First tooth:
Old teeth have been found. (Modern Humans’ First European Tour SCIENCE vol. 334 no. 6056 November 4, 2011 page 576) Two molars found in southern Italy have been dated to between 43,000 and 45,000 years old apparently as reported by Stefano of the University of Vienna and his colleagues. That means that early tools previously thought to be made by Neanderthals were probably made by early modern humans. It means things happened a few thousand years earlier than had been thought.
I am always a little edgy when the evidence about very early humans is up for discussion. My own guess is that it takes 2,000 generations for two populations of mammals to be separated before they become effectively different species. I could stretch that to 4,000, but I feel more comfortable with 2,000. That is except when dealing with early humans, when I get very uncomfortable.
It is generally agreed that humans are all the same species and are able to have fertile offspring without eventual hybrid infertility no matter where the elements of a couple come from. For those who left Africa early on, the time seems good. We were mostly sitting around the same campfire 60,000 years ago, so it was possible for the children of Montezuma to marry into the Spanish aristocracy and have offspring to this day even though their ancestors had gone opposite ways around the world. Along the way, those of us who were doing the traveling picked up a significant number of Neanderthal genes, and there was another archaic form of human that contributed detectably to a swath that went across Asia. But these I take to be exceptional events. Mostly we were modern humans.
This of course ignores the question of the “most recent common ancestor.” That turns out according to Wikipedia to be people who lived only 5,000 to 15,000 years ago estimating from the sharing of genes. I confess I am baffled. It seems much too recent. But it is not a problem for the notion that it takes 2,000 generations to speciation.
What would be a problem would be if it were determined that we had already separated into different groups 2,000 generations or 60,000 years ago. But I have not yet encountered that. If we find groups that have been separated for 120,000 years and still can mate with fertile offspring, then there is trouble. I do not expect we shall see that, but of course I shall remain on the alert. Evidence that you are wrong is more valuable than no evidence at all and actually often more valuable than evidence that does not contradict you.
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