Help me out, evolutionary theorists. I’m a bit puzzled at the conclusion that homo habilus cannot be ancestral to homo erectus because the two species apparently lived alongside each other for 500,000 years or so.
Isn’t it possible for one population of a species to be isolated from other populations because of relatively short-lived changes in geography or environment and to diverge just enough in that time period that speciation takes place? And after that the two populations co-habit again, but rarely or never interbreed?
With hominids, it even seems possible for me to imagine that something we could broadly define as “culture” playing a role in this kind of isolation. Reading the work of some primatologists, it seems to me that even chimps have culture in some sense, that particular social groups of chimps may have distinctive habits and practices whose roots are at least somewhat arbitrary rather than specifically adaptive. (E.g., one day an influential member of one group starts doing a common task a different but basically equivalent way, and the others eventually follow.) Exaggerate that aspect even a little bit and maybe you could get two groups of habilis living near to each other who rarely if ever interbreed simply because their day-to-day routines were culturally different, because the temporal organization of their lives amounted to the isolation of their populations from each other.
There are so many just-so stories that strike me as reasonable about the evidence that we have. As an outsider, I’m always a bit surprised at final and direct claims from the big players in the study of hominid evolution after significant fossil or genetic evidence comes to light.
To be fair, Leakey’s comment in the NYTimes article is a bit more circumspect than “cannot be ancestral.”
The speciation scenario you describe is definitely possible, but the odds of divergence get markedly better the greater the separation. It’s worth noting that the indigenous people of Tasmania are thought to have been completely isolated from the rest of humanity for ten thousand years without having developed sufficient differences from other people to have speciated.
That said, I think that there’s another factor operating in the Times’ coverage, namely the persistence of the Great Chain of Being idea. It might be that Homo habilis is being shifted slightly off the branch that leads from mudfish to Dick Cheney, but that sort of thing happens fairly often in cladistic analysis. The difference here I think is that even some educated people find it hard to grasp that there were once a whole lot of different species of us, that our current existence was more the luck of the draw than the result of an inexorable evolutionary force. The notion that human species didn’t arise and give way to the next in stately fashion, like a cascade of dominoes, still causes a bit of cognitive dissonance in a lot of folks.
Or, instead of reading my comment you could just go read an excellent post by PZ in which he actually answers your question.
I just replied in that thread. PZ and some of his commenters seem to me to offer the “Scientists misquoted by the media” argument, which in this case doesn’t seem quite right to me–Spoor and the Leakeys seem to me to have stepped right into the “misquote” quite voluntarily.
I don’t know what Leakey and Manthi are thinking. Both have made absolutely irresponsible and unfounded statements to the press, like this one:
“They were kind of sisters, if you like,” said Frederick Manthi, the
scientist who discovered the fossils. “Homo habilis never gave rise to
Homo erectus. These discoveries have completely changed the story.”
This is directly contradicted by the Nature paper itself, which says:
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.
Your scenario an entirely reasonable resolution of the situation. For some reason, the authors are saying one thing in the paper and something very different to the media.
Oh, and I replied directly here.
Isn’t it possible that habilis and erectus are both varieties of the same parent species?
And isn’t it also possible that habilis is a variety of erectus?
And isn’t it also possible that erectus is a variety of habilis?
I think so.
And isn’t true that I have a really hard time understanding how anybody can distinguish a variety from a species this far removed in time and based on such scanty evidence?
I was once in Ralph Holloway’s (expert in paleoneuroanatomy) lab at Columbia, looking at his skulls, bones, and endocasts. He offered the thumbnail estimate that perhaps 10% of the evidence about the brains of our ancestors was in that lab. And it wasn’t a big room. He also observed that the number of individuals represented by the collectivity of fossile remains couldn’t be more than a small fraction of 1% of the total population. There’s no particular reason to believe that that faction is a random sample of the population, and no way to estimate what biases are in play. Conclusion: we make generalizations at our (intellectual) peril.
I forgot to ask: could someone please help me understand what a “just-so story” is?
Just-So Stories by Rudyard Kipling. Fables that are intended to give a humorous explanation of how an animal acquired an anatomical feature that it’s well-known for through its actions (often its misbehavior). Some people (including me) would accuse some work in evolutionary psychology, archaeology and paleontology of verging on being a just-so story at times.
Wandering off topic with you: This is my big problem with Richard Dawkins when I’ve heard him interviewed. He makes a series of generaly cogent arguments critiquing religious belief, and then slides into these entirely speculative Just-So Stories in evolutionary psychology, to the point where I can’t see any difference between his version of religious and scientific belief except for the symbolic order they inhabit.
Yeah, absolutely–Dawkins and some of the other evo-psychs have a real tendency to tell what strike me as possible but by no means necessarily true fairy tales about early human existence that are based on a bad melange of reading and extrapolating from a few works of anthropology that happen to stroke their favored ideas about human nature (and ignoring anything that contradicts), Clan-of-the-Cave-Bear level visions of prehistoric hominid life, and the heavy use of particular kinds of contemporary survey research whose results are usually derived from studying 18-22 year old North American college students.
I once heard a presentation by a young evo-psych scholar who got very very angry about the just-so stories tag. He said, “Look, you can only offer an evo-psych analysis if you can demonstrate that the behavior you’re talking about is fairly universally distributed kinds of behavior in most or all human populations”. He then went on to argue for an evolutionary explanation of a behavior which had been documented extensively in the US and Western Europe…and, he added, there’s a study of 22 Kenyan women which suggests they do it too. Nothing about any human population before today (because you’d have to develop the hermeneutical skills to read documents for evidence of the behavior: no survey results available) or any contemporary human population outside of the US and Western Europe except for this one piddling and unsatisfying study of Kenyans.
And that was “universal”, established.
So, when the APA, in its Diagnostic and Statistical Manual, merely lists behaviors that, they say, are suffient for a diagnosis of “Antisocial Personality Disorder,” for which there is no lab test, aren’t they tellling a “just-so” story?
What makes these behaviors, “disordered?” Why aren’t they simply natural, and “ordered,” for some of us. Why aren’t the people who the APA proclaims have “Antisocial Personality Disorder,” nothing more than members of one variety of Homo Sapiens? Why aren’t these people simply “varietas antisocial?”
Evolutionary psychology is a very small and particular branch of psychology. Contemporary psychiatric and psychological diagnostics don’t draw much on evolutionary thinking (which is one of the complaints of evolutionary psychologists, occasionally with some merit).