The Appearance of Modern Behavior:
There is a recent paper, Late Pleistocene Demography and the Appearance of Modern Human Behavior by Adam Powell, Stephen Shennan and Mark G. Thomas, SCIENCE vol 324 no 5932 June 5, 2009 page 1298, I find very interesting.  It turns out that there is a package of accomplishments, such as art, body decoration, systematic use of small stone tools, functional and ritual objects of bone and stone and ivory, stone grinding and pounding, better hunting and trapping technology, long distance transfer of raw materials, and recognizable musical instruments, that together mark the stage of human development called the Late Stone Age, or LSA.  The earliest LSA finds come from various places Africa sporadically starting about 90,000 years ago and possibly as far back as 160,000 years ago.  Then they disappear between 75,000 years ago and 60,000 years ago and then disappear there until about 40,000 years ago.  By that time they had emerged in Europe about 45,000 years ago.  Other places followed.

The explanation offered by Powell et al is that it takes a certain density of population to support this modern behavior.  As a skill is handed from person to person, the transfer is not perfect and later practitioners are less and less adept until the skill vanishes.  Occasionally someone makes an improvement, which everyone in contact then adopts, and the process starts over.  If the population is small and isolated, skills vanish.  If the population is sufficiently large and connected to other populations, skills advance.  This explanation they support with ample data, and it is hard to quarrel with it. 

An enigma they point out is that such skills vanished in Africa by about 60,000 years ago at a time when there was no precipitous decline in population, although there was some environmental stress.  The LSA package presumably made it easier to survive and should have encouraged denser populations and a more secure hold on the technology. 

So what happened?

As I have pointed out on this web site, big populations collapse for genetic reasons.  The very population size and connection with other populations that made it possible to achieve and maintain the LSA package also predisposed the community of the skilled to low birth rate.

All that is necessary to account for the finding is to assume that not absolutely everybody bought into the new way of doing things.  Sub populations that were less dense and less connected would have retained or have reverted to the cultural level that existed before the Late Stone age.  Then when the environmental stress occurred, the LSA people, with their low birth rate, would have been forced over the edge.  The others continued in substantially the same numbers and rather rapidly expanded into the areas previously boasting the LSA package. 

In fact, the scattered and episodic distribution of LSA skills during the long period from 90,000 years ago and possibly before down to as late as 60,000 years ago could be accounted for by the same process.

So how does one account for the success in Europe?  The short answer is that they managed to reach the requisite numbers.  But the LSA package tended to emerge rather slowly in most places.  In Europe the whole package evolved quickly, and once they had it they never looked back.  If the modified demographic argument – that a larger interacting community can develop and maintain a technology but at the cost of a lowered birth rate – can account for the relative failure in Africa, it must also account for the relative success in Europe. 

In Europe at the time, the Cro-Magnon anatomically modern humans were expanding at the expense of the Neanderthals.  The moderns were expanding from a very small founder population, while the Neanderthals had been at equilibrium for a very long time.  If the high fertility from the less diverse population effect works, it should have worked the same in Europe as it did for the pioneers in the early United States.  The younger more homogeneous population simply swamped the old diverse one with an irresistible flood of babies.  This led to a dense enough population to permit the rapid adoption of the LSA package.

Of course that leaves another puzzle.  How did the Europeans manage to subdivide themselves enough to postpone the inevitable collapse until so recently?  Maybe they got lucky on geography.  While Europe is the most genetically homogenous population of its size (So I am told.  I can’t help wondering about the Han Chinese.), it is also greatly subdivided.  Although there are vast habitable areas, they tend to be chopped up by the fact that Europe is a collection of islands and peninsulas, is subject to severe winters to hamper travel, has had impenetrable forests and has formidable mountain ranges.  The high rainfall might mean that even rivers, generally the highways of pre industrial societies, could also have been barriers. 

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