(Picture from here.)
I spoke in my blog (see here) about the recent Neanderthal sequence effort and the resulting evidence that Neanderthals and humans interbred.
Now, mitochondrial DNA of Neanderthals was sequenced some time ago. (2008: See here.) What’s interesting about that is there is no evidence in that DNA of interbreeding. This new information is about nuclear DNA and has several common points. I suggest you go to the previous blog mentioned above sequences to get the original information.
The high points, then:
- Neanderthals are 99.7% identical to humans. (Chimps are 98.8% identical to humans.)
- Neanderthals and humans split about 400,000 years ago.
- Interbreeding could have occurred as recently as 80,000 years ago, when modern humans migrated out of Africa, but more likely about 60, 000 years ago.
- Approximately 2% of the genomes of present-day humans living in Europe to Asia comes from Neanderthals.
- No appreciable DNA in present-day humans in Africa appear to have derived from Neanderthals.
- There is evidence of selective sweeps: DNA that was found to be sufficiently selectively advantageous to move through the population.
From this point on we’re indulging in pure speculation. No data to support any of this and the sample size is too small for prediction. We’re just gonna’ have fun.
The first interesting thing is the differential inheritance between nucleic and mitochondrial DNA. Mitochondria derive solely from the mother. Now this cannot mean that human men would not mate with Neanderthal females. Humans will copulate with anything: dogs, cats, trees, garbage cans. If it can be done, human males will do it.
It can mean Neanderthal females would not accept human male advances. There’s some evidence that sexual dimorphism between males and females were not as pronounced as between human males and females. Neanderthals were incredibly strong in comparison with humans. It’s fairly likely that a single Neanderhal woman could easily take care of herself against human men. This doesn’t preclude gang rape or possible infanticide of the offspring. But we have to start building some pretty high air castles to account for no interbreeding.
Although there is one interesting scenario. At some point in human history we started hiding estrus. We don’t know when that happened– none of the characteristics of an animal being “in heat” fossilize. But it did and it did long before recorded history. Chimps, our closest living relatives, have the standard primate estrus cycle. The female is not particularly receptive to sex when she’s not ovulating. The males are excited by the physiological signals she puts out when she is. Humans don’t do any of that.
It’s not a far speculation to think that Neanderthals were like chimps in this respect rather than human. Like other primate species, males are always producing viable sperm even when no available female is in the vicinity. It’s not too far off to think that Neanderthal women, even if willing, would not necessarily be viable outside of estrus periods. And it’s also not so far off to think of this being the point when they would be most vigorously defended by Neanderthal men.
Another possibility is the differential representation is biological. Possibly the cross of male Neanderthal and female human was viable but the cross of male human and female Neanderthal was not. Perhaps female Neanderthal reproductive systems rejected human embryos as non-viable or irretrievably damaged and female human reproductive systems were not so selective.
So we have back in the past two human groups cohabiting the same landscape where males from both species are competing for females of one species. An interesting dynamic.
Of course, it could all just be the result of genetic drift. That’s the basis for the mitochondrial eve hypothesis.
Another curious feature of the research is the differential between African and non-African human groups. The answer is obvious: the humans that moved north didn’t move back south. Consequently, any genetic sweep that occurred stayed north of the Sahara.
The two questions that immediately leap to mind are 1) What were the selective advantages of the genes in the sweep? and 2) Do they have any bearing on the world now?
Now this is the sort of thing a blogger might say: “here be dragons.” After all, it doesn’t take very much to get a good racist attitude going. The Eugenics Movement started with less.
Still, where we came from is an indicator of where we’re going. So, let’s dive in.
Some of the genes implicated as “sweep” genes are DYRK1A, CADPS2 and AUTS2. RUNX2 is also implicate. It’s already known that Neanderthals share the FOXP2 gene, though it is thought that this is more an artifact of common heritage than interbreeding. DYRK1A is implicated in Down Syndrome. CADPS2 has to do with managing the vesicles in neurons and has been implicated in autism. AUTS2 is also one of the candidates for autism. RUNX2 is one of the regulatory genes for skeletal development. FOXP2 mutations have been associated with speech and language disorders.
So: we have two regulatory genes (DYRK1A, regulating cell proliferation and possibly brain development, RUNX2, skeletal development) and two genes strongly correlated with neurological development (CADPS2 and AUTS2).
Why would they sweep?
It can’t have much to do with intelligence– Africans show little difference from Europeans when corrected for test deficiencies. It’s also an unlikely candidate since intelligence is highly selected among human beings. Any thing that swept on basic intelligence would be driven strongly towards maximum. I.e., we’d see a huge difference.
The skeletal development gene might be interesting. Are there cold adaptations in skeletal development between African and Eurasians? Hard to say.
It must also be remembered that whatever selected for the sweep of these genes through the population may no longer be active. We could easily be seeing an artifact of a past sweep and the relic has no present bearing at all. Or, it could be that the group that moved north had some disadvantages that were remedied by the introduction of Neanderthal DNA and the net effect is zero.
But I’m a science fiction writer. I like to play with dangerous things.
I’ve read Jared Diamond‘s two books: Collapse and Guns, Germs and Steel. When I read both books, I kept coming back to the idea of the Tragedy of the Commons, Garret Hardin’s article from 1968. To encapsulate a very interesting idea: private gain trumps common good. This is something I see as an American all the time. (Of course, America is the encapsulation of the world, right?)
When I read about other cultures I see it considerably less than I see it in Euro-centric cultures. In many other cultures, tribe and/or family trump individual gain. What if this was something we gained from the Neanderthals, a change in the boundary condition between tribe and extended family versus local family and individual? A minor reduction in the ability of the individual to defer gratification based on inhibitory cultural patterns? A tiny reduction in the ability to sensing tribal/familial place? Something that might have served us back when we had to fight mammoths and glaciers but which has little place now?
Now, let me say on record that I think any genetic differences between races, genotypes and gender are completely overwhelmed by the noise of cultural adaptation. Whatever we use in our heads to navigate the internet or solve differential equations is not derived from any tiny differences accumulated from 80,000 years ago when one small group of humans moved away from another small groups of humans.
I expect there are differences between peoples based on genetic heritage. I also expect that they rarely manifest themselves as conveniently as simple differences in abilities. Do Kenyans have an innate genetic ability to win the Boston Marathon? Or are Kenyans predisposed by their upbringing and geography to win the Boston Marathon? My money is on the latter more than the former. But the former has more juice for a science fiction story than the latter.
That said, genetic sweeps don’t just happen; they’re the result of something useful propagating through the biology of the population. They are evidence that different subgroups of the human population have, historically, evolved differently from one another.
They are relics of where we came from.