Wednesday, December 16, 2009

Shellfish & the human bottleneck   posted by Razib @ 12/16/2009 06:02:00 PM

How shellfish saved the human race:
Turns out, somewhere between 130,000 to 190,000 years ago, the human species was reduced to less than 1000 breeding individuals--just a few thousand people in total. Ancient, naturally driven climate change pushed our species to the brink, said Curtis Marean, Ph.D., a professor with the Institute of Human Origins and the School of Human Evolution and Social Change at Arizona State University.

What saved us? According to Marean, the answer may be "shellfish".

"They're a great source of protein," he said. "And shellfish are immune to colder ocean temperatures. In fact, when the water gets colder, those populations go up."

Marean used climate models to pinpoint locations in Africa where human hunter-gatherers could have hunkered down during a long glacial period that dried out the continent and expanded deserts. Of the four-to-six possible locations, he focused in on an area along the coast of South Africa.

This is probably wrong. Though one of the many ideas about this period in human history have to be right. And there's always the tricky problem of falsification and testing alternative hypotheses when it comes to models of human evolution. From what I know humans are a relatively homogeneous species which underwent some sort of demographic expansion within the last 100,000 years. The tacit assumption seems to be that the proto-modern African lineage nearly went extinct, and then bounced back, possibly due to an exogenous shock. My own question: why couldn't it just be that a few bands simply exterminated or marginalized all the other African lineages? This is an assumption by many about modern humans as they expanded into Eurasia, more or less, so why not in Africa? Humans can certainly reproduce up to the Malthusian limit. In 1800 1.2 million humans were resident in New England, the vast majority of whom were descended from the 20,000-30,000 who arrived in the 1630s. And remember that the reproductive population is going to be a fraction of the census size.

I could speculate on what gave a small subset of African humans an advantage, but I'll leave that to the comments.