Exodus of the ancestral Pueblos:
Since childhood I have been acquainted with the haunting beauty of the cliff face dwelling in the American Southwest.  In the old days we just called the people “The Cliff Dwellers,” and always assumed the inconvenient location was used for defensive reasons.  Nobody ever asked, “What became of them?”  It was assumed that either they were overrun or the need for such a formidable defensive posture came to an end.

Now we call them Ancestral Pueblos, acknowledging a connection between the ruins and the contemporary Pueblos, of whom we were also aware.  But a mystery has now appeared: where did they go?  It turns out (Richard Monastersky, And Then There Were None Nature vol. 527 no. 7576 November 5, 2015 page 26) that it is rather a mystery.  Over a short period of time 30,000 people who had lived there apparently moved away.  Now in my book, when I hear “moved away” I think “died out.”  But this time the explanation is plausible.  Something like half of them did indeed turn up elsewhere. 

What drove them away seems complex.  There was the matter of a large population competing for limited agricultural resources, a severe drought and violence, lots of violence.  The drought of notion of course leaves me skeptical.  The population was already falling when the drought hit, and since the drought was estimated on the basis of tree ring width, I suspect that as among those of Long House Valley, trees were cultivated.  As the population fell the cultivation was no longer so necessary, tree growth declined, and a drought is inferred. 

As for running out of available land, that would indeed be a strong incentive for moving out.  And I assume those moves were of whole extended families, not nuclear families, as the Long House Valley evidence also suggests.  But this is massive exodus. 

As for the violence, some was indeed perpetrated by outsiders, but much appears to have been caused by the people turning against each other.  Yet when they moved, they became the peace loving Pueblos and preyed upon each other no longer.  There were political changes that the article describes and no doubt go far to explaining the rise in violence.

But of course when a population crashes I generally shout, “Demographic collapse!  They got so many people together in a single social pool that the babies stopped.”  And indeed the population seems to have undergone two phases of growth and contraction of about 300 years each, which is consistent with my own estimate of how long a single gene pool can go between kickoff and disaster.  One of the cycles only reaches a modest size, some 10,000, which is enough to cause infertility, but it does not have the characteristic notch visible in Long House Valley.  The second cycle, with growth to 30,000 or so, does have the notch.  So all that works our well enough for evidence that needs must be indirect. 

Then there’s the violence.  I strongly suspect that when a population is collapsing for lack of babies, there comes a time when people get violent.  An ancient city state in which all the members are very old could be sacked by a much smaller population of young men.  They were, indeed, attacked, but I suspect that it was lack of babies that caused the local violence.

Of course when they moved as extended families, their fertility would have rapidly risen as they would have tended to remain isolated from the strangers they went among.  So some of those thousands who seem to have moved away may in fact reflect a higher birth rate in their new digs.

It is, as we radiologists are wont to say, “Consistent with but not diagnostic of” the diagnosis.

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