Two fossils unearthed in Kenya have added a new dimension to our view of life at the birth of our Homo genus. They show that two ancestral human species seem to have lived cheek-by-jowl in the same area, much as gorillas and chimpanzees do today. . . .
Anthropologists have tended to see the evolution of Homo species as a linear progression, beginning with H. habilis and passing through H. erectus before ending up with modern humans. But it seems the path through time was broad enough for more than one species to walk abreast, with H. erectus and H. habilis living in the same place at the same time for as much as half a million years. Spoor and his colleagues argue that this makes it less likely that H. erectus was a direct descendant of H. habilis, instead suggesting that there is a common ancestor yet to find.
The creationists, of course, have decided this proves once and for all that . . . well, it proves something. (Mmm, no -- this is creationism -- it has to disprove something, but it doesn't do that, either.) Actually, as PZ Myers points out, it doesn't prove much of anything.
Myers (and I think any evolutionary biologist who knows what he/she is talking about will concur) points out that there is absolutely nothing to prevent parent and offspring species from overlapping for a greater or lesser amount of time. In that regard, even the Nature news summary is little over the top. There may be a common ancestor, or the current view may be correct. That's one of the joys of evolution for biology geeks like me -- we get to poke around some more and find out.
Myers' summation of the authors' conclusions is helpful:
The two species are anatomically distinct, and they don't see signs of a blending between the two.
The two species were sympatric, or living in the same territory at the same time. This suggests that they probably had different lifestyles, or conflict would have driven out one or the other.
They did not have an anagenetic relationship, that is, one species did not gradually and imperceptibly change into the other. The Homo lineage had branched at some earlier date.
That branch occurred elsewhere and earlier, and the H. habilis→H. erectus→H. sapiens line of descent is still tenable; it's just that KNM-ER 42703 would then be a member of a dead-end branch that did not leave descendents in modern times (of course, KNM-ER 42700 is probably also not a direct ancestor — it's representative of a population that may have led to us.)
Experience tells me that this concept, that individual fossils can't be arranged in a simple, linear, lineal relationship, is going to be very hard for many people to grasp, and is going to fuel quite a few creationist shouts of triumph in the near future, and the media aren't helping. It's misplaced. Evolution predicts a great many branches, with only a few twigs here and there preserved in the fossil record, exactly as we see in this discovery.
This is not a new concept at all -- nothing revolutionary. In fact, it's a staple of evolutionary thought. Repeat after me: evolution does not have a plan.
See Myers' smackdowns of Casey Luskin and Vox Day, who seems to be a real, certifiable cretin. Get this quote:
It doesn't matter what the evidence is, evolutionary biologists are happy to change their story to suit.
As Myers points out, that's the way real science works -- you draw your conclusions after you look at the evidence. New evidence may mean that you have to change your conclusions.
Duh.
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