what do you guys make of this is this the stepping stone from ape to man (well besides old jimbo that is )
Small fossil skull has big implications for modern humans
By John Timmer | Published: August 08, 2007 - 12:02PM CT
Credit: Nature/National Museums of Kenya
F. Spoor and J. Reader.
Based on a quick reading of a text book or a short trip through a natural history museum, it's easy to think of the origin of modern humans as a linear trip through a series of progressively more human hominins. Biology is rarely that simple, so it's not really been much of a surprise that, over the past few decades, our picture of hominin evolution has changed as new fossils reveal a complicated tangle of overlapping species. Instead of a linear change where an entire population gradually transformed into a distinct species, hominins seem to have followed the pattern found elsewhere in biology, with new species arising as a subpopulation and coexisting with their relatives for long periods of time.
Today's issue of Nature will contain the latest example of some fossils that causes this sort of revision to our thinking, courtesy of a team that includes members of Kenya's Leakey family. First up is the fantastic skull shown at right. It's a nearly mature Homo erectus, and would be a fascinating find in its own right. It's shown on top of a more typical skull of the same species, which reveals that this individual was remarkably small for the species. This sort of variation is typical of gorillas, but does not occur in either humans or chimps. The authors suggest that it may represent a difference in size between the sexes.
The other unusual thing about the fossil is that it has features that are normally associated with Homo erectus fossils found in Asia. The study's authors argue that many of the characteristics that have been considered "Asian" are simply consequences of a difference in cranial size, rather than distinct features. Thus, they seem to be suggesting that all Homo erectus fossils belong to a single population, and that most variation within that population is simply due to differences in a single characteristic.
On its own, this fossil would be pretty notable, but it was accompanied by a jaw from a member of the species Homo habilis. Prior to these finds, habilis was thought to have gradually evolved into erectus on the population level, the sort of linear relationship termed "anagenetic" by scientists. Both of these fossils come with strong dating information, and they reveal that the two species overlapped in this region of Africa for at least a half-million years. This makes the linear relationship rather unlikely, and it suggests that erectus arose as an offshoot of the then-dominant habilis population.
Overall, it's hard to consider this revision a radical one. In most areas of biology, we've found that species originate by branching and diversification far more often than via an anagenetic process. The only reasons I can see for expecting entire populations to transform in a way that produces a linear progression of hominins lie in outdated notions of there being some sort of "progress" towards Homo sapiens.
Incidentally, for those of you wondering about the "hominin" designation, it's the modern term that arose once we realized that the orangutans and gorillas are outliers in the formerly used "hominid" group. Hominins exclude that branch, but include everything else along the route to both humans and chimps.
Nature, 2007. DOI: 10.1038/nature05986
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