UCSC Review Winter 1996

The Anatomy of Evolution: Fleshing Out the Fossil Record

By Jennifer McNulty

His name means "Sir" in Swahili, and as the senior male gorilla at the San Francisco Zoo, Bwana commanded respect. A majestic animal, he kept watch over the troop and intervened when his own son Kubie harassed females. He was playmate and protector to infant gorillas. And two generations of zoo visitors admired him.

One of those visitors was UCSC anthropologist Adrienne Zihlman, one of the world's preeminent primatologists. Zihlman's history with Bwana goes back to the 1960s, when she spent time observing him while she was a graduate student at UC Berkeley. In 1980, she had the opportunity to study him up close as part of the team that measured, weighed, and examined him when the gorillas were tranquilized and moved to a new enclosure.

Bwana died eighteen months ago of an acute intestinal infection. His death at the age of 36 made headlines, and even San Francisco Chronicle columnist Herb Caen wrote a loving tribute to Bwana. When Zihlman heard the news, she called the zoo and received permission to dissect his 380-pound body after autopsy.

So little is known about the anatomy of gorillas that the only complete description is a book published in 1950 that was based on a single animal from Chicago's Lincoln Park Zoo. Without Zihlman's intervention, only Bwana's skeleton would have been saved for study.

Zihlman recovered Bwana's body over Labor Day weekend in 1994, and the information she is gleaning will enable her to do the male-female and primate comparisons that make up the core of her research.

"People have different kinds of dreams, and this is one of mine," Zihlman said in the first hectic days after Bwana's arrival. Paleoanthropology is a field riddled with debate and dissent. Each major discovery of early human fossils triggers discussion and controversy. Researchers hotly debated the question of the closest ape relative of humans until the 1960s, when molecular evidence revealed that blood proteins in humans, chimpanzees, and gorillas varied by less than 2 percent. Subsequent research on DNA in the 1980s pointed directly to chimpan-zees. And yet, gorillas offer a valuable counterpoint because humans, chimps, and gorillas are like branches of the same tree. Zihlman is studying the branches in search of those characteristics that are shared by the three animals--and those that set them apart.

"We know humans evolved from apes, but the question is how that transition occurred," says Zihlman. She believes the key to understanding how our ape ancestors functioned lies in the muscles, flesh, and movement of modern animals. Ultimately, by studying the distribution of such tissue and applying it to the interpretation of the fossil record, she hopes to reconstruct the course of ape and human evolution during the last 5 to 10 million years.

Zihlman, who joined the UCSC faculty in 1967, began her career studying locomotion and became an expert in primate anatomy as she analyzed the underpinnings of how animals get around. The changes that occurred when our ancestors began walking upright account for the biggest anatomical differences between ancient apes and early humans.

"Bipedalism and the structures that go with it are the defining features of early humans," says Zihlman. "It's what separated us from the apes. If you really understand functional anatomy and living apes, you can make some pretty good guesses about the early hominids."

It was Zihlman's groundbreaking work on chimp anatomy, combined with her study of fossils, that prompted her and three colleagues to propose in 1978 that pygmy chimps are the best living model of a common ancestor of African apes and humans. The hypothesis, based in part on the pygmy chimp's heavy lower limbs and ready bipedalism, drew a lot of criticism from her peers. But Zihlman takes satisfaction today in the fact that many of these critics have since come to respect this point of view.

In 1983, Zihlman dissected her first gorilla, a female named Missus who died of natural causes at the San Francisco Zoo. In 1986, she received a call from the Fort Worth Zoo, where Sue, an obese arthritic female gorilla, had died of old age. With two gorillas, she was able to make some comparisons, but it was not until she got Bwana that the stage was set for Zihlman's comparative studies of male and female gorillas, and of gorillas, chimps, and humans.

When Bwana died, Zihlman put out an urgent call to associates far and wide. Timing was critical because Bwana's body was not preserved, and Zihlman hoped to avoid the use of chemicals that alter the color and texture of flesh. Anthropologist Robin McFarland, who has collaborated with Zihlman on primate anatomy and locomotion for several years and who helped dissect Sue, came in a flash. Lynda Couch, a trained anthropologist who worked on the dissection of Missus and is now a money manager in San Francisco, traded her business suits for a lab coat for several days. And John Gurche, an artist/anthropologist who specializes in the reconstruction of early humans and works frequently for National Geographic magazine, flew in from the Denver Museum just to dissect the head.

Zihlman recruited a team of talented undergraduates (see related story), and the work began. The atmosphere in the lab was charged as everyone pitched in, helping each other trace muscles, tendons, and ligaments, and referring frequently to the 1950 text for guidance. "We had six people at one point," says Zihlman. "Lynda, Robin, and I were dissecting. We each had an assistant, and we were working as hard as we could. The students were great. When something like this happens, everyone just pitches in."

Following the initial segmentation, which took about three days, the dissection of Bwana stretched out over more than nine months as Zihlman and her team carried out the tedious work of examining, classifying, and cross-checking each type of tissue.

Zihlman's analysis of primate anatomy involves a labor- intensive method of dissection that includes all four types of tissue--muscle, bone, fat, and skin. On one side of the animal, the functional segments are removed muscle by muscle. On the other side, the segments are removed--head, upper and lower arm, hand, thigh, leg, and foot--and then dissected. Each tissue type is identified, separated, weighed, and some are measured.

This method of dissection is rarely used, in part because it requires whole animals, specialized facilities, and enormous amounts of time. But it's the perfect tool for Zihlman, who is pursuing very specific questions about anatomy that few other researchers are asking.

Even at this early stage of analyzing her data, Zihlman says hints about locomotion are emerging. For instance, the different functions of hind limbs are evident in Zihlman's comparisons of the tissue weights of the upper and lower limbs of gorillas, pygmy chimps, and humans. The large-bodied quadrupedal gorilla has almost equal amounts of weight in its arms and legs; the smaller pygmy chimp, which is more bipedally oriented than the other African apes, has considerably more weight in its lower limbs than its arms; and the bipedal human has a dramatically higher proportion of weight in its legs than the chimp or gorilla.

The gorilla's more laid-back way of life is evident in the percentage of body weight made up of muscle: only 38 percent in the two healthy gorillas compared to nearly 45 percent in pygmy chimps. On the other hand, Sue, at the age of 38, had less than half as much muscle as Bwana or Missus, in part because her arthritic joints reduced her activity level and also because muscle mass typically declines with age.

Sue's arthritis and obesity have made some comparisons tricky. Unlike the other gorillas, for example, she had slightly more muscle in her upper limbs than lower limbs because her body adapted to the arthritis by relying more on her arms for locomotion. For practical purposes, Zihlman's male-female gorilla comparisons are based on Bwana and Missus.

Zihlman and McFarland have presented some preliminary findings at professional meetings, and they expect that analyzing the data will keep them busy through the rest of this year. Zihlman anticipates that some critics will question the work because it is based on only three animals. She readily acknowledges that she will probably never get a comfortable statistical sample of gorillas, which are endangered. "Ideally, you want a large sample, but when you're working with rare animals that are also long-lived, that's just not possible, and some information is better than no information," she says.

Over the years, Zihlman has challenged some of the most established theories in physical anthropology. Her work on the role of women in evolution, begun in the 1970s, prompted her to revisit the long-held assertion that male hunting activities gave rise to bipedalism, and hence were the basis of the transition from ape to human. Instead, Zihlman and her late UCSC colleague Nancy Tanner, a cultural anthropologist, proposed a scenario in which female activities, such as making and using tools and gathering food, spurred the transition. It is a hypothesis that integrates fossil evidence, molecular discoveries, and chimp behavior. Yet Zihlman and Tanner were accused by some critics at the time of shaping their science to fit a feminist agenda--a charge that galls Zihlman to this day.

Zihlman thrust herself into the center of another controversy when she questioned some of the basic assumptions about "Lucy," a small 3-million-year-old hominid fossil skeleton discovered by Donald Johanson in 1974. Johanson, who is president of the Institute of Human Origins in Berkeley, and his colleague Tim White of UC Berkeley, have declared the remains those of a female; moreover, they have combined Lucy's remains with similar bones from other hominids to form a species they claim is the ancestor of all other hominids--Australopithecus afarensis. Finally, they maintain that there is a big size difference between males and females of the species.

Zihlman has doubts about these assertions, based on her research on gorillas and chimps. First, she says, it is impossible to determine sex without other pelvises to compare to Lucy's, and she believes Johanson and White have erred by using the modern human pelvis as the basis for their work on Lucy's pelvis. On the evolutionary time line, Lucy's 3-million-year-old remains are closer to apes than modern humans; as such, apes would be a better yardstick for comparison, contends Zihlman.

Neither is she convinced that Johanson hasn't patched together two or more different species in creating Australopithecus afarensis. Some of the remains he has combined vary more in size than do the limbs of pygmy chimps and gorillas, notes a wary Zihlman, who has seen more similarity between species than there is within Australopithecus afarensis.

What intrigues Zihlman about Lucy is that the remains are strikingly similar to the pygmy chimpanzee. In 1982, Zihlman published an illustration of Lucy's left side juxtaposed with the right side of a pygmy chimp. That image clearly showed the parallels between the early human fossil remains and pygmy chimps. "It was so dramatic, you couldn't deny that our early ancestors were chimplike," says Zihlman.

Unlike some physical anthropologists, Zihlman is not content to study only the fossil record for clues to human evolution. "You can read into those bones anything you want," she says, adding that the vast majority of fossils are teeth or bone fragments from the jaw. There's only about a 10 percent chance of finding a bone from a limb, or anything below the neck, says Zihlman. Moreover, fossilized bones are usually incomplete, and rarely is more than one bone from an individual recovered. "Without a sample of the same bones from a number of individuals, there's no way to establish variation within a population, which makes it difficult to reconstruct a range of body weights, limb lengths, and proportions," says Zihlman. "That's the kind of information you need to interpret how the animals were moving about, using tools, gathering food, raising the young, and doing all the activities that were part of life on the savanna."

As a scientist leaving the safety of charted waters to explore new territory, Zihlman is in good company. Her commitment to research is unwavering, even in the face of criticism and rebuke. Studying Bwana has provided an enormous influx of information. Because he is the first male gorilla that Zihlman has dissected, he represents a major step forward in her work. And one of the most striking aspects of Zihlman's career is that no matter how focused her research gets, she consistently applies what she learns to the "big picture" questions of human evolution and human existence.