Two thousand generations:
Yes, two thousand human generations is a long time. It’s about sixty thousand years. They say it was only about seventy thousand generations ago that the worlds human population was reduced to a few thousand, just a few villages. Recently the whole genetic clock that this estimate was based on has been seriously challenged so that perhaps we should say we numbered in the low thousands a hundred and fifty thousand years ago. I don’t know. I have recently seen the old time scale used once again. Either the report I read has simply been ignored or old codgers in white coats are scowling and throwing spitballs at each other even as I write.
Either way, if it takes two thousand generations of speciation to occur, we are still the same species. There is no surprise there. Certainly for those who left Africa there is solid evidence of being the same species. Upon reaching Palestine modern humans (so they have said … it’s hard to keep up) interbred with Neanderthals and some of the hybrids went eastish across Asia and entered North America in the American Northwest. Others went westish into Europe, eventually sailing west (after a number of rather abortive efforts) and establishing colonies in the Western Hemisphere. Cortez precipitated a massive local uprising against the Aztecs and their leader Montezuma was killed. The Spanish thought that wasn’t right; Montezuma was rightful king and you weren’t supposed to kill them. The Aztec’s children were rescued and brought back to Spain where they married into the Spanish nobility and descendants remain to this day. So there was no species barrier; there was no hybrid breakdown. As far as those who remained in Africa, the evidence is more equivocal. I’m sure it will be worked out some day.
I settled on two thousand years of isolation leading to speciation after looking at what evidence I had. This included times to speciation for rabbits in the Azores and mice in the canary islands and well as camels. It included an apparent limitation of mating pool size to something under 1,000 or two thousand copies of a chromosome, the mechanism assumed to be there to keep species from being wiped out if they achieved mating pool size of 1,000 fertile adults and remained there.
So I had my evidence and gravitated toward 2,000 generations for speciation. There is now support for this. In Daniel Lord Smail On Deep History and the Brain, University of California Press Los Angeles 2008) there is a reference (Edward O. Wilson Sociobiology: The New Synthesis. Harvard University Press Cambridge, Mass. 1975 page 569) Wilson also suggested two thousand years to speciation provided selective pressure is strong enough. I’m not sure selective pressure is ever turned off, but even if it is it surely has to be on when a species is beginning to exploit a new niche. That’s when it matters, so that is what the mechanism to protect a population from speciation effects must be ready to protect against.
It’s nice to know that there is support. Obviously Wilson’s data set is completely different from my own. None of the references I cite is as old as 1975 except for some of the historical data and nobody has ever analyzed that data the way I do. (Although why not is a total mystery.)
So I have some reading to do. Perhaps I shall return to the topic after I have had a chance to study what promises to be a rather hefty book.
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