Living Alongside Ancestors?

Out of Africa and University College London comes National Geographic fieldwork describe in a NYT article (BBC).

"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."

The NYT article quotes Dr. Leakey as saying that the "co-existence makes it unlikely that Homo erectus (1.55 Mya) evolved from Homo habilis (1.44 Mya)." Certainly, an earlier erectus specimen could not have evolved from a later habilis specimen, but I am not convinced that a species could not co-exist with an ancestral species.

The article states that, "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."

This seems eminently possible, though difficult or impossible to demonstrate. The conclusion of divergence from a common ancestor seems excessive when based on only two specimens.

What's wrong with the exclusory picture? Based on DNA evidence, Homo sapiens (us) and chimpanzees, who still live contemporaneously, arose from a common ancestor somewhere around 7 to 5 million years ago. There is simply no way to know how long individuals of the divergent lineage might have lived alongside the ancestral lineage, nor of how different the lineages might have appeared half a million years after diverging genetically.

The ancestors of chimpanzees and bonobos are estimated to have split again between 0.89 and 0.86 million years ago, and the two common chimpanzee subspecies are estimated to have diverged about 0.46 million years ago. The two common chimpanzee subspecies are quite similar morphologically after only half a million years.

The modern definition of a species is that sexual intercourse between different species does not produce fertile offspring. Clearly, Wilt Chamberlain and jockey Willie Shoemaker could not have mated (for other reasons than species difference), but would a paleontologist place them in different species on the basis of skeletal fragments some 1.5 million years hence? Although the image of two males of very different height is striking, Wilt Chamberlain would tower over women and Mike Tyson is bulkier than women. Paleontologists refer to this size difference as sexual dimorphism and skulls of different sizes can be expected in males and females of the same species.

The DNA data suggests that an evolutionary split does not necessarily immediately eradicate the pre-split ancestral line. However, the greater reproductive success of humans does indicate that one divergent species may be much better adapted to survival and that passage of sufficient time will probably eliminate (replace) the common ancestor.

Several challenges face paleontologists studying hominids. The number of specimens is small and their preservation is typically incomplete (imagine trying to reconstruct a person from only a tooth or the top of their skull). Most of the species or apparent species studied are extinct, so we have no DNA evidence to indicate their evolutionary relationships. Further, it is more interesting for the paleontologist (and the media) when the latest discovery appears to diverge from previous analyses.

Certainly, the notion that hominid ancestry resembled a bush rather than a direct linear descent is nothing new.

Other blogs: New Fossils and Our Understanding of Human Evolution, New Hominid Fossils Reported