I wish I could quit you, chimpanzee
The analysis, by David Reich, Nick Patterson and colleagues at the Broad Institute in Cambridge, Mass., sets up a serious conflict between the date of the split as indicated by fossil skulls, about 7 million years ago, and the much younger date implied by genetic analysis, as late as 5.4 million years ago.In other words, the phenotypic split doesn't line up with the genotypic split, and we've got primates with human-looking heads running around before genetic evidence of speciation. Reich's interpretation is that the early fossils don't indicate an independent species, but two related species that regularly interbreeded to improve adaptation and fertility -- manpanzees would have combined the adaptive traits of chimps and people, but females might have had to back-breed with chimps instead of their sterile bybrid cousins. The chimp and chumanzee lines might thus have been intertwined for a good long time.
The conflict can be resolved, Dr. Reich's team suggests in an article published in today's Nature, if there were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split.
Hybrid populations often go extinct because the males are sterile, Dr. Reich pointed out, so hybrid females may have mated with male chimps to produce viable offspring. The human lineage finally re-emerged from this hybrid population, Dr. Reich suggests, explaining the younger genetic dates, while the very early fossils with humanlike features may come from the earlier period before the hybridization.
There have been a number of blog responses to this study. Jason at Evolutionblog makes a preemptive strike against IDers who might misuse the findings by claiming that "Darwinists" are ignoring the inconsistencies in their data. Jason calmly explains the difference between phenotype and genotype, and how we can use those concepts to understand the disagreement between fossil records and gene records -- it's too smart for IDers, but it's pleasant for the rest of us to read. Carl Zimmer at The Loom draws a comparison to the ethical quandaries surrounding human-animal chimeras; his definition of "chimera" differs from what I'd understood it to mean, but Carl is one of the best science writers out there, so he probably knows what he's talking about. Razib at Gene Expression explains why it's not absurd to imagine a human-chimp hybrid. And John Hawks, who seems to be the only one among us who's actually gotten to look at the pre-pub paper, takes serious issue with the science. Oh John, you're ruining all the fun we've been having, breaking out the portmanteau words like "humanzee" and "manimal." But these are serious objections, especially the one about insufficient citation of previous studies. John argues that "The result was entirely anticipated by earlier work on smaller datasets. There's nothing new here, other than the addition of more data," and on this one I disagree -- the addition of more data is a significant improvement, and there's not a huge difference between "the result was entirely anticipated by earlier work" and "the result confirms earlier work." Most of the critiques are very informative, though; for instance, John implies that the study is claiming "hybridization after a long prior differentiation," which is not what I'd understood after reading the NYT piece, and is harder to imagine. Go to his blog and pick up your grain of salt before you get too into the idea of our chumancestors.
Finally, there's Lynne's take on the article, from her email:
My favorite part:This, unsurprisingly, is my favorite interpretation of the data. And here's the kicker... not only did chimps manage to overcome their knuckle-walking awkwardness long enough to seduce some hominids, but they actually maintained a pretty stable relationship -- the lines didn't fully split for over a million years, and hell, that's longer than most marriages. Those humans just kept coming back! Plus, this all happened millions of years before any primate learned to wear shoes. Hairy, knuckle-dragging chimps can get laid without shoes? What have we women been thinking all these years?
' "If the earliest hominids are bipedal, it's hard to think of them interbreeding with the knuckle-walking chimps — it's not what we had in mind," said Daniel E. Lieberman, a biological anthropologist at Harvard.'
To me this sounds like, 'Chimps are totally not sexy enough for humans!' which I think is pretty hilarious.