Homo interbreeding?

I've always been a believer in interbreeding between Cro Magnon man and Neanderthals. Even when the scientific establishment insisted it didn't (and in some cases couldn't) happen. Now we have evidence that it did, and also evidence that anatomically modern humans in Asia interbred with another related species, called Denisovans.

Shock, horror! The common belief is that Neanderthals would have been unattractive...short, stocky and with weird faces. And, of course, that belief can be reversed, with an argument that they would not have found us attractive either. And yet, it happened. In fact, it may have brought with it distinct benefits for the descendants of those crosses.

So. How did it happen? I'm going to bring together two things here.

First, genetic studies have indicated that human biodiversity in Africa is considerably greater than outside Africa. This is evidence of a genetic bottleneck. A bottleneck occurs either when a species is dramatically reduced in numbers or when a relatively small population is isolated. In this case, we're dealing with the latter. The vast majority of humans stayed in Africa. Only a relatively small number, fueled perhaps by population pressure, perhaps by curiosity, came up through the Middle East to be the ancestors of all non-Africans. The common theory is that all of the differences are adaptations; pale skin to absorb more vitamin D, slanted eyes in plains dwellers might allow for better vision of objects coming over the horizon.

Hold that thought for a second. Because the second thing I would bring up is a social science study done a few years ago in Washington, D.C. Through anonymous questionnaires and also some face to face interviews, social scientists did an intensive study of mate choice amongst teenagers in the city. One of their more interesting results was a significant minority of young people preferentially choosing to date outside their ethnicity. This is despite lingering stigmas against mix-raced relationships, although the miscegenation taboo has faded, it is not completely gone. If they were just looking for more choice, then that would explain not caring. But what explains a sixteen year old Caucasian male who will only date African American girls?

The answer is, of course, instinct. A lot of people would like to think we freely choose our mates, in a society in which most marriage taboos have broken down. But falling in love generally makes little conscious sense. Cupid has a bow...and a blindfold. Studies have indicated that chemistry has a strong impact, and one of the most striking results was that humans tend to choose mates with different MHC genes. These effect the immune system, and being heterozygous for these genes is a definite advantage in terms of not getting sick.

Is that white kid in Washington instinctively looking for a mate with a lot of different genes for some reason? Is he part of a mechanism designed to increase biodiversity in the species by pushing individuals who pass a certain point of homozygosity into choosing a mate from an outcross line?

If that is true, and here we are in speculation, we have a population of homo sapiens coming out of Africa. This is a relatively small population that has now become isolated from other tribes. Homozygosity, therefore, increases. As it does, so do individuals affected by the drive to find an outcross.

This, surely, would have resulted in at least some of these affected individuals selecting mates outside their species. As the numbers of Neanderthals declined thanks to climate change and competition from us, then their homozygosity would also have increased. Interbreeding would have increased and eventually sealed the death knell of the less numerous species, leaving only a few marker genes.

Maybe. We still don't know exactly what effects that ancient outcrossing had on modern man. But for those interested in science fiction, there are all kinds of ways you can go with the outcrossing instinct. For example, would it drive people on an isolated colony to leave to seek a mate? Would it have an effect on interstellar trade?

All food for thought.