Speculating About Neanderthals and Humans

(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:

  1. Neanderthals are 99.7% identical to humans. (Chimps are 98.8% identical to humans.)
  2. Neanderthals and humans split about 400,000 years ago.
  3. 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.
  4. Approximately 2% of the genomes of present-day humans living in Europe to Asia comes from Neanderthals.
  5. No appreciable DNA in present-day humans in Africa appear to have derived from Neanderthals.
  6. 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.

To paraphrase Randy Newman, “I’d sell my soul and your soul for a story idea”.

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.

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Speculating About Neanderthals and Humans — 8 Comments

  1. Overall, interesting article. A few points regarding genes and their regulation, which are my domain of research:

    I happen to work with DYRK1A. It codes for a dual-specificity kinase that has nothing to do with cell proliferation.

    Genes do not encode the nervous system. Even less do they encode complex behavior. Each is like a cog of a complex instrument: if suboptimal, it can make the instrument break down, but (with few important exceptions) it doesn’t make it run.

    More on this:

    Miranda Wrongs: Reading Too Much into the Genome
    http://hplusmagazine.com/articles/bio/miranda-wrongs-reading-too-much-genome

    Bonobos, which are as close to us as chimpanzees, are sexually receptive most of the time. This implies that all hominids had hidden estrus, though it can never be unequivocally proved. Incidentally, in your phrase “Neanderthal women, even if willing, would not necessarily be viable outside of estrus periods.” I assume you mean receptive… otherwise Neandearthals would have lasted the grand total of one generation.

    The suggestion that Neanderthal genes might increase individuality goes against the evidence that they were almost exclusively hunters (versus the gatherer-hunter pattern of the Cro-Magnons). Hunting requires coordination, even the herding or stampeding type they almost certainly practiced.

    My brief take on the Neanderthal sequencing results:

    Neanderthal Genes: The Hidden Thread in Our Tapestry
    http://www.huffingtonpost.com/athena-andreadis-phd/neanderthal-genes-the-hid_b_568211.html

    David Shenk’s The Genius in All of Us is a recent “sweep” (grin) of the inextricable interweaving of nature and nurture. For more detailed takes, especially pertaining to gender issues, I recommend the works of Anne Fausto-Sterling and Sarah Blaffer Hrdy.

  2. Steven – re Guns, Germs and Steel, I was compelled to dispute David Friedman on an embarrassing panel in San Diego a couple of years ago. At the time, I had no idea of Mr. Friedman’s education or pedigree. All I knew was that here was this guy saying that Guns, Germs and Steel was a bad book, based on his own background in Icelandic history. After some back and forth, Mr. Friedman admitted that his negative conclusion about the book and Dr. Diamond was based upon not the book itself, which he had not read, but rather upon a book review, which he had read some months earlier.

    But great thoughts – the two books are full of food for thought and information to absorb.

  3. This was years ago, but I had an anthropology professor who speculated that blonde hair, blue eyes, and fair skin came from neanderthals. Her reasoning was that their presence in Europe predated that of humans and that fair features, normally disadvantageous (less because of skin cancer – which is a genetic consequence, like Huntington’s disease, that usually doesn’t kick in until after 50 and thus doesn’t matter much in an evolutionary sense – and more because of the fact that paler skin is thinner and some kind of folic acid processing problems occur in people with too much UV exposure) are only strongly advantageous in very cold climates (because fair skinned people get much more vitamin D from UV rays than dark skinned people and are much less likely to get rickets even when they get very little sunlight).

    FWIW, my understanding about Kenyans winning the Boston marathon was that it was neither genetic nor entirely cultural. I thought it was that Kenya is at high altitude but, unlike, say, Everest, the land is still flat enough to run on. People living and training at high altitude develop more red blood cells in response to lower oxygen. Artificially transfusing additional red blood cells before a race is called blood packing and is illegal, but anyone who trains in Kenya has the same advantage without having to do anything extra.

  4. Thanks for all the comments. A couple of responses:

    With great trepidation I have to push back on a couple of Athena’s comment: “Bonobos, which are as close to us as chimpanzees, are sexually receptive most of the time. This implies that all hominids had hidden estrus, though it can never be unequivocally proved.” I don’t think Bonobos receptivity says anything about human estrus cycles. Chimps split from the hominid line about 6 mya and bonobos split from chimps about 1 mya. Both bonobos and chimps have visible estrus cycles regardless. This argues to me the reduced estrus cycle in humans arose after the split, not before, and therefore could be placed anywhere along it barring other evidence. “Other evidence” would include a genetic determinate of the estrus cycle which I am not privy to and don’t know the research.

    The data from DYRK1A came from gene cards (http://www.genecards.org/cgi-bin/carddisp.pl?gene=Dyrk1a):
    “It may play a significant role in a signaling pathway regulating cell proliferation and may be involved in brain development. This gene is a homolog of Drosophila mnb (minibrain) gene and rat Dyrk gene. It is localized in the Down syndrome critical region of chromosome 21, and is considered to be a strong candidate gene for learning defects associated with Down syndrome.”

  5. Enna, that is interesting, as I have a friend who believes the same re: Neanderthals – i.e. pale skin, eyes and hair are Neanderthal-derived, not modern-human-derived characteristics.

    I find this very pleasant although unknowable, as I enjoy thinking about taking my club and crushing others at least once or twice a week.

  6. Oh – re gene expression, even to a layperson, it should be self-evident that while there is an *association* between specific genes and specific expressed characteristics and disease vulnerability, the simple fact that not everyone who has the BRCA gene has or gets breast cancer, and the fact that actual clones often look very different from their clone-parents – should indicate there is something else we do not yet understand, and may never understand, at play.

    DNA worship – well, why has RNA gotten such short shrift?