Body Bipedal

Our large brains, our upright posture and two-legged gait, our ability to use tools-- these are a few of the features that separate humans from the apes. Still, we are not as far removed from our fellow primates as we might imagine. Kevin Hunt, assistant professor of anthropology at Indiana University Bloomington, has found that careful consideration of our nearest "relative"--the chimpanzee--reveals a host of similarities. Furthermore, Hunt's painstaking observations of chimps in the wild have yielded fascinating insights into the anatomy and behavior of our earliest known ancestors (after our lineage branched off from the apes), the hominids classified as Australopithecus afarensis.

A. afarensis lived approximately 3 to 4 million years ago. The anatomy of these early hominids became clear in 1974, when anthropologists in Ethiopia discovered "Lucy," an amazingly complete skeleton of an adult female. Curiosity about this ancient forerunner of Homo sapiens has raged ever since. Both scientists and the general public wonder, "What was Lucy like?" In many ways, Hunt answers, she was like a chimpanzee.

One piece of evidence that supports this assertion is the size of the australopithecine brain. Hunt notes that their brains were scarcely larger than those of chimpanzees, only around 400 cubic centimeters, or about a third of the size of a modern human's brain. Their skulls were also shaped like those of chimpanzees. "The overall proportion of the brain case to the size of the face, the fact that there's no chin, that there is a procumbent--or protruding--jaw, in all of these ways the australopithecine skull resembles a chimpanzee skull," Hunt says.

But australopithecines were definitely not chimpanzees, as Lucy's fragmentary pelvis makes clear. "The pelvis bone is very humanlike," Hunt says. "It's not indistinguishable from the pelvis of a modern human, but it's much more like the pelvis of a modern human than the chimpanzee pelvis is." Fossils of A. afarensis skeletons indicate that the hominid also had the narrow shoulders and cone-shaped chest characteristic of chimpanzees, as well as thighs, feet, knees, and ankles that were, Hunt has written, "strikingly similar to those of modern humans."

One of the most significant advantages of their lower anatomy was that the australopithecines could stand erect and walk steadily and continuously on two feet. They are our earliest known bipedal ancestors. At times chimps can be bipedal. Hunt has observed, for example, that chimps stand on two legs when they eat tiny fruits, some of them no larger than a grape seed. These findings support a hypothesis advanced in 1970 by anthropologist Clifford Jolly. But Hunt's observation of chimps has added a new wrinkle: chimps are bipedal while eating tiny fruits both when on the ground, consistent with Jolly's hypothesis, and when in trees. Bipedalism is useful to chimps when feeding in either location.

Even though australo-pithecines were bipedal, Hunt surmises that they, too, spent a good deal of time climbing in trees rather than walking on the ground. He speculates that this behavior was especially common for the smaller and lower-ranking australo-pithecines, which probably would have included females. (Among A. afarensis, adult males were about five feet tall and weighed around 120 pounds; females were no more than four feet tall and weighed approximately 60 pounds.) Hunt bases his supposition--along with other theories he has developed about australopithecines--on information gathered during his own observations of chimpanzees in the field.

Beginning in September of 1986, Hunt spent a year studying chimpanzees and baboons at two sites in Tanzania. Since then he has twice spent six months in Uganda, most recently in 1992, observing chimpanzees and other primates at the Kibale Forest Reserve. Though it was previously known that chimps spend about half of their time and most of their energy in trees, feeding mainly on fruits, Hunt was the first researcher to document their actions so minutely. In the Tanzania study, Hunt says, "Every two minutes, I wrote down exactly what the chimps were doing, observing thirty-five different variables, from their height in the tree to which parts of their bodies were touching a branch, to the size of the branch that each body part was touching, to the angle of the branch, to what they were eating at the time, to relationships of parts of their bodies to one another."

In the publications resulting from the data he gathered in the field, Hunt focuses on several topics of significance to human evolution. One of these topics is chimpanzee posture during food-gathering. Hunt has noticed that the chimpanzee's upper anatomy, with its narrow shoulder blades and upward-facing socket joints, lends itself to one-armed hanging. He pictures Lucy, whose upper body was so similar to that of the chimpanzee, feeding herself in a similar posture, standing on a branch or beneath a tree, with one hand gripping a branch above her. "Some would say that Lucy's anatomy is especially adapted for walking around on the ground, and I agree with that," Hunt says. "But I also think it's important to consider the evidence suggesting that australopithecines were terrestrial animals that climbed trees. In looking at what Lucy is like above the waist, I would estimate she spent about a quarter of her time in trees."

It is likely, Hunt suggests, that like chimps, Lucy and other females spent more time in trees than their male counterparts did. This suggestion stems from Hunt's consideration of chimpanzee positional behavior, another topic related to questions about human evolution. Hunt's records from all three of his Africa studies depict a complex social system among chimpanzees. Rank is based on a combination of factors, including size and fighting ability. A particular chimp's physical location while feeding--whether it feeds from high branches, from low branches, or on the ground--is directly connected with the individual's social rank. The most desirable spots, Hunt has observed, are those that require the least effort to occupy, so high-ranking males tend to feed on or near the ground. Because female chimpanzees are smaller than males and more concerned with caring for offspring than with social dominance, they are relegated to locations associated with lower social rank. Females, therefore, are more likely to climb trees in order to feed.

Was early australopithecines' social behavior similar to that of modern chimpanzees? "Maybe the big males were able to bully everybody into letting them have all the food that they could collect from the ground," Hunt says. "Then the females, the young, and the lower-ranking males would have had to be able to get into trees, or they would have starved."

This system for distributing resources may seem unfair--not to mention sexist-- but perhaps it contains clues about how the large-brained, technologically skilled humans of today evolved from those early hominids. Hunt says that among chimpanzees "females eat foods that require more manipulation, foods that are small. So they eat termites, and males eat meat. Females eat small, young leaves, and males eat piths--large grass stems. Even when they eat the same species of fruits, males eat bigger fruits.

"The intriguing thing to me is that because females spend so much time processing their food, anything they can do to save time will be helpful," Hunt says. "They can't increase their rank because they're smaller than males. But they can use shortcuts to harvest foods that males don't eat. One of these shortcuts is the use of tools. There are differences in the ways male and female chimpanzees use tools." Researchers have observed females using rocks to crack open nuts more frequently than males do, to crack open harder nuts, and to crack open nuts while they are perched in trees, something males apparently are unable to do. For females, tools provide an attractive alternative to "fighting with males over foodstuffs when they're not going to win anyway, and possibly being injured or having their infants injured in the process," Hunt says.

Hunt's ideas that technology originated in the "kitchen," rather than on the battlefield or the hunting ground, and that the most productive early tool-users were female, have met with some enthusiasm and some skepticism, he says. "But this hypothesis certainly fits with the data, which show that since female chimpanzees can't compete against males for the same foods, they're using these tools to help with other foods, foods that require more processing."

In Hunt's vision of the distant past, the earliest humans were more gatherer-hunters than hunter- gatherers--much like chimpanzees of today. In addition to Hunt's field observations, microscopic analysis of australopithecine fossils supports that view. "They have scratches on their teeth that are consistent with their eating a lot of fruit, some piths and leaves, and very little meat," he says. This description sounds remarkably like the chimpanzee diet, but Hunt believes there are significant differences. "Chimpanzees appear to specialize on low-carbohydrate, low-secondary-compound forest foods (secondary compounds are poisons or nutrient-neutralizers) that are found in locally dense patches," Hunt has written. "Humans, I hypothesize, specialized on high-carbohydrate, low-secondary compound fruits found in drier habitats than those in which chimps live." Hunt proposes that a shift to this higher-carbohydrate diet--a diet that also had a low incidence of toxic compounds that would interfere with the energy gain that carbohydrates provide--may have contributed to the gradual growth of human brain capacity. "Big brains demand a high-energy, low-poison diet," Hunt says. "Twenty percent of our calories go to run our brains; the percentage is only a fraction of that in most animals." And the increase in brain size has led, over the eons, to the need for ever more energy, and to the evolution of tool-making and tool-using to produce it.

But, like chimpanzees, Lucy and her companions probably used no tools more sophisticated than stones for smashing nuts and perhaps cracking open the bones of scavenged animal carcasses. The oldest known human-fashioned tools are only 2 million years old. Thus our earliest ancestors were not, Hunt has written, "spear-carrying hunters striding across the grassy savannas, fixing skittish mammals with a steely hunter's eye, but fruit eaters, spending much of their day standing and gathering rather than striding."

--Karen Grooms