Ecoscience: Primate Evolution, Humans and Gorillas

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Illustration by Fotolia/AlienCat
Research into primate evolution shows humans are about equally related to chimps and gorillas.

Although there’s still considerable debate about details, the basic outlines of the past few million years of primate evolution are clear for the human line. The study of fossils has taught us a great deal about our distant relatives.

About four million years ago, our ancestors were fully upright creatures only about four feet tall, with brains of about 350 to 450 cubic centimeters (cc) in volume — roughly a third as large as those of modern people. Because the first remains of these ancestors were discovered in southern Africa and were thought to be of apes, the creatures were placed in a genus named Australopithecus — the “ape of the south.”

The oldest fossil Australopithecus found so far is a creature from the Afar triangle of northern Ethiopia, who lived about three and a half million years ago, A. afarensis, discovered by paleontologist Don Johanson and his colleagues, became famous when the first fossil specimen was nicknamed Lucy, and Johanson published a fine book under that title.

Australopithecus afarensis gave rise over several million years to two subsequent australopithecine species: A. africanus and A. robustus — both of which were a little taller and had somewhat larger brains than Lucy. The last Australopithecus died out perhaps one and a quarter million years ago. But a couple of million years before that, the australopithecines had given rise to an important branch — the one leading to us.

Tracing the Human Line

The earliest known member of the Homo line was a creature called Homo habilis. It was a contemporary of the australopithecines, lived some two million years ago, stood about five feet tall, and had brain size of around 460 cc. Homo habilis was followed, about a million years ago, by Homo erectus. Members of H. erectus were quite similar to modern people; they average about five and a half feet tall and had brains overlapping size with today’s Homo sapiens (700 to 1,250 cc as opposed to 1,000 to 1,800 cc)

Fossils of H.erectus are abundant and widespread, and historically have been given a variety of names including ” Pithecanthropus erectus” and “Sinanthropus pekinenses.” With a shave, a haircut, and modern clothing, an average male Homo erectus would probably cause little comment if seen on the street today. And there’s good reason to believe that our progenitors of a million years ago also had a highly developed culture.

From H.erectus to modern H.sapiens, and its minor “neanderthal” variant, was a mere hop, skip, and jump. Indeed, the basic Australopithecus/H. erectus/H. sapiens sequence constitutes a superb fossil record, one with essentially no “missing links.” It’s only because of a keen interest in our own history that people even bother to search for more fossils from the past four million years of our ancestry.

Our Primate Cousins

There are, of course, no paleontologists among the gorillas and the other great apes, but extensive searches for their fossils have been made by their human cousins. However, we still lack a recent fossil record of the apes. Consequently, the exact relationships among gorillas, chimpanzees, orangutans, gibbons, and human beings are not known. The fragments of fossil apes that have been found date from about 15 million years ago and are adequate to sort out the history of the branching lines. On the hominid side, there’s about a 10-million-year gap before Lucy, and on the ape side, the gap runs right up to the present.

But comparisons of living forms make the general relationships among the high primates crystal clear. Evidence from anatomy, shared malarial organisms and parasitic worms, chromosome structure, biochemical similarities, behavior, and so on, all indicate that people, gorillas, and chimpanzees have a more recent common ancestor than the African apes have with the Asiatic gibbons and orangutans.

After separation from the Asian apes, an African stock evidently split into the hominid and ape lines, and then the ape line split again into gorillas and chimps. Human beings are probably about equally related to chimps and gorillas, and that relationship is “close” by almost any taxonomic standard.

Although the gorillas are the largest of the great apes, they were the last to be discovered by science — indeed, they were long confused with the chimps. It wasn’t until 1847 that they received a formal scientific description based on skulls and some other bones collected in Gabon by Thomas Savage, a medical missionary. He and Harvard zoologist Jeffries Wyman gave the gorilla the Latin name Gorilla gorilla.

The original geographic distribution of the gorilla appears to have constituted an arc from Zaire, Rwanda, and Uganda in the east up through the Central African Republic to the West Coast —Cameroon, Gabon, and Congo-Brazzaville. The distribution of the gorillas appears to have been limited by their inability to swim (the Congo River seems to have been a barrier to the spread of gorillas into the rain forests south and east of the river) and by their avoidance of open country.

Three subspecies of gorillas are recognized: Gorilla gorilla gorilla on the West Coast; G. g. graueri in the central African lowlands; and G. g. beringei, the “mountain gorilla,” best known in the Virunga volcano area. The differences among the three races are small. For example, the western and mountain races differ in the alignment of the “thumb” of the foot and in the shape of the head and face. Gorilla g. graueri tends to be intermediate between the other two.

How We Compare

The differences between gorillas and human beings are more striking. The average cranial capacity of gorillas is less than half of ours, and their posture is predominantly quadrupedal. Like most other mammals, the gorilla’s backbone forms a bridge between fore and hind legs, from which the internal organs are slung. The hole through which the spinal column enters the skull is at the back. In contrast, all hominids are bipedal, with the skull balanced on top of the spine (and the hole at the bottom) and the internal organs dropped into the cradle of the pelvis—making us susceptible to hernias.

There are, of course, also many minor anatomical differences. Gorillas, for example, have large canine teeth. These are especially prominent in the males, who threaten and fight with them. And gorillas have much bigger molars than we do, which they use to grind up coarse vegetarian diet. They are also hairier, heavier, and stronger than Homo sapiens.

Next to people, the gorillas are the most terrestrial of the apes — the mountain gorillas, especially, spend little time in the trees. And unlike humans or chimps, they are strictly vegetarian. It’s ironic that their fearsome reputation is totally undeserved. Gorillas ramble in small groups through the forest, devouring a wide variety of plants (including the bark of trees) and posing no threat at all to other animals.

Gorilla groups ordinarily consist of a dominant “silverback” male, sometimes a second silverback, often a younger “blackback” male three or four mature females and three to six immature individuals. If a group is approached by surprise, the silverback will interpose himself between his family and the intruder, giving the others a chance to flee.

The defending silverback is an imposing creature, weighing perhaps 350 pounds and standing almost six feet tall. He can produce an impressive display: among other things, hooting, standing on his hind legs, beating his chest, and thrashing his arms in the vegetation. Then he may charge. If the intruder doesn’t retreat, the charge will typically end in front of its target. Then the silverback will drop to all fours and move off to the side.

A fleeing intruder, however, is pursued and bitten, often on the buttocks. A gorilla bite is considered a great disgrace among poachers — as well it might be, since a person need only stand his ground with a spear in front of him to have an excellent chance of slaughtering a charging silverback.

The Intelligence Question

The question of the relative intelligence of gorillas, chimps, and people is a controversial one. It’s now generally agreed that apes don’t speak because they lack the appropriate vocal apparatus. Both chimps and gorillas have been taught to communicate in sign language, although the similarity of that communication to human speech is still debated.

The gorilla Koko, trained by Penny Patterson, is disarmingly humanlike in some of her communication. For example, she called a zebra, on first sight, a “white tiger.” And after she had broken a sink, Koko attempted — in all too human a fashion — to fix the blame on one of Penny’s assistants, signing “Kate there bad.”

When she was six and a half, Koko had a vocabulary of almost 400 signs and scored about 85 on a Stanford-Binet I.Q. test — even though the test was human-biased. (For example, to the question “Where does one shelter from the rain?” Koko answered “tree;” the “correct” answer was “house.”)

Gorillas and chimps both show the ability to solve problems with flashes of insight — such as piling up boxes and climbing on them to get bananas otherwise out of reach. While for a time people believed chimps to be smarter than gorillas, the consensus now is that the two African apes are similar in intelligence but different in character.

Whatever the evolutionary relationship of these gentle giants to Homo sapiens, their ecological relationship is precarious. As we shall see a subsequent column, humanity may be in the process of exterminating one of its two closest relatives.

For an introduction to gorillas, see Dian Fossey’s Gorillas in the Mist (Houghton Mifflin, Boston, 1983) and George Schaller’s The Mountain Gorilla: Ecology and Behavior (University of Chicago, Press, Chicago, 1976). More technical information can be found in A.F. Dixon’s The Natural History of the Gorilla (Columbia University, New York, 1981) and T.L. Maple and M.P. Hoff’s Gorilla Behavior (Van Nostrand Reinhold, New York, 1982). Money is badly needed to help preserve the mountain gorillas. Donations may be sent to African Wildlife Leadership Foundation. State that you wish to support the “Mountain Gorilla Project in Rwanda.”

Paul Ehrlich (Bing Professor of Population Studies and Professor of Biological Sciences, Stanford University) and Anne Ehrlich (Senior Research Associate, Department of Biological Sciences, Stanford) are familiar names to ecologists and environmentalists everywhere. But while most folks are aware of the Ehrlichs’ popular writing in the areas of ecology and overpopulation (most of us —for instance—have read Paul’s book The Population Bomb), few people have any idea of how deeply the Ehrlichs are involved in ecological research (the type that tends to be published only in technical journals and college texts). That’s why we’re pleased to present this regular semitechnical column by these well-known authors/ ecologists/educators.

The Ehrlichs’ work is supported in part by a grant from the Koret Foundation of San Francisco.