Today’s New York Times has this interesting article about some recent hominid fossil finds. Alas, it falls into the familiar trap of reporting every mundane find as if it is a scientific revolution:
Two fossils found in Kenya have shaken the human family tree, possibly rearranging major branches thought to be in a straight ancestral line to Homo sapiens.
Scientists who dated and analyzed the specimens — a 1.44 million-year-old Homo habilis and a 1.55 million-year-old Homo erectus — said their findings challenged the conventional view that these species evolved one after the other. Instead, they apparently lived side by side in eastern Africa for almost half a million years.
If this interpretation is correct, the early evolution of the genus Homo is left even more shrouded in mystery than before. It means that both habilis and erectus must have originated from a common ancestor between two million and three million years ago, a time when fossil hunters had drawn a virtual blank.
Fascinating! But I smell a version of the “If humans evolved from apes, why do we still have apes?” argument here.
The fact that erectus and habilis coexisted for a substantial period of time hardly implies that the former could not have evolved from the latter. It is certainly possible that both evolved from a common ancestor, but these two fossils hardly force such a conclusion. There is nothing implausible in suggesting that a species managed to coexist with one of its evolutionary descendants. Furthermore, I find this statement suggestive:
Although the findings do not change the relationship of Homo erectus as a direct ancestor of Homo sapiens, scientists said, the surprisingly diminutive erectus skull implies that this species was not as humanlike as once thought.
Based on this it sounds like the earliest erectus fossils were more ape-like (and more habilis-like) than the erectus fossils that came after them. That seems perfectly in accord with the idea that erectus is a descendant of habilis.
The article goes on to play up this idea that the fossils challenge “linear” evolution:
In any case, Dr. Leakey said, “Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis.”
Dr. Spoor, speaking by satellite phone from a field site near Lake Turkana, said the evidence clearly contradicted previous ideas of human evolution “as one strong, single line from early to us.” The new findings, he added, support the revised interpretations of “a lot of bushiness and experimentation in the fossil record,” rather than a more linear succession of species.
I have already explained why I am unconvinced by Dr. Leakey’s argument. Dr. Spoor, on the other hand, seems to be exaggerating the significance of these finds. The idea of a simple linear progression of species leading from ape to man was discarded decades ago. I don’t think you will find many paleontologists who are suprised to hear that the path of human evolution was more complex than the standard popular misconceptions.
We already know that starting roughly six to seven million years ago and proceding to the present there were a very large number of hominid species existing on various African plains. The fossils we have show a clear progression from the mostly ape-like oldest fossils through increasingly more human-like fossils, culminating in modern Homo sapiens roughly 100,000 to 200,000 years ago. No doubt the fossils we have represent only a fraction of all such species that actually existed. So there is an inherent implausibility in the idea that the ones we have just happen to be the ones that are on a direct ancestral line linking ancient Australopithecines to modern humans. From fossils alone it is essentially impossible to distinguish between a direct ancestor and a closely related side-branch. That notwithstanding, I fail to see anything in this article that requires a change in our thinking on human evolution.