“Modern humans have been around for at least 160,000 to 200,000 years but there is no archaeological evidence of any technology beyond basic stone tools until around 90,000 years ago. In Europe and western Asia this advanced technology and behaviour explodes around 45,000 years ago when humans arrive there, but doesn’t appear in eastern and southern Asia and Australia until much later, despite a human presence. In sub-Saharan Africa the situation is more complex. Many of the features of modern human behaviour — including the first abstract art — are found some 90,000 years ago but then seem to disappear around 65,000 years ago, before re-emerging some 40,000 years ago.[High Population Density Triggers Cultural Explosions]
So why did those cultural and technological explosions happen at those particular moments in time? Was it a boost in brain power or advance in language? It turns out that the answer is population density.
The research, which is published in the June 5 issue of the journal Science, suggests that tens of thousands of years ago, as human population density increased so did the transmission of ideas and skills. The result: the emergence of more and more clever innovations. [Party Animals: Early Human Culture Thrived in Crowds – Yahoo! News]
Using genetic estimates of population size in the past, the team went on to show that density was similar in sub-Saharan Africa, Europe and the Middle-East when modern behaviour first appeared in each of these regions. The paper also points to evidence that population density would have dropped for climatic reasons at the time when modern human behaviour temporarily disappeared in sub-Saharan Africa.[High Population Density Triggers Cultural Explosions]
Here is the original paper