Time of Isolation

Recent direct measurement of the human mutation rate  strongly suggest that it is about half as large as previously estimated.  Neutral genetic differences between groups would have taken longer to accumulate.

Therefore, estimates of population splits need to be recalibrated. Generally, this suggests that everything in human prehistory happened considerably earlier than previously thought.  For example, using the old value of the mutation rate, researchers had estimated that the split between modern humans and Neanderthals/Denisovans took place 272,000-435,000 years ago – which seemed odd at the time, since Europe has plenty of hominid fossils that look to be on their way to becoming  Neanderthals, yet are  considerably older than that split estimate.   The new mutation estimates now fits the fossil evidence.  Along the same lines, the previous estimate had modern humans exiting Africa about 60,000  years ago, which meant that the fairly-modern looking Skhul/Qafzeh, fossils found in Israel, some 80,000-120,000  years old, must have died out without contributing to modern Eurasians.  Which was possible, of course, but not as simple and appealing  as a single Out-of-Africa expansion.  The revised mutation rate, along with  recent evidence of an African middle stone age tool tradition (Nubian complex) in South Arabia, makes for a simpler story, one in which the Skhul/Qafzeh population eventually expanded into Eurasia.

But  the story  still isn’t all that simple, because we’ve never found
anatomically modern fossils  that are more than 50k years old further out into Eurasia.  Somehow, anatomically modern humans (AMH)  didn’t yet have the moxie to displace archaic humans.  They apparently spent  tens of thousands years stuck in the Middle East.

Maybe that makes sense. We know that  people outside sub-Saharan Africa have lower levels of genetic variation than Africans: maybe an extended stay in the southern Middle East could have that effect.  Consider that the area habitable by early hunter-gatherers isn’t very big, at least during the desertification  stages. The Sahara, and Neanderthals in the Levant, could have blocked gene flow with sub-Saharan Africa.  That’s the scenario you need to reduce genetic variation: small numbers and isolation.

Why the lag?  It may help to think of AMH as an invasive species. Very often, some new pest shows up and spends quite a while hanging around the point of origin, not yet seeming very formidable.  At some point, it transitions into an irresistible menace, like crabgrass. Our best guess is that the budding pest species  spends that lag time  responding to selection, becoming better adapted to its new environment.  Sometimes the newcomer manages to steal useful genes from related species, which naturally are already adapted to  that environment.  Starting with potential advantages over the locals, and  now prepared for local conditions, the invasive species set forth to conquer.

You might also think of south Arabia as the wading pool,  a place different from sub-Sahara Africa,  but not too different.  AMH needed to make some changes: lose expensive immunological defenses against African zoonoses, acquire the necessary defenses against Eurasian pathogens,  begin to adjust to lower temperatures and more variation in the length of the day, etc..  Neanderthal admixture likely helped.

AMH really needed some kind of trump card: it is not easy to displace a competitor with similar abilities that’s already locally adapted.  Although their toolkit is not yet much different from Neanderthals, the Skhul/Qafzeh population already showed some signs of greater cultural capabilities, like a ritual burial with grave goods.  Somehow, though, that wasn’t enough – success against Neanderthals waited another 50,00o years.

 

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5 Responses to Time of Isolation

  1. redzengenoist says:

    Ridley Scott has a theory. He recently made an uncharacteristically shitty movie about it.

  2. Greying Wanderer says:

    Numbers?

    http://upload.wikimedia.org/wikipedia/commons/d/d1/Relief_Map_of_Middle_East.jpg

    If you look at Arabia and think of land routes out it seems to me you have three main ones: Egypt, fertile crescent, and the valley between Israel and Jordan. If the density of hunter-gatherers in those three spots was particularly high then weight of numbers might have held AMH in place. If wetlands, deltas etc (which i assume all three of those spots were before irrigation?) can support particularly high HG population densities (?) then that might explain it. Especially if at some point those wetlands (if they were wetlands) dried up neutralizing that numbers advantage?

    Another possibility might be crossing the desert directly which might mean a long time adapting to the desert first (and in the process self-selecting themselves a competitive advantage or two because of the difficulty of adapting?)

    or learning to sail.

  3. albatross says:

    Could the ultimate expansion of AMHs over Neanderthals be based on some technology the AMHs got and the Neanderthals couldn’t? Given the time scales involved, this would probably have to be something that the AMHs could handle and the Neanderthals could not handle. That might be more complex language allowing for better coordination that required brain adaptations the Neanderthals didn’t share, or better missile weapons that required hand-eye coordination or brain adaptations the Neanderthals didn’t have, or something else.

  4. dave chamberlin says:

    “AMH really needed some kind of trump card”

    Increased cognitive skills which were the result of hybridization of early AMH and neanderthal. There never was a great leap forword, instead there was a period tens of thousands of years long in which AMH took to find the right genetic recipe that included those genes from neanderthals that made man truly modern in inventiveness. There may appear to be a great leap forward because at the time we became capable of crafting an entire tool box of stone and bone tools we spread very fast all the way to Australia.

  5. typal says:

    If EMH brainpower wasn’t enough and they needed to acquire Neanderthal’s local adaptations then the the Neanderthals would be edged out rather slowly I think . But there is some doubt about the overlap between Neanderthals and EMH now. It’s being claimed that Neanderthals disappeared as soon as EMH got to an area. It’s easy to displace a competitor with similar abilities that’s already locally well adapted if you eat their young, like brown rats did to black rats. One certain EMH trump card was projectile weaponry, and William H. Calvin has a theory about projectiles and intelligence.

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