Climate history in tropical Africa

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Climate history in tropical Africa

Postby Anonymous » Mon Oct 08, 2007 1:35 pm

Message sent to the mailing list of the Society of Africanist Archaeologists a few days ago. I thought it would be of interest here as well.

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Last year I sent a message to this list about the raising of a 380-m core from the center of Lake Malawi in 2005. Yesterday I heard a presentation by my colleague Andy Cohen, one of the leaders of the team, on the analysis of the top 70 m of the core, which spans the last 130,000 years. (This was just published as Christopher Scholtz (and 16 others!) , “East Africa megadroughts between 35 and 75 thousand years ago and bearing on early modern human origins”, Proceedings of the National Academy of Sciences, 4 September 2007).

The results, which are corroborated by similar results from Lake Tanganyika and Lake Bosumtwi (Ghana) are very surprising, and contradict much that has previously been assumed about the climatic background to the exodus of early modern humans in Africa. For one thing, there is no sign at all of volcanic ash in the core at ca. 70,000, so it seems very unlikely that the eruption of the massive volcano Toba in Indonesia at this time had any effect on humans in Africa (as argued by Stan Ambrose ). Secondly, Lake Malawi was clearly pretty full (how full they can’t tell) throughout the last 75,000 years. The last glacial maximum (at around 22,000 calibrated) doesn’t even register in the core records. Between 75,00 and 135,00 (the bottom of this section of the core) the center of Lake Malawi was reduced for much of the time to a shallow bowl, 50m or less in depth (i.e. about 550 m lower than its present level). The pollen spectrum at this time is wholly grass. Clearly tropical Africa was superarid for about 20,000 years. After 95, 000 there was a slow recovery, and the last 75,00 years have by comparison been quite humid. The suggestion is that perhaps human populations went through a severe population bottleneck cased by drought, and that rapid expansion of human populations probably dates after 75,000 years ago.



These finding also pose a huge problem for the use of “molecular clocks”. Much of the calibration of the evolution of the cichlid fishes of Lake Malawi (one of the hottest topics in molecular biology) has assumed that the adaptive radiation of the cichlids dates after the last glacial maximum (i.e. 18 – 15,000). This is quite clearly wrong – the recovery of Lake levels that lead to the adaptive radiation of cichlids must now be redated to around 95-75.000. I don’t know enough about the calibration of the molecular clocks for modern humans to know whether these findings have any implications for human evolution.



David Killick
Associate Professor
Department of Anthropology
University of Arizona
Tucson, AZ 85721-0030
U.S.A.
Anonymous
 

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