Ancient climate change and human change:
Of course there is enormous interest (J. R. Stewart and C. B. Stringer Human Evolution out of Africa: The role of Refugia and Climate Change SCIENCE vol. 335 no. 6074 March 16, 1012 page 1317) in just how evolution of humans proceeded and produced what we have today: a single global species. 

The article is one to stretch the mind, quite difficult to follow because of its complexity and subtlety but not buried in jargon so there is hope for the intrepid reader.  Without trying to write a précis of such a refined work let me just say that most scientist agree that anatomically modern humans appeared first in Africa, whence they spread into areas that had been long occupied by “archaic” humans, with whom they rarely interbred but mostly seem to have swept them away.  For the whole story, get the article from 

I shall pretend you have read it.  The thing is that there are some puzzles that are not pointed out but are clear.  I am sorry to say that although I can bring a unique perspective – that humans need to marry kin to survive long – I do not at this point find that this perspective clarifies everything.  But I’ll try.

First a word about a couple of words:  Referring to all humans before anatomically modern humans as “archaic” makes my flesh crawl a bit.  It sounds like there was something wrong with them.  Well they survived longer than we have survived or are likely to survive so it is hard for me to consider them failures.  I don’t have a pet term to offer in place of archaic, so I leave that to a cleverer mind.  The second term of interest is “refugium,” pleural “refugia,” which is a small area where a species survives during an ice age if it is a warm adapted species or where a species survives between ice ages if it is a cold adapted species.  The high north is now a (vanishing) refugium for polar bears; when things were colder hereabouts, their range was bigger. 

I shall address four things that I find a bit puzzling.  1) What causes local extinctions of a species?  2) What happened to the Neanderthals we replaced (having picked up some of their genes along the way)?  3) How we could mate successfully with Neanderthals?) Why Africa?  What was the secret power?

It turns out that there are a lot of local extinctions according to DNA studies.  I have long believed that.  Consider a very benign landscape in which some animal can eat, avoid predators and disease, mate and move about freely.  There has to be a balance.  Otherwise we would expect the prediction of Malthus – that a population will increase without limit until starvation or some other environmental limit is reached.  This should be happening in animals all the time.  But we do not see it.  Wild, they are lean.  Semi tamed, like pigeons, they are fat.  But starving, no.

What I believe is going on is that the populations are not rigorously defending their mating pools.  A local population will flourish, grow past some critical level, experience the inevitable fertility and population decline and vanish taking a pit of diversity out of the picture.  Other populations will drift in from the edges.  Neither the animals nor careful observers will notice that anything has happened.  The numbers just fluctuated a bit.  But when they go look at the genes there is not as much diversity as would be expected.  That is an inherent problem.

The way the problem is being resolved is to say “bottleneck” rather than “local extinction.”  Bottleneck means the population was reduced to a very small size, maybe an ice age eliminated all but a refugium here or there.  No question such things happen, but the paradox is resolved by substituting a theory for evidence – a theory for which the evidence may not always be there.  Cheetahs for instance are thought to have gone through a recent bottleneck, but there are not many ice ages in Africa or the Middle East. 

The Neanderthal people survived a very long time and than vanish just as anatomically modern humans arrived.  The circumstantial evidence is very strong that we wiped them out.  Some say that there was an ice age blooming at the time, and maybe it was the environment.  I notice two things. 

One is that the final refugia were far to the south, along the southern fringe of Gibraltar for instance.  That would be consistent with them being driven thither by the climate, but the Neanderthals were very well adapted to cold weather.  It seems odd to think of them dying out from the cold just when the Africa adapted moderns showed up. 

The other oddity is that archaic humans had been drifting out of Africa for a long time.  The Homo erectus species flourished far and wide, as did the later Heidelberg Man species.  The pathway out of Africa might not have been so much difficult as impeded by the fact that there were only inhabited lands beyond.  In that case the replacement of the Neanderthals might have been a large scale local extinction, just as what happens in animals.  They got better at what they were doing, traveled more widely, met a larger social pool, expanded their gene pool and thus died out, now throwing the door open to us moderns.

People are disturbed by the fact that we moderns were able to mate with archaic humans (from whom we had been separated much longer than it should take for speciation to occur) successfully and have long term surviving progeny.  That used to bother me, too.  My opinion today is that although humans are generally hyped as the ultimate generalists, able to go anywhere and do anything, we are in fact extreme specialists.  What we do is nurture our young.  We do it beyond belief, beyond all reason, beyond sanity.  Nothing else on earth cares for young the way we do.  All our other accomplishments are simply by products of this prime directive.  We can do anything anywhere any time simply because we have to be able to do that in case your young should need it.

So a handicap in fertility that might be overwhelming for any other animal hardly fazes us.  If there is a first generation offspring – and inter-specific viable forms are not rare – that offspring will probably grow up well and strong and have a pretty good chance of finding a mate.  In the rare event of a second generation offspring, that person will also have advantages no other animal has in the wild.  And so it continues. 

What is the secret of us final wave African emigrants?  What did we bring from Africa that made so much difference?  (Obviously it must be something we share with those who never left Africa at all.)

If you look at what is going on in Africa, tribalism dominates everything.  They go for mating with the extended family to a far greater degree even than we, their progeny, do.  That seems to be the African magic.

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