial Complex seems to have represented the limit of Homo habilis’s new cognitive capacity and inventive powers. Far from being followed by further innovations, it remained unchanged for 800,000 years. This lack of development in stone tool-making may reflect a similar conservatism in the lifestyle of its maker.
The emergence of bipedalism and the beginnings of a larger brain were two major genetic steps in the process of morphing the chimplike ancestor into modern people. A third genetic revolution occurred 1.7 million years ago in the form of the physical and behavioral changes shown by a new species known as Homo ergaster. Ergaster was presumably a descendant of habilis, though the fossil record is too scant for proof. It’s the first creature whose skeleton shows most of the features of human identity even though its brain volume, at 800 ccs, is way below the modern capacity.
FIGURE 2.2. EVOLUTION OF HUMAN STONE TOOL KITS.
The first stone tools, made by Homo habilis, appeared 2.5 million years ago. The kit remained in use, unchanged, until 1.7 million years ago, when it was replaced by a more sophisticated set of implements, the Acheulean industry, made by a more advanced species, Homo ergaster.
The time axis, expressed in units of millions of years ago, is not to scale. The lower two bands occupy a time span of 2.25 million years, the upper two bands one of just 0.25 million years. During this latter period, the conservatism of the previous 2 million years was replaced by a much brisker tempo of innovation. The Acheulean gave way to the Middle Stone Age tool kit, made by both the Neanderthals in Europe and the human lineage in Africa. Then from 50,000 years ago, the modern humans who replaced the Neanderthals in Europe started making the highly refined artifacts of the Upper Paleolithic age. These included smaller tools, some designed to be set in wood handles or weapons, as well as decorative and artistic objects.
Ergaster’s arms were of human length, not ape length, suggesting it had made a final farewell to the trees and was committed to living at ground level. Its chest cavity had the human barrel shape, not the cone shape of an ape’s, the indication of a major change in diet. Apes need enormous guts to digest masses of plant material and their rib cages are cone-shaped because they must be wide enough at the bottom to cover the stomach compartment. The barrel shape of ergaster’s chest was positioned above a smaller belly, showing that it was eating a richer diet, consisting of meat and maybe also tubers, the starchy roots that served as storage devices for plants living in dry environments.11
Tubers, a staple of hunter-gatherers, are a likely new food because ergaster had learned for the first time to inhabit dry, hot areas in East Africa where tubers flourish. Aridity, shown by the presence of dust in sea-floor sediments, increased sharply at the time of ergaster’s emergence. Ergaster may even have learned to cook the tubers, a significant advance if so because cooking releases nutrients from foods, making them more digestible.12 There is no evidence that ergaster did in fact use fire, but since ashes do not preserve well, the evidence of ergaster’s cookouts may have been lost.
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bsp; Paleoanthropologists see signs in ergaster’s body structure that it had made a significant transition from an apelike to a more human mode of social organization. Ergaster is the first species along the human lineage to show a sharp reduction in male size compared with female, although the females are still smaller. This is a hint of some important change in social structure, very possibly a switch from the separate male and female hierarchies of chimp communities to the male-female bond that characterizes human societies.
Such a shift does not imply anything as extreme as monogamy, but it could mark at least the beginning of a family structure in which males took some interest in protecting and feeding the mothers of their children. Tubers, assuming they had become part of ergaster’s diet, are items that women can collect. A greater role for women in providing food might have promoted the pattern of greater cooperation between the sexes, as is implied in the decreased male-to-female size ratio.
Further evidence of change in relations between the sexes comes from ergaster’s anatomy. Its pelvis was narrower than habilis’s, and the smaller birth canal meant that much of the infants’ extra brain size had to be acquired by growth after birth. This in turn means that infants would have had to be carried, making the women more vulnerable. The fathers might have found that in order to protect their genetic legacy they needed to spend more time guarding their specific offspring and the children’s mother, not just the group’s general territory. Such behavior, and a closer bond between parents, would have been genetically favored if it led to more surviving offspring. That’s a long chain of inference from the simple anatomical fact of a narrower pelvis, but not so unreasonable.13
Ergaster’s brain was only slightly larger, in relation to body size, than that of habilis, its assumed predecessor, but it was nonetheless capable of a whole new level of stone tool-making. The more sophisticated tool kit, called the Acheulean Industrial Tradition, included the lozenge-shaped stones thought to be hand axes, although their precise use is not known for sure, as well as cleavers and other large tools. From the microscopic pieces of material on the stones, as well as modern experiments in stone knapping, the tools seem to have been designed for a wide array of tasks, such as heavy and light duty butchering, slitting hides, breaking bones, cutting grass, and woodworking. Ergaster doubtless used many tools and materials made of wood and other perishable substances.
The Farewell to Fur
The humanlike species that evolved during the first three million years after the split with chimps probably looked far more apelike than human. Their bodies were of ape proportions, with long arms, and doubtless covered with hair from head to toe. Not until ergaster’s arrival on the scene did the lineage’s physical appearance assume a more recognizably human form. Ergaster, unlike its predecessors, possessed an external nose. This acquisition was its most prominent adaptation to hot, dry climates, the role of a nose being to conserve water by cooling and condensing moist air from the lungs before it leaves the body.
After the differences in body proportions, humans differ most strikingly from their ape cousins in the distribution of body hair. Over most of the body people are essentially hairless, possessing just vestigial hair that is largely invisible. Nakedness is a complex issue, for which there may be several layers of explanation. Hairiness is the default state of all mammals, and the handful of species that have lost their hair have done so for a variety of compelling reasons, such as living in water, as do hippopotamuses, whales and walruses, or residing in hot underground tunnels, as does the naked mole rat. With humans, the prime cause may have been the need to sweat. Ergaster may have been the first of the human line to shed its fur in favor of naked skin, in the view of the paleoanthropologist Richard Klein.14 His inference is based on the idea that if ergaster were living in dry, hot places, it would need to have evolved a way of cooling the body and its larger brain. Sweating, an efficient way to do this, requires a naked skin. Besides, humans must have lost their hair at some time, and the most plausible period is when they traded the shade of the trees for the heat of the savanna.
Another, perhaps secondary, reason for human nakedness may have to do with sexual preferences. Darwin, who first suggested the idea, gave the matter serious attention in his book of 1871, The Descent of Man. “May we then infer that man became divested of hair from having aboriginally inhabited some tropical land?” Darwin asked. (He had already assumed that humans originated in Africa, because that is where the great apes are found, but there was then no fossil evidence to confirm the idea.) Yet that couldn’t be the whole story because other primates in tropical countries have retained their hair. Perhaps shedding the body hair freed humans from the burden of parasites like lice, fleas and ticks. But that didn’t seem a decisive enough advantage to Darwin. “The view which seems to me the most probable,” he concluded, “is that man, or rather primarily woman, became divested of hair for ornamental purposes.” 15
Darwin believed that sexual selection was an important factor in evolution because it determined mating success. Sexual selection arises in two distinct forms, intersexual and intrasexual. The first is the way that men and women choose each other as mates; the second is the competition within each sex, between men for women and, sometimes more discreetly, between women for men. Hairlessness would have been favored, in Darwin’s view, if men and women had preferred partners with less hair. Two biologists, Mark Pagel and Walter Bodmer, have recently reinvoked Darwin’s idea of sexual selection as the driver of human hairlessness. They suggest that lack of hair was favored among early humans because it was a sure signal that no parasites were lurking in their fur.16
The date proposed by archaeologists for when humans lost their hair is based on the guess that it coincided with the emergence of Homo ergaster. But an actual fix on the date has been supplied by geneticists. Their research illustrates the wealth of information that can be extracted from a single gene if the right questions are asked.
The gene in question is one that makes the melanocortin receptor, a protein that helps determine skin color. It does so by controlling the proportions of different-colored melanin pigments that are synthesized in a person’s skin cells. Some versions of the melanocortin receptor produce black skin and hair, others generate ginger or brown or yellow.
Rosalind Harding, of the University of Oxford in England, recently analyzed the order of the DNA units in the melanocortin receptor gene possessed by people from Africa, Europe and Asia. She and her colleagues found that all Africans had essentially the same version of the gene but that people outside of Africa possessed many different versions.17
An obvious explanation for the receptor gene’s constancy in Africa is that it is under fierce selective constraint there, meaning that natural selection prevents any significant change. The African version of the gene is set to produce maximum blackness; any change in its DNA sequence is likely to make the skin lighter and its owner more vulnerable to the sun’s ultraviolet radiation, which destroys an essential nutrient known as folic acid. (Ultraviolet radiation can also cause skin cancer, but it is the destruction of folic acid that is more likely to reduce fertility and hence to shape the evolution of the gene.) Anyone with a changed melanocortin receptor gene is likely to leave fewer or no descendants, and the variant gene will in time be eliminated from the population. Hence everyone living under the African sun has the same version of the gene, no deviations allowed.
Before human ancestors lost their hair, however, their skin was almost certainly pale, according to Nina Jablonski, an expert on the evolution of human skin color.18 This can be inferred from the skin color of chimpanzees, the reliable surrogate for the joint human-chimp ancestor. Beneath their dark hair, which protects them from the sun, chimpanzees have light skin. They too have a melanocortin receptor gene, but it exists in many different versions, as if natural selection does not mind letting it vary, and all produce pale skin. (Chimps have dark-skinned faces, but that is from tanning of the pale faces they have at birth.)
Reading Harding’s article, Alan Rogers, a population geneticist at the University of Utah, wondered how African populations had all acquired the same version of the gene. The process must have started, he supposed, when the human lineage first started to lose its apelike hair, dangerously exposing the pale skin beneath. Any mutation in the melanocortin receptor gene that led to a blacker, more protective skin would have conferred a great advantage on its owner. In several generations the new version of the gene would sweep through the population.
Genetic sweeps can often be dated because after a must-have gene has become universal it starts to accumulate what are known as silent mutations, ones that don’t alter the structure of the gene’s protein and so are not eliminated through natural selection.c Since the silent mutations accumulate at a known rate, the number of them is a measure of the time that has elapsed since the new version of the gene swept through a population.
Rogers realized that from the silent mutations in the African version of the melanocortin receptor gene, he could calculate the date at least of the gene’s most recent sweep. He estimates that this event took place about 1.2 million years ago. 19
There may have been several earlier such genetic sweeps, each one producing a progressively more effective version of the melanocortin receptor gene. The gene, after all, had to make a very significant transition, from producing the pale skin of the joint human-chimp ancestor to the black skin that protected the newly hairless body from the sun in the scantly shaded African savanna. If the first of these sweeps had started several thousand years before, that would fit well with the archaeological evidence for the emergence of Homo ergaster 1.7 million years ago.
Before the Dawn: Recovering the Lost History of Our Ancestors Page 3