The Origin of Humankind (7 page)

In the late 1980s, these assumptions were shattered by the work of several researchers. Holly Smith, an anthropologist at the University of Michigan, developed a way of deducing life-history patterns in fossil humans by correlating brain size and the age of eruption of the first molar. As a baseline, Smith amassed data for humans and apes; she then looked at a range of human fossils to determine how they compared. Three life-history patterns emerged: a modern human grade, in which first-molar eruption occurs at six years of age and life span is sixty-six years; an ape grade, with first-molar eruption at a little over three years and a life span of about forty years; and an intermediate grade. Later
Homo erectus
—that is, individuals who lived after about 800,000 years ago—fit the human grade, as did Neanderthals. All the australopithecine species, however, slotted into the ape grade. Early
Homo erectus
, like the Turkana boy, was intermediate: the boy’s first molar would have erupted when he was a little more than four and a half years old; had he not met an early death, he could have expected to live about fifty-two years.

Smith’s work showed that the australopithecines’ pattern of growth was not like that of modern humans; instead, it was apelike. She further showed that early
Homo erectus
was intermediate between modern human and ape in its growth: we now conclude that the Turkana boy was about nine years old when he died, and not eleven, as I’d initially supposed.

Because these conclusions were contrary to a generation of anthropologists’ assumptions, they were highly controversial. There was a possibility, of course, that Smith had made some kind of error. In these circumstances, corroborative work is always welcome, and in this case it came quickly. The anatomists Christopher Dean and Tim Bromage, both then at University College, London, devised a way of directly determining the age of teeth. Just as tree rings are used to calculate how old a tree is, microscopic lines on a tooth indicate its age. This method of calculation is not as easy as it sounds—not least because of some uncertainty about how the lines are formed. Nevertheless, Dean and Bromage initially applied their technique to an australopithecine jaw identical to the Taung child’s in terms of tooth development. They found that the individual had died at a little over three years of age, just as his first molar was erupting—right on cue for an apelike growth trajectory.

When Dean and Bromage surveyed a range of other fossil human teeth, they, like Smith, found three grades: modern human, ape, and something intermediate. Once again, the australopithecines were squarely in the ape grade, late
Homo erectus
and the Neanderthals were in the modern human grade, and early
Homo erectus
was intermediate. And once again the results stirred debate, particularly over whether australopithecines grew like humans or apes.

That debate was effectively ended when the anthropologist Glenn Conroy and the clinician Michael Vannier, at Washington University in St. Louis, brought high technology from the medical world into the anthropological laboratory. Using computerized axial tomography—the three-dimensional CAT scan—they peered into the interior of the Taung child’s petrified jaw and essentially confirmed Dean and Bromage’s conclusion. The Taung child had died when it was close to three years old, a youngster on an apelike trajectory of growth.

The ability to infer biology from fossils through research in life-history factors and tooth development is enormously important to anthropology, because it allows us metaphorically to put flesh on the bones. For instance, we can say that the Turkana boy would have been weaned a little before his fourth birthday and, had he lived, would have become sexually mature at about fourteen years old. His mother probably had her first baby when she was thirteen, after a nine-month pregnancy; and thereafter would have been pregnant every three or four years. These patterns tell us that by the time of early
Homo erectus
, human ancestors had already moved in the direction of modern human biology and away from ape biology, while the australopithecines remained in their ape grade.

The evolutionary shift by early
Homo
toward modern human patterns of growth and development occurred in a social context. All primates are social, but modern humans have developed sociability to the highest degree. The change in biology we inferred from the evidence of teeth in early
Homo
tells us that social interaction in this species had already begun to intensify, creating an environment that fostered culture. It appears that the entire social organization was significantly modified, too. How can we know this? It is evident from a comparison of the body size of males and females, and from what we know of such differences in modern primate species, such as baboons and chimpanzees.

In savanna baboons, as noted earlier, males are twice the size of females. Primatologists now know that this size difference occurs when there is strong competition among mature males for mating opportunities. As in most primate species, male baboons, when they reach maturity, leave the troop into which they were born. They join another troop, often one nearby, and are from then on in competition with the males already established in the group. Because of this pattern of male migration, the males of most groups are usually unrelated to each other. They therefore have no Darwinian (that is, genetic) reason for cooperating with each other.

However, in chimpanzees, for reasons that are not fully understood, males remain in their natal group and females transfer. As a consequence, the males in a chimpanzee group have a Darwinian reason for cooperating with each other in acquiring females, because as brothers they have half their genes in common. They cooperate in defending against other chimp groups, and on occasional hunting forays, when they usually try to corner a hapless monkey in a tree. This relative lack of competition and enhanced cooperation are reflected in the size of males compared with females: they are a mere 15 to 20 percent bigger.

With regard to size, australopithecine males follow the baboon pattern. It is a reasonable assumption, therefore, that social life in australopithecine species was similar to what we see in modern baboons. When we were able to make a comparison of male and female body size in early
Homo
, it immediately became obvious that a significant shift had occurred: males were no more than 20 percent bigger than females, just as we see in chimpanzees. As the Cambridge anthropologists Robert Foley and Phyllis Lee have argued, this change in body-size differential at the time of the origin of the genus
Homo
surely represents a change in social organization, too. Very probably, early
Homo
males remained in their natal groups with their brothers and half brothers, while the females transferred to other groups. Relatedness, as I’ve indicated, enhances cooperation among the males.

We can’t be certain what prompted this shift in social organization: enhanced cooperation among males must have been strongly beneficial for some reason. Some anthropologists have argued that defense against neighboring troops of
Homo
became extremely important. Just as likely, and perhaps more so, is a change centered on economic needs. Several lines of evidence point to a shift in diet for
Homo
—one in which meat became an important energy and protein source. The change in tooth structure in early
Homo
indicates meat eating, as does the elaboration of a stone-tool technology. Moreover, the increase in brain size that is part of the
Homo
package may even have
demanded
that the species supplement its diet with a rich energy source.

As every biologist knows, brains are metabolically expensive organs. In modern humans, for example, the brain constitutes a mere 2 percent of body weight, yet consumes 20 percent of the energy budget. Primates are the largest-brained group of all mammals, and humans have extended this property enormously: the human brain is three times the size of the brain in an ape of equivalent body size. The anthropologist Robert Martin, of the Institute of Anthropology in Zurich, has pointed out that this increase in brain size could have occurred only with an enhanced energy supply: the early
Homo
diet, he notes, must have been not only reliable but nutritionally rich. Meat represents a concentrated source of calories, protein, and fat. Only by adding a significant proportion of meat to its diet could early
Homo
have “afforded” to build a brain beyond australopithecine size.

For all these reasons, I suggest that the major adaptation in the evolutionary package of early
Homo
was significant meat eating. Whether early
Homo
hunted live prey or merely scavenged carcasses, or both, is a highly controversial issue in anthropology, as we will see in the next chapter. But I have no doubt that meat played an important part in our ancestors’ daily lives. Moreover, the new subsistence strategy of obtaining not just plant foods but meat as well probably demanded significant social organization and cooperation.

Every biologist knows that when a basic change occurs in a species’ pattern of subsistence, other changes usually follow. Most often, such secondary changes involve the species’ anatomy, as it adapts to the new diet. We have seen that the tooth and jaw structure of early
Homo
is different from that of the australopithecines, presumably as an adaptation to a diet that includes meat.

Very recently, anthropologists have come to believe that, in addition to dental differences, early
Homo
differed from the australopithecines in being a much more physically active creature. Two independent lines of research converged on the conclusion that early
Homo
was an efficient runner, the first human species to be so.

A few years ago, the anthropologist Peter Schmid, a colleague of Robert Martin’s in Zurich, had an opportunity to study the famous Lucy skeleton. Using fiberglass casts of the fossil bones, Schmid began assembling Lucy’s body, with the full expectation that it would be essentially human in shape. He was surprised with what he saw: Lucy’s rib cage turned out to be conical in shape, like an ape’s, not barrel-shaped, as would be seen in humans. Lucy’s shoulders, trunk, and waist also turned out to have a strong apelike aspect to them.

At a major international conference in Paris in 1989, Schmid described the implications of what he had found, and they are highly significant.
Australopithecus afarensis
, he said, “would not have been able to lift its thorax for the kind of deep breathing that we do when we run. The abdomen was potbellied, and there was no waist, so that would have restricted the flexibility that’s essential to human running.”
Homo
was a runner;
Australopithecus
was not.

The second line of evidence that bore on this issue of agility flowed from Leslie Aiello’s work on body weight and stature. She obtained measures of these features in modern humans and apes and compared them with similar data gleaned from human fossils. Present-day apes are heavily built for their stature, being twice the bulk of a human of the same height. The fossil data, too, fell into a clear pattern—one that by now was becoming familiar. The australopithecines were apelike in their body build, while all
Homo
species were humanlike. Both Aiello’s findings and Schmid’s work are consistent with Fred Spoor’s discovery of the difference in anatomical structure of the inner ear in australopithecines and
Homo:
a greater commitment to bipedality goes along with the new body stature.

I hinted in the previous chapter that major changes other than that of brain size occurred with the evolution of the genus
Homo
. We can see now what it was: australopithecines had been bipeds, but were restricted in their agility; species
of Homo
were athletes.

I argued earlier that bipedalism evolved initially as a more efficient mode of locomotion in a changed physical environment, enabling a bipedal ape to survive in a habitat unsuited to conventional apes. Bipedal apes were able to roam more terrain as they foraged for widespread sources of food in open woodland. With the evolution of
Homo
, a new form of locomotion emerged, still built on bipedalism but with greater agility and activity. The lithe stature of modern humans permits sustained striding locomotion and promotes effective heat loss, which is important for an animal that is active in open, warm environments, as early
Homo
was. The efficient, striding biped represented a central change in hominid adaptation. And that change surely involved some degree of active hunting, as we shall see in the next chapter.

The ability of an active animal to dissipate heat is especially important for the physiology of the brain, a point emphasized by the anthropologist Dean Falk, of the State University of New York, Albany. In her anatomical research in the 1980s, she demonstrated that the structure of the vessels that drain blood from the
Homo
brain is conducive to efficient cooling, while in australopithecines it is much less so. Falk’s so-called radiator hypothesis is one more argument in support of the magnitude of the
Homo
adaptation.

That the
Homo
adaptation was successful scarcely needs to be said: we are here today as evidence. But why do we not have other bipedal apes for company?

Two million years ago,
Homo
coexisted with several species of
Australopithecus
in East and South Africa. But a million years later,
Homo
was in splendid isolation, the various australopithecine species having slipped into extinction. (We tend to think of extinction as a mark of failure—as something that happens to a species that is somehow not up to the challenge that nature presents to it. In fact, extinction appears to be the ultimate fate of all species: more than 99.9 percent of all the species that ever existed are now extinct—probably as much a result of bad luck as of bad genes.) What do we know about the fate of the australopithecines?