Two new Homo fossils are described in this week’s Nature, and here they are.
This is KNM-ER 42700. It’s a very well preserved brain case, it has been dated to 1.55 million years ago, and it has been positively identified as belonging to Homo erectus. It’s a little unusual in being particularly small, but otherwise, definitely H. erectus.
This is KNM-ER 42703. It’s a broken maxilla, or upper jaw, and it has been dated to 1.44 million years ago — it’s over 100,000 years more recent than the KNM-ER 42700. This specimen has been identified as Homo habilis.
This is very cool and there are some interesting things to learn about human evolution from them. Unfortunately, one fact seems to be dominating the news about them, and is being consistently misinterpreted: the H. erectus specimen is older than the H. habilis specimen, yet the most common models of human evolution have H. habilis giving rise to H. erectus which in turn was the progenitor of H. sapiens. Even the Nature news summary makes a big issue of this difference.
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.
These are all wrong (I think; I don’t hang out with anthropologists much—they don’t really see evolution in that simplistic and linear fashion, I hope?). These discoveries do not put any seriously held theories in doubt. They do nicely demonstrate that a linear progression is not to be seriously held.
Just as your mother’s life most likely substantially overlapped with your own, the persistence of a parental species so that it overlaps in time with its daughter species is not a challenge to evolution at all. That’s the case here; the authors certainly do not regard this work as casting any doubt on the evolution of humans at all. Here’s their conclusion:
Although some characters previously regarded as diagnostic of
H. erectus differ from H. habilis simply on the basis of overall cranial
size, the two taxa are nonetheless
metrically and nonmetrically distinguishable throughout their
lengthy co-occurrence through time. Moreover, during this period
of nearly half a million years the dento-gnathic morphology of
H. habilis shows relatively little change. The long period of sympatry
suggests the existence of some form of niche differentiation between
H. erectus and H. habilis, one that may have included foraging or
dietary differences. Taken together, these new fossil data highlight
that an anagenetic relationship between the two taxa is implausible.
As the earliest secure evidence of Homo is found outside the known
region of overlap, it is nonetheless possible that H. erectus evolved
from H. habilis elsewhere, and that the Turkana basin was a region of
secondary contact between the two hominin taxa.
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.
Spoor F, Leakey MG, Gathogo PN, Brown FH, Anton SC, McDougall I, Kiarie C, Manthi FK, Leakey LN (2007) Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya. Nature 448:688-691.