Archaic admixture – what is it good for ?

It now looks as if everyone outside of sub-Saharan African has two or three percent Neanderthal ancestry. Australoids have, in addition,  somewhat more Denisovan admixture, while Pygmies and Bushmen  have a couple percent admixture from an unknown, very divergent African archaic population.  The question is, what kind of effect – if any – might such admixture have had?

The first-cut answer is that anatomically modern humans, the invasive population, picked up adaptive alleles from those archaics. Such a new allele is like a favorable mutation: a single copy with a fitness of s  has a 2s probability of fixation.  Admixture at the few-percent level would have allowed  the expanding mostly-AMH population to pick up most such alleles.  The most obvious kind of beneficial archaic allele would be one that conferred some kind of local adaptation.  After all, the mostly-AMH population was moving into areas that been occupied by those archaic humans for hundreds of thousands of years: obviously the archaics were well-adapted to local circumstances.  Loci involved in defense against regional pathogens ought to have been acquired by the invaders: Parham thinks he has found that kind of adaptive introgression in HLA loci, and he may be right.

It’s also possible that some archaics had some alleles that were better generally, not just in their locale.  Even though these archaic humans were competitively inferior, since they were largely replaced,  they could have had, indeed almost certainly did have generally superior versions of some genes. Some people find this incomprehensible.  For example, they don’t see how a presumably dumber human subspecies could possibly have an allele that could boost human intelligence in AMH.  Well, a T-34 was a better tank overall than a PZ-III, but would have been been better yet with a German radio.  Adaptive introgression can combine the best of both designs: we see this in agriculture every day.

In the short run, admixture almost certainly increased fitness, at least after natural selection and recombination had winnowed out the advantageous alleles.  I say almost certainly because AMH likely picked up any and all archaic driving genes in this assimilation process:   some might have been harmful.

In the longer run, the effects are unpredictable. In principle, alleles entering via admixture might change the contours of the adaptive landscape, changing the long-term direction of adaptive evolution.   Changes that strongly increase fitness in the short run can dead-end:  others that yield smaller fitness advantages in the short run can have a better upgrade path. Natural selection can’t tell and we’re not that good at it ourselves.

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2 Responses to Archaic admixture – what is it good for ?

  1. blert says:

    Funny you should mention T-34 radios…
    The Germans destroyed ALL of Russia’s radio tube production capability during Typhoon — and Stalin was able to fool everybody!
    His agents told America they sure could use some ADDITIONAL radio tubes to ramp up production; so we flew them express priority — and also by ship.
    Until they arrived Stalin cannibalized EVERY tube Russia had for the High Command. This left every Russian tank with no radio during the 1942 Summer Campaign. The only tanks with radios were LendLease British and American tanks. ( M3’s ) It was a disaster.
    This is why inferior German Mark III’s ran riot over the Russians that summer.
    However, by the time of Uranus American tubed radio sets were in every T-34 from then on.
    ( Previously only command tanks had radios! This explains the weird, lousy performance of early T-34 units. It also explains the German shock when the Russians were up to par on the radio net. Everything flipped on a dime. )

    WRT to your thesis: I suspect that Hss up out of Africa were extremely close to the San. Upon contact, the San males were ambushed — and done in. The Hss XX became Cinderella booty for the Neanderthals — and a very low cost sex slave, too.

    Male Neanderthal and female Hss is the only way to explain the pervasive mDNA coming up out of Africa. Such mating would be, obviously, almost a 50:50 blend. ( The Y not carrying the full range of ‘switch-power.’ )

    I just don’t see any San XY’s getting through at all. The Neanderthal DNA sequence is still massively incomplete. However, the figures reported are for variations — only.

    Neanderthals MUST have contributed MUCH more than we can ever know: those DNA sets that are identical to both San and Neanderthal. They arrived by way of Neanderthal in the mating — but are ‘lost’ against the common backround.

    What Neanderthal DNA we are finding would seem to have come from XY.

    What is confusing is what happened when Neanderthal XY mated to produce half-blood XX daughters. Somehow, for the Neanderthal X its mDNA has been lost to Darwin.

    This is strongly indicative that Neanderthals were losers against at least one disease that was countered exclusively by Hss X DNA, perhaps Hss mDNA.

    As for what ended the Neanderthals: they were the Paul Bunyons of their day — and almost certainly the source of the truly ancient tree clearing legend.

    It must be obvious by now that Neanderthals were ‘people of the wood’ — and that they were tricksters and trappers. Their number one gambit was to fell a tree by fire — or to steal one from a beaver — and then use it as a hide. Their prey would be starving come the snows — and that browse would look mighty sweet. Then, bam! The perfidious Neanderthals would lunge forth from the branches with killer stabs.

    This is the method for bringing down even the largest prey: you keep your mouth shut and lure them into a kill zone.

    And this is how Neanderthals would’ve destroyed any San daring enough to head north.

    The San NEVER saw the Neanderthals until it was too late. The San are not at all oriented towards hunting from the shadows — quite the opposite, in fact.

    When the climate changed enough to destroy the forests — the jig was up for Neanderthal.

    As for Cro Magnon: they’re the half-breed sons of Neanderthals. I suspect that each became king in his own niche. The Cro Magnon blended the hunting styles of both — and had the resistance of both.

    It would also be entirely possible for Neanderthals to re-hybridize over and over with Cro-Magnon. This seems logical.

    Then, time and again Darwin washed out the Neanderthal mDNA from the maternal line. The last surviving pocket of Neanderthals really represent the last location that was free of the killer disease. It well could’ve been small pox.

    It fits the killer profile perfectly.

  2. Gorbachev says:

    We need to know more about neanderthal DNA. This is the trick.

    Much will be clearer.

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