by Donald c. Johanson
(Natural History, 5/93, pages 52-53)
The Black Skull (or KNM-WT 17000, as it is formally called) was recovered in the summer of 1985, in the desolate wastes west of Africa's Lake Turkana, where it was first identified by Alan Walker, of Johns Hopkins University. The skull-more properly a cranium because the mandible is missing-has been stained blue black by manganese-rich minerals. It has cheekbones like stout flying buttresses, a projecting face, and a swept-back braincase with a prominent longitudinal (or sagittal) crest, presenting an overall architecture reminiscent of a Boeing blueprint. The International Herald Tribune called it "the 'most exciting' discovery in the evolution of early man in years." Nevertheless, the truth is that it represents a dead end on the tree of human evolution. So why all the fuss about a creature that wasn't even our ancestor?
When I first laid eyes on a cast of the Black Skull, I was immediately impressed by its mixture of advanced and primitive traits. By advanced traits I don't mean more humanlike ones but simply specialized characteristics, indicating that the anatomy has diverged from the ancestral condition. On the one hand, the projecting face, very small cranial capacity, cresting pattern on the rear of the cranium, and a host of other features recall the more ancient species Australopithecus afarensis, which includes the well-known skeleton Lucy. On the other hand, the prominent sagittal crest, the dish-shaped (flat, concave) face, and enormous molars and premolars (inferred from the large tooth roots) are typical of later creatures known as robust australopithecines.
With their massive jaws, greatly enlarged chewing muscles, and huge crushing and grinding cheek teeth, the robust australopithecines were once successful members of the hominid family-the group that includes humans and their extinct close relatives. Their distinctive cranial architecture was forged by natural selection to process a tough, fibrous, vegetarian diet. Robert Broom first recognized this adaptation in fossils from South Africa, which he christened Paranthropus robustus, meaning a "robust near relative of man." In 1959, Mary Leakey found an East African version at Olduvai Gorge that was first named Zinjanthropus boisei by Louis Leakey. Zinj derives from an Arabic word that means "East Africa," and boisei honors Charles Boise, who financed the work at Olduvai.) Most anthropologists now lump these two species in the genus Australopithecus, but there is an increasing tendency to use Broom's genus, Paranthropus.
When Frank Brown, of the University of Utah, a recognized expert on the geology of the Lake Turkana Basin, reported an age of roughly 2.5 million years for the Black Skull, there were lots of raised eyebrows in the anthropological community. Hominid fossils in East Africa from between 2 and 3 million years ago are exceedingly rare, so the Black Skull was a welcome addition to the fossil record. At the same time, it called into question many people's pet theories concerning the timing and manner of change in hominid evolution.
In 1979, Tim White, my colleague at the University of California, Berkeley, and I had proposed a version of the human family tree in which we placed A. afarensis, at 3.5 to 3 million years ago, at the base of a simple Y pattern. We considered it to be the primitive, generalized species ancestral to both the robust australopithecines and the genus Homo (the line culminating in modem humans). There was a limited choice of fossils to guide us in the interval between late A. afarensis and the good samples of fossil hominids a million years later. Perhaps the most controversial aspect of our tree was how we treated A. africanus, a "gracile" australopithecine species discovered by Raymond Dart in South Africa and dated between A. afarensis and the robust australopithecines. Others considered this fossil species to be, if anything, on the line leading to Homo, but we saw it as ancestral exclusively to the robust australopithecines. In my opinion, A. africanus, with its heavily buttressed mandible and expanded chewing teeth, forecasts the vegetarian specialization of the robust forms.
At that time we placed all the robust types in a single species, but the East African vegetarians have a distinctive facial anatomy, as well as larger cheek teeth and stronger cresting patterns than their South African relatives. They are sometimes called hyper-robust. Yoel Rak, of Tel Aviv University, subsequently convinced us we should therefore distinguish two species-A. robustus in South Africa and A. boisei in East Africa.
This is the way things stood until the Black Skull appeared, forcing anthropologists to go back to the blackboard and draw some new family trees. The first task was to establish the relationship of the Black Skull to the other robust types. To resolve this, Walker and his colleagues looked at the degree of anatomical resemblance of forty features common to the Black Skull and other species of Australopithecus. This is standard procedure-the more traits that are shared between any two species, the more trust we have that there is a close evolutionary relationship between them. This exercise led Walker and his colleagues to conclude that "for most of the features the new specimen resembles A. boisei."
Using that list as a guide, Bill Kimbel, of the Institute of Human Origins, Tim White, and I embarked on a similar exercise incorporating thirty-two cranial features (some were also on Walker's list, but we added a few and combined those we felt were redundant). In some respects our conclusions were similar. Since twelve of the traits of the Black Skull were shared with both boisei and robustus (and not with other hominids), the robust nature of the skull was certain. But we felt it could not be tied exclusively to either species: of the remaining traits, the Black Skull shared only two exclusively with boisei. What most surprised us was the close relationship between the Black Skull and afarensis: they shared a total of twelve of the remaining traits.
The Black Skull thus seemed to us a potential evolutionary link between afarensis and the later robustus and boisei. The timing was also right-at 2.5 million years, the Black Skull was also intermediate in age. Calling it afarensis would obscure the evolutionary importance of the enigmatic cranium as much as calling it boisei. This sort of reasoning prompted a number of anthropologists to say, "If you don't give this thing a new name when you publish it, someone else will."
In their paper, Walker and his colleagues had referred to a nearly forgotten robust mandible, found in 1967 in the Omo River deposits in southern
This is one of the problems in naming different parts of a continuum. On the scale from black to white, there are innumerable shades of gray, but at the ends of the scale, black and white are very distinct. Walker and his associates saw the Black Skull and boisei as two shades of gray. But we saw the differences as black and white, and so went with the designation A. aethiopicus. Although any tie between the toothless old mandible from Ethiopia and the new cranium is at best tenuous, most anthropologists appear satisfied with this decision. To be sure, when intermediates are recognized between aethiopicus and boisei, this distinction will become less clear.
The evolutionary implications of the Black Skull are far more important than the nomenclature debates. KNM-WT 17000 is the oldest robust fossil ever found. This means it could be ancestral to boisei or robustus or possibly both. Even more important, the South African A. africanus lacks the primitive features shared by afarensis and aethiopicus and can no longer be considered an ancestor to all the robust forms.
In the process of redrawing, the human family tree, generated a number of plausible hypotheses. The discovery of the Black Skull does not alter the position of A. afarensis as the last common ancestor of all the later hominids. Some researchers have argued that another hominid ancestor lived at the same time as afarensis, but at the moment here is no convincing evidence. Others have argued that the afarensis fossils may themselves represent more than one species, but we remain confident of our original interpretation. But after afarensis, what?
I lean toward the view that A. africanus is the ancestor of A. robustus, and A. aethiopicus is the ancestor of A. boisei An interesting implication of this view is that the specialized features that characterize these robust hominids evolved independently in East and South Africa. We call this parallel evolution: descendants of a common ancestor diverge and subsequently evolve similar adaptations independently. The development of a robust, vegetarian specialization was a viable hominid adaptation. Although we are not certain exactly when our robust relatives died out, the last of their fossils date to about 1.2 million years ago in East Africa, while those in South Africa may be slightly younger than 1 million years old. In terms of our existence, it is humbling to think that they survived for more than 1.5 million years and still went extinct. If, as I believe, the robust adaptation arose twice, then both times it met with extinction.
Most monkeys and apes are plant eaters. The vegetarian
specialization of our robust relatives was a continuation of a long-standing
dietary adaptation and feeding strategy among hundreds of species of primates.
Perhaps the real lesson we can learn from the Black Skull is the reminder
that we, Homo sapiens, are clinging to a side branch of the primate family
tree.