| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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feature A Face from the Past Lucy finally found a mate. Lucy, you'll recall, is the diminutive, 3.2-million-year-old fossil skeleton from Ethiopia that spurred a flurry of new ideas about our origins after her discovery 20 years ago. Her remains, along with other fossils from Hadar in Ethiopia and Laetoli in Tanzania, led to the naming of a new hominid species in 1978, one with the tongue-twisting moniker Australopithecus afarensis. The announcement of afarensis met with controversy in part because the new species was known mainly from teeth, jaws, and limb bones. Just as we recognize people by their individual faces, paleoanthropologists, the scientists studying human origins and evolution, distinguish extinct ancestors more easily from their anatomy above the neck, so hominid species have historically been described and debated on the basis of one or more skulls. Lucy herself has only a lower jaw and a few skull fragments, and frustratingly, a skull of her species eluded anthropologists. Until now. In March, Academy Fellows William Kimbel and Donald Johanson of the Institute of Human Origins, in Berkeley, California, and Yoel Rak of Tel Aviv University, announced in the British journal Nature the first skull from Lucy's species, which is to say the oldest known skull from any hominid, any member of the human evolutionary family. The skull joins 52 other hominid fossils collected at Hadar since 1990 that the team reported. "We had hoped ever since Lucy's discovery in 1974 to find a skull of Australopithecus afarensis," says Johanson, who secured his place among the annals of fossil hunters by finding the famous Lucy. Now add to that list Rak, who found the new skull in 1992 on the steep slope of a gully about a mile from where Lucy's bones had lain. Painstakingly pieced together from some two hundred fragments, the new skull is three-quarters complete. Much of the top and back of the cranium have been preserved as well as large portions of the cheek bones, both sides of the upper jaw, and part of the lower jaw. The skull has an essentially ape-like face, with large front teeth in projecting jaws but without the depression above the eye sockets found in all apes. The cranium is the largest known from any Australopithecus species, based on a single measurement across the back of the braincase, but a size estimate of the brain once held within the skull awaits further analysis. Wear on the teeth indicates that this individual lived a long life in early hominid terms, perhaps reaching an age of 30 years. The presence of prominent bony crests that once anchored powerful jaw muscles suggests to its discoverers that this skull belonged to a male afarensis. (Unlike Lucy, named for the Beatles song "Lucy in the Sky with Diamonds," the skull bears only an identifying number, A.L. 444-2, but the press has called the skull "Son of Lucy" and "Lucy's Grandson." During Johanson's appearance on "Larry King Live," his host suggested Larry as an appropriate name.) Regardless of the popular names that fall to individual fossils, the naming of a new hominid species draws criticism and controversy. It's been that way from the beginning, since the Neandertals first came to light in Europe and were described in 1864. When Raymond Dart named Australopithecus africanus in 1925 from a child skull, it was dismissed as a young chimpanzee or dwarf gorilla. It was eventually accepted--so much so that when Louis Leakey and colleagues proposed the species Homo habilis in 1964, some critics considered it too similar to Australopithecus to warrant the distinction. The same reticence to split the family tree greeted Johanson and colleagues Tim White and Yves Coppens when they proposed Australopithecus afarensis. Several factors made the afarensis announcement particularly contentious. One complaint was that samples of different ages from Hadar and Laetoli, separated by more than one thousand miles, were lumped together, an act that one critic, writing in Nature in 1982, called "a folly." Others insisted that there was too much variation in the sample, that the large and small individuals couldn't possibly come from one species. These criticisms continued ten years ago after Kimbel, White, and Johanson published their composite reconstruction of an afarensis skull made from bits and pieces of twelve Hadar hominid individuals. The scientists brought their knowledge of anatomy to filling in the gaps. Some anthropologists responded that the reconstruction grafted fossils from two distinct species, and the question of how many different hominids lived at Hadar remained in dispute. Yet just as each of the previously contested hominid species became eventually accepted by scientists, a consensus has emerged that afarensis is also a biologically valid species, regardless of whether it sits at the roots of the family tree or hangs from one of its branches. The new group of fossils affirm that view, particularly the skull, which bears a striking resemblance to the 1984 reconstruction and should end discussions that Kimbel and colleagues inadvertently combined two species. "We think we have really strengthened the idea that there was only a single [hominid] species at the Hadar site," says Johanson. However, there remain a few holdouts. Ironically, one of the nay-sayers is Yves Coppens, the prominent French paleoanthropologist who co-authored the 1978 paper that brought afarensis into the scientific literature. The sticking point for most critics has been how to interpret the wide range in size of the Hadar fossils. Skeletal variation could be due to an animal's age, sex, or species, or to just a certain amount of natural variation in a population. Unfortunately, it's not easy to distinguish between, say, sex and species using fossil bones alone. One scientist's evidence for gender differences becomes another's evidence for species diversity. As for afarensis, no one is complaining about a lack of evidence to study. The Hadar hominid collection alone now numbers more than three hundred specimens, with the latest crop increasing the total sample of lower jaws, or mandibles, by half and the number of measurable teeth by more than 60 percent. Throughout the 400,000-year history captured in Hadar's sediments, fossils from large individuals, like the new skull, and small individuals, like Lucy, are present. In life, Lucy may have weighed about 60 pounds, while her larger cohorts would have topped one hundred pounds. Kimbel, Johanson, and Rak contend that this size variation can be attributed to differences in sex rather than species. Differences between males and females in body size and shape, known as sexual dimorphism, are not unusual in living apes. Gorillas and orangutans are the most dimorphic apes, with males attaining twice the body weight of females. Chimpanzees show only moderate dimorphism, as do humans, in which females reach 89 percent of average male body weight. What about afarensis? Academy Fellow Henry McHenry, a paleoanthropologist at the University of California at Davis, agrees that the new fossils solidify the single species argument, but his studies of the afarensis limb bones have revealed a striking pattern of sexual dimorphism. When McHenry calculated the degree of dimorphism from hind limb joint size, afarensis exceeded modern chimpanzees but not gorillas. The afarensis forelimbs, however, show an extraordinary amount of dimorphism. "It's very intriguing," McHenry enthuses. "I think that's a lot of fun." McHenry speculates that this differing degree of dimorphism could be due to the hominid having its arms and hands free from propelling the animal about. Whatever afarensis did with its forelimbs, they were exceptionally powerful, as muscle attachments on a new partial humerus from Hadar attest. Perhaps, says McHenry, the forelimbs were under some sort of sexual selection, because afarensis males contested each other with their hands rather than with canine teeth as do chimpanzees. In many ways, afarensis is a mosaic of chimp-like and human-like anatomy, though it displays more marked dimorphism than either humans or chimps. Kimbel sees no reason to expect that the level of dimorphism in afarensis should parallel that in chimps or humans, since each australopithecine species had a distinct life history that influenced its anatomy. It's still tough, though, to sort large females from small males in the fossil sample. "The trick," says Kimbel, "is to know, or to try to figure out, what the average male and female body size is, and not just take the largest and the smallest." Until recently, the extent of variation could only be measured from teeth and limb bones. Now scientists can begin to study variation above the neck in afarensis. In addition to the skull, another new fossil is a partial face some 3.25 million years old. It is smaller and less projecting, or prognathic, than the male skull's face, has smaller upper canine teeth, and may come from a female. "It's exactly what you'd predict from the great apes," says Johanson. Further evidence supporting afarensis as a single, albeit dimorphic, species appeared last November in Nature, where Academy Fellow Tim White of the University of California at Berkeley and several colleagues reported new fossils from Maka, Ethiopia. In 1982, this site, located 37 miles south of Hadar, yielded the upper end of a hominid femur, or thigh bone, that remains the oldest known evidence (3.8 million years) for upright walking or bipedalism. The newly announced Maka fossils collected in 1990 include four complete or partial mandibles and parts of two arm bones from afarensis. All the fossils are about 3.4 million years old and vary in tooth and body size as much as the Hadar and Laetoli hominids. The most complete afarensis mandible from Maka, assembled from 109 pieces, is much larger than Lucy's lower jaw but matches in size and shape a specimen from Laetoli originally found in the 1970s that became the type specimen for the species. Like the new Maka fossils, the oldest Hadar hominids date to 3.4 million years ago. Another Academy Fellow, geologist Robert Walter, recently used a sophisticated laser dating technique to home in on Lucy's age and determined that she is 3.2 million years old, plus or minus 10,000 years. The youngest Hadar fossils, including the new skull, are about 3.0 million years old, so there's solid evidence for the species surviving for 400,000 years in this one area. At another site in Ethiopia called Belohdelie, a small skull fragment that had been loosely attributed to afarensis now has a more certain identity based on comparison with the same region on the new male skull. This forehead fragment dates to 3.9 million years ago. All these numbers add up to suggesting that afarensis existed from three to four million years ago, perhaps as the only hominid species around. And the fact that the Belohdelie skull fragment matches a specimen fully 900,000 years younger indicates that the hominid skull underwent little anatomical change during that time. Says Kimbel, "This is a hallmark of a stable or static pattern of evolution in a species." What's more, the species survived through dramatic local environmental changes that turned Hadar from an evergreen forest to an arid grassland and back during the 400,000-year tenure of afarensis. The rock record reveals that other mammal species changed or disappeared, but afarensis occurs from one end of the sequence to the other. So, if afarensis was the only hominid that lived between three and four million years ago, and one that managed to be widespread and adaptable as well as enduring, what lessons could that hold for the pace and pattern of human evolution? Ironically, the cover story in the Nature issue with the new Maka fossils proclaimed "Punctuated equilibrium comes of age." Does our own family tree provide evidence for that idea, put forth in 1972 by paleontologists Stephen Jay Gould and Niles Eldredge? They argued that rather than a pattern of steady, gradual change out of which new species evolve, the fossil record displays long periods of no change marked by episodes of rapid, species-spawning change. Species tend to remain conservative (the equilibrium), until some environmental pressure drives them to change (the punctuation) or go extinct. An important theme in punctuated equilibrium is the notion that "stasis is data" and not an absence of evidence for evolution. The growing sample of afarensis fossils speaks strongly for stasis in this earliest known hominid, as the authors of both of the recent Nature papers pointed out in their respective articles. Evidence for a later punctuation has also accumulated. Sometime after three million years ago, several new hominid species evolved and diversified (see Pacific Discovery, Summer 1993), including the first members of our own genus, Homo. Around three million years ago, the eastern African environment began to grow dryer, and perhaps this climatic change influenced the spurt of new hominids. Johanson, Kimbel, and colleagues plan to focus on sites at Hadar younger than three million years. Not far from where Lucy and the new skull were found, there is already good evidence for 2.5-million-year-old tools—possibly the earliest examples of hominid stone technology. Though no hominid fossils have yet emerged from these younger sediments, a skull of the toolmaker may lie buried beneath a layer of volcanic ash, ready to be exposed by rain.
Blake Edgar is Assistant Editor of California Wild. |
Summer 1994
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