It is just this kind of environmental stress that has forced evolutionary change since the beginning of life on earth. In response to new conditions, a species must adapt, by drawing on the genetic variation available in the members of its population. The individuals better endowed to meet the new conditions will thrive and leave more descendants. As the descendants continue to adapt to the new conditions, their genetic constitution, over the course of the generations, will differ increasingly from that of their ancestors.
The drought that occurred in Africa 5 million years ago may have been the agent that forced evolutionary change on the apes ancestral to both chimps and humans. Despite the serious lack of fossil evidence, much can be inferred about the joint human-chimp ancestor. Its population numbered somewhere between 50,000 and 100,000 breeding individuals, according to genetic calculations.6 Assuming that its way of life resembled that of chimpanzees, it would have lived in communities about a hundred or so strong, structured around a group of related males. By inference from chimpanzee society, the males would have defended their territory aggressively against the males of neighboring communities, with frequent fatalities. Each community's survival strategy lay in defending as large as possible an area of fruit trees for its females to feed in. The male apes, again on the assumption the species resembled chimpanzees, were much larger than the females and paid little attention to them except to mate. There was no particular bond between male and female. Each sex had its own hierarchy, with the females subservient to the males. The males spent their day, when they weren't fighting, building alliances with other males and trying to work their way up the male hierarchy of their community. The risks of being alpha male were considerable, but the payoff considerable, at least in Darwinian terms: the alpha male and his allies got to father most of the community's offspring. So consider this population of 100,000 chimplike apes somewhere in the eastern side of equatorial Africa 5 million years ago. Times are tough and their forest homeland is shrinking. The trees no longer carry enough fruit. The apes are forced to spend a lot of time on the ground searching for other sources of food. Large cats stalk or ambush the unwary. Each generation is tested by this harsh new environment, and in each generation the better adapted produce more offspring. There are two kinds of survivor. One, clinging to the remnants of forest, manages to continue in much the same way of life: this is the lineage that leads to chimpanzees, and because it clings to the same habitat it has no great need to change its way of life or physical form. The other manages to survive by venturing into a new niche—it learns to occupy both the trees and the new spaces that have appeared in between them. Helping it survive on the ground is the emergence of a critical new ability—that of walking on two feet.
Two-footedness—bipedalism in paleoanthropologists' parlance—is the first great stride toward becoming human. Standing upright is in fact not so big an adjustment for apes, who move in the trees by hanging under branches and swinging from one arm to the other. Monkeys, on the other hand, prefer to run along the tops of branches; so when some of them started to live on the ground—becoming baboons—they preserved their four-footed style of travel.
Chimpanzees get around on the ground by knuckle-walking—using the knuckles of the hand as front feet. So why did those on the human side of the split prefer bipedalism? Many advantages of bipedalism have been cited as decisive—it frees up the hands for carrying things, it allows better surveillance of the surroundings—but the most likely reason for its emergence is simply that upright walking is more efficient than knuckle-walking. For the same expense of energy, a chimp can knuckle-walk 6 miles a day but a man can walk 11. Bipedalism probably evolved because it was a better way of getting about, and its other benefits were at first incidental.
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. 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.b 9
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.^° 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.
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