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Family feud: enter the 'black skull;' an ancient African skull has stepped into a dispute over the purported 'common ancestor' of hominids, including humans.

Family Feud: Enter The 'Black Skull'

A fossil skull known only by its museumnumber, KNM-WT 17000, made quite a splash last summer. The 2.5 million-year-old hominid, or human-like creature, was discovered in Kenya by Alan Walker of the Johns Hopkins University in Baltimore, Richard Leakey of the National Museaums of Kenya in Nairobi and their colleagues (SN: 8/16/86, p.100). There were great expectations that this ancient fellow would throw a monkey wrench into previous notions of the course of early hominid evolution. The mix of primitive and advanced features on the skull has indeed suprised some paleontologists who, as a result, have rearranged branches of their proposed hominid "family trees."

But ironically, the new fossil -- found inmanganese-rich sediment that darkened it and led to its being dubbed the "black skull" -- has also been used to buttress, rather than overturn, two opposing explanations of the transition from apes to humans.

On one side of this long-running debate(SN:7/23/83, p.8) stand the discoverers of the famous partial skeleton known as Lucy, which was found with a number of other hominid fossils in Hadar, Ethiopia, about 10 years ago. Shortly after that excavation, Lucy's locators--Donald Johanson and William Kimbell of the Institute of Human Origins in Berkeley, Calif., and Tim White of the University of California at Berkeley -- assigned all the remains to one species, Australopithecus afarensis. They maintained that smaller Hadar individuals such as Lucy were females and larger ones were males.

Johanson and White say that the blackskull helps to confirm their position that A. afarensis, which dates to between 3.7 million and 3 million years ago, is the common ancestor for later hominids, including the human line. They acknowledge that WT 17000 forces them to reconsider the placement of some later species, but view the skull as an evolutionary link between A. afarensis and the more specialized A. bosisei, a large-boned, "robust" australopithecine dated at 1.2 million to 2.2 million years old.

Other researchers, however, say thatWT 17000 supports their previous contentions that the Hadar fossils represent two different species: a slender, "gracile" type exemplified by Lucy, which was an early member of the Homo line, and a robust type. The appearance of a 2.5-million-year-old robust australopithecine with some features of A. afarensis increases the likelihood that Lucy coexisted with a robust species, say anatomist Todd R. Olson of the City University of New York Medical School and Dean Falk of Purdue University in West Lafayette, Ind. Therefore, they argue that the common ancestor of hominids appeared sometime before 3.7 million years ago.

Johanson and White say that WT 17000provides no basis for this contention. "When you look at its mosaic of characteristics," says White, "the new cranium is intermediate between A. afarensis and the robust australopithecines. This thing is distinct from A. boisei, and the only other species to which it might belong is A. aethiopicus."

The latter species is controversial andbased on a single lower law found in Ethiopia 20 years ago. Many scientists have held that one jaw does not a species make. Although WT 17000 is missing the corresponding jaw area, White and Johanson say it would need one like the original A. aethiopicus specimen, only larger.

In their initial report, Walker andLeakey suggested the new skull might be example of A. aethipicus, but said it was more likely an early A. boisei specimen. They argued, therefore, that A. boisei was a separate line evolving in parallel with an africanus-robustus line. All robust forms, whether they evolved on one or more lines, eventually becme extinct.

White, however, sees a clear link to A.aethiopicus. "The size difference between the original [aethiopicus] jaw and that required for the new skull is analogous to the size differences between A. afarensis specimens," he says. Just as Lucy was a female and her considerably larger contemporaries were males, contends White, the A. aethiopicus jaw is from a female and the black skull represents a male of the same species.

Johanson and White previously heldthat A. africanus, found only in southern Africa and estimated to have appeared between 2.5 million and 3 million years ago, linked A. afarensis to the progressively larger robust forms, robustus and boisei. The new skull suggests that A. aethiopicus may have to led to boisei, say the researchers, while africanus may have led either to robustus or to the human line.

Nevertheless, adds White, "there was alot of hype about the black skull when it was first announced and it has been used as an inappropriate vehicle to attack afarensis."

Olson and Falk are at the forefrontof the afarensis attack. Evidence from a number of features on Hadar specimens, including the base of the skull, teeth, and cranial sinuses used for blood drainage, has led them to conclude independently that this was not a unified species. At this point, though, it becomes difficult to follow the hominid players without a scorecard.

Lucy and her ilk, says Olson, were theearliest members of the human line. He assigns them to another species of aethiopicus based on a distinction in the Hadar fossils made several years ago by Phillip V. Tobias of the University of the Witwatersrand in Johannesburg, South Africa, Tobias, however, classified Lucy-sized creatures as a subspecies of A. africanus (A. africanus aethiopicus). Olson says they were a full-fledged species, Homo aethiopicus, linked to later members of the human lineage by A. africanus, which in his scheme were actually Homo africanus.

Lucy's larger colors at Hadar were, inOlson's view, the most primitive members of the robust australopithecine lineage, which he prefers to call Paranthropus. He labels the initial robust Hadar species Paranthropus africanus (not to be confused with Homo africanus), based on terminology used in 1950 by a German anthropologist who, according to Olson, studied East African fossils of the same species. He concludes that the black skull links P. africanus to later boisei and robust forms.

"Prior to the new find, the oldest robustskull was around 2 million years old," says Olson. WT 17000 "puts into further question the argument that the Hadar material represents one species." He estimates that the early homo and Paranthropus lines split from a common ancestor around 4 million years ago.

Johanson and White's assignment ofthe black skull to A. aethiopicus, adds Olson, is unjustified, at least until more hominid material from the same time period is discovered. "The original aethiopicus jaw has no crowns on its teeth, only roots," he says, "and there is no basis for a [structural] comparison to WT 17000. The two specimens are from the same time period, but that's not enough to show that they're the same species."

Johanson and White's colleague WilliamKimbel agress that it may be too early to label the new skull as A. aethiopicus. Only the discovery of more fossils from the period between 2 million and 3 million years ago will help to resolve the issue, he says.

On the other hand, Kimbel says he is"mystified" by efforts to use WT 17000 in the debate over whether the Hadar remains represent one species or two species. "You can't make conclusions about 3- to 3-1/2 million-year-old fossils from a 2-1/2 million-year-old skull," he contends. "It's irrelevant."

Frederick Grine of the State Universityof New York at Stony Brook also sees no reason to use WT 17000 in the "afarensis attack." But, he adds, "I've told Johanson and White that calling it A. aethiopicus is a silly interpretation. Based on its differences from other known specimens, if this isn't a new species, we're in serious trouble as far as identifying any new species."

Grine supports a family tree drawn upby Eric Dolson of the City University of New York. In that scheme, A. afarensis is accepted as a single species that split into two lines, one becoming A. africanus and evolving into modern humans, the other becoming the species represented by the black skull, which then split to form the now-extinct A. robustus and A. boisei.

Grine hopes some kind of consensuscan be hashed out at a major workshop on robust australopithecines that will be held, under his direction, at Stony Brook toward the end of March.

At about the same time, an announcementwill be made concerning a partial hominid skeleton discovered by Johanson, White and their colleagues at Olduvai Gorge in Tanzania last summer. The fossils were unearthed in sediments that have been dated at 1.8 million years old. "The partial skeleton is very fragmentary and at the moment somewhat enigmatic," says Johanson.

So, it might be added, is the significantof the black skull for the hominid family tree.
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Author:Bower, Bruce
Publication:Science News
Date:Jan 24, 1987
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