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

Science News, Jan 24, 1987 by Bruce Bower

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.