White also predicted that “the Laetoli prints will eventually be shown to be subtly distinct from those left under analogous conditions by anatomically modern humans” (White and Suwa 1987, pp. 510, 512). But as far as anyone can see now, they are indistinguishable from those of modern humans. Even White himself once said: “Make no mistake about it. They are like modern human footprints. If one were left in the sand of a California beach today, and a four-year-old were asked what it was, he would instantly say that somebody had walked there. He wouldn’t be able to tell it from a hundred other prints on the beach, nor would you. The external morphology is the same. There is a wellshaped modern heel with a strong arch and a good ball of the foot in front of it. The big toe is in a straight line. It doesn’t stick out to the side like an ape toe” (Johanson and Edey 1981, p. 250).
And Tuttle (1985, p. 130) noted: “in all discernible morphological features, the feet of the individuals that made the G trails are indistinguishable from those of modern humans.”
11.11 Black Skull, Black Thoughts
In 1985, Alan Walker of Johns Hopkins University discovered west of Lake Turkana a fossil hominid skull stained dark by minerals. Called the Black Skull, it raised questions about Donald Johanson’s view of hominid evolution.
According to Johanson, Australopithecus afarensis gave rise to two lines of hominids. This arrangement can be visualized as a tree with two branches. The trunk is Australopithecus afarensis. On one branch is the Homo line, proceeding from Homo habilis to Homo erectus to Homo sapiens. On the second branch are the australopithecines arising from Australopithecus afarensis.
Johanson and White claimed that Australopithecus afarensis gave rise to Australopithecus africanus, which in turn gave rise to Australopithecus robustus. The trend was toward larger teeth and jaws, and a larger skull with a ridge of bone, the saggital crest, running lengthwise along the top. The saggital crest served as a point of attachment for the powerful jaw muscles of robust australopithecines. Australopithecus robustus then supposedly gave rise to the superrobust Australopithecus boisei, which manifested all the above-mentioned features in an extreme form.
In an article titled “Baffling Limb on the Family Tree,” Walker’s wife Pat Shipman, also of Johns Hopkins University, explained the evolutionary significance of the Black Skull, designated KNM-WT 17000.
The first specimens of Australopithecus robustus were, it was thought, about 2 million years old (Johanson and Edey 1981, p. 283). But the Black Skull, with its Australopithecus boisei features, including the largest cranial crest of any hominid (Shipman 1986, p. 91), was 2.5 million years old. Shipman believed this meant that Australopithecus boisei and the boisei-like Black Skull could not be descended from Australopithecus robustus, as believed by Johanson and others.
So where does that leave us? Here is one possibility suggested by Shipman. On our hominid family tree, we could now go from Australopithecus afarensis up one branch to Australopithecus africanus. Then from Australopithecus africanus could come two separate branches. On one branch is Australopithecus robustus and on the other Australopithecus boisei and the boisei-like Black Skull. In other words, instead of deriving Australopithecus boisei from Australopithecus robustus, both originate from Australopithecus africanus.
But perhaps not. “All known africanus skulls share many features that are derived, i.e, advanced, relative to those of the new skull, such as a moderate flexion or angling of the base of the cranium and a deep jaw joint with a bony lump in front of it,” said Shipman (1986, p. 91).
So, according to Shipman, another possibility now emerges—that Australopithecus africanus, although ancestral to Australopithecus robustus, might not have been ancestral to Australopithecus boisei and the boisei-like Black Skull.
This leaves us with a three-branched family tree. Down at the bottom we still have Australopithecus afarensis. Above are three branches—the Homo line on the first, Australopithecus boisei and the Black Skull on the second, and then Australopithecus africanus on the third, leading to Australopithecus robustus.
But Shipman pointed out that it then becomes difficult to account for the fact that Australopithecus boisei and Australopithecus robustus are so similar. If Australopithecus robustus came from Australopithecus africanus and Australopithecus boisei from Australopithecus afarensis, then Australopithecus boisei and Australopithecus robustus would have had to develop their robust similarities independently by parallel evolution, something that is possible but unlikely.
According to Shipman, another way to explain the similarities between Australopithecus boisei and Australopithecus robustus is to propose that Australopithecus robustus was not descended from Australopithecus africanus and that Australopithecus robustus and Australopithecus boisei had a common ancestor besides Australopithecus africanus—perhaps Australopithecus afarensis.
So now we have a four-branched tree, with Australopithecus afarensis at the bottom. Above are the Homo line, Australopithecus africanus, Australopithecus robustus, and Australopithecus boisei, all separate from each other.
Shipman found it very hard to believe that a single species, Australopithecus afarensis, could have given rise to four separate lineages. So where did the four new species come from?
Shipman suggested that one should take a very hard look at the idea that Australopithecus afarensis represents just one sexually dimorphic species. She pointed out, as we have discussed in Section 11.9.8, that some scientists have concluded that “at least two species of Australopithecus and possibly Homo are mistakenly lumped together into afarensis” (Shipman 1986, p. 90).
Walker said it is likely that “the specimens identified as Australopithecus afarensis include two species, one of which directly gives rise to Australopithecus boisei” (Walker et al. 1986, p. 522).
How did Johanson respond to the discovery of the boisei-like Black Skull? He admitted that the Black Skull complicated things, making it impossible to arrange Australopithecus africanus, Australopithecus robustus, and Australopithecus boisei in a single line of succession coming from Australopithecus afarensis. Johanson proposed 4 possible arrangements of these species, along the lines we have been discussing, without suggesting which one was correct (Johanson and Shreeve 1989, p. 126). There was, he said, not yet enough evidence to decide among them.