October 21, 2007

From Fred Colbourne:

Re: Trip to beach a milestone in human evolution: study (Oct. 17): The Mossel Bay finds dated 164,000 years ago and other later finds near shorelines around 125, 000 BP do suggest the presence of modern humans. These finds fit the time-frame provided by genetic evidence indicating that modern humans evolved earlier than 100,000 years ago and later than 200,000 years ago.

But is it plausible that the modern people identified at Mossell Bay left any descendants? Two problems stand in the way of plausibility, both related to climate change. The period 164,000 years ago was followed by colder and drier conditions for about 40, 000 years. The later finds were followed by the depths of cold and drought of the last glaciation. This can be seen clearly from the data presented by Martrat and others (Science 317, 502 (2007). Data from lake Malawi and other lakes illustrate the same point. So did these people have enough fresh water to survive millenia of drought?

Did the Mossel Bay people did have enough water to survive until the the warm interglacial that started around 125,000 years ago, the one before the one in which we now live? That’s the problem and why the inferred bottleneck in human population around 70, 000 years ago is so important. At the time of the bottleneck, the climate plunged to become cold and dry, some say triggered by the eruption of Mount Toba, an event that probably only made matters a little worse and only for about 5 years or so. In any event, the genetic evidence has been interpreted as a bottleneck occurring 70,000 years ago, when the “effective” size of the human population declined to about 10,000 pairs of adult men and women. The actual total population may have been 100,000 people or more, spread over an area larger than a million square kilometers. Until recently, Ethiopia was a possible candidate refuge area, but recent evidence from Lake Malawi now points to the big African lakes as possible refugia. Unfortunately, lake levels may be slightly higher now than 164,000 years ago.

Nevertheless, the coastal areas of some African lakes may be the key to puzzle of modern human origins. Louis Leakey and his generation were not so specialized that they did not take into account paleogeography, something that seems to have been lost today.

Fred Colbourne


Post a Comment

<< Home