African Mega-Drought Rewrites Out of Africa

By Guest Column Joshua Hill —— Bio and Archives October 13, 2007

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The words 'tropical' and 'lush' are not necessarily the descriptive words you would normally associate with Africa, especially when presented with the choice of 'barren' and 'arid'. But the continent of Africa is not all desert and dust; at least, not any more.

Scientists who have been studying sedimentary cores from Lake Malawi have discovered that, 135,000 to 90,000 years ago, areas of Africa that would now be classified as tropical--such as Lake Malawi--were hit by a mega-drought. These droughts were so extreme and subsequently widespread that they were unlike anything the region had ever encountered previously. The importance of this discovery lends new weight to the Out of Africa model for human evolution, also known as the Recent Single Origin Hypothesis. The RSOH or Out of Africa hypothesis states that all human beings descended from one small evolutionary branch appearing in Africa between 200,000 and 100,000 years ago that then spread out from Africa 55,000 and 60,000 years ago. "We've got an explanation for why that might have occurred --tropical Africa was extraordinarily dry about 100,000 years ago," said Andrew S. Cohen, a University of Arizona in Tucson professor of geosciences. "Maybe human populations just crashed." The Lake Malawi Drilling Project was responsible for providing sedimentary cores for the scientists to examine. The project was landlocked due to the depth at which they would have to be drilling; a slightest movement would snap the drills. And at 2,316 feet (706 meters) in depth, and drilling a further 1247 feet (380 meters) in to the sediment, snapping was a real possibility. The team was able to essentially use the cores as a timeline for Africa. "What's unique about the Malawi, Tanganyika and Bosumtwi cores is that they're continuous records. We can see what happened in one place over a long period of time," Cohen said. Such lake cores contain a high-resolution record of the things that fell in or died in the lake -- plankton, aquatic invertebrates, charcoal from fires on land, pollen from the surrounding vegetation. Scientists analyze those materials to figure out what the vegetation and the lake conditions were like at a particular point in time. The samples that were taken during the mega-drought period--time determined by radiocarbon and other dating techniques--was found to have little pollen or charcoal, indicating that there was very little vegetation and even less that would burn. Cohen said, "The area around Lake Malawi, which today is heavily forested and has rainfall levels comparable to the southeastern U.S., at that time would have looked like Tucson." In addition to ecological evidence, "Archaeological evidence shows relatively few signs of human occupation in tropical Africa during the megadrought period," added Cohen. Comparably, 70,000 years ago there is recorded evidence for Africa being much wetter. In addition, there is evidence of more people in the region and of populated movement northwards. This would suggest that when the human population in the area was provided with greener and wetter surroundings, they were once again able to breed without nature's harsh winds and climates decimating their populations. With increased populations would naturally come the need to spread out and seek greener pastures; so to speak. From there, the Out of Africa hypothesis springs, with populated movement tracked in to current locations such as North and South America, in through Asia, and matched with the arrival of humans in Australia 50,000 years ago. Joshua Hill, a Geek’s-Geek from Melbourne, Australia, Josh is an aspiring author with dreams of publishing his epic fantasy, currently in the works, sometime in the next 5 years. A techie, nerd, sci-fi nut and bookworm.