The drying of East Africa’s climate since 5 Ma ago shifted the distribution of its ecosystems towards more widespread savannah. In the most general sense that probably created conditions for ape speciation towards an upright gait and the potential for tool-using and growing consciousness that palaeoanthropologists visualize at the core of human evolution. The apparently dominant influence of North Atlantic circulation changes on climate fluctuations since then has suggested to many climatologists that the shift to glacial-interglacial and dry-humid cycles, at high and low latitudes, stems from some trigger for a fundamental shift in that circulation. The favoured process is the closure of open connection between Atlantic and Pacific Oceans when the Isthmus of Panama formed about 5 Ma ago. That transformed Atlantic circulation, and probably set in motion the Gulf Stream. However, there are several such gateways whose affects on ocean circulation link to plate movements.
One is the narrow passage between Indonesia and Australasia, which transfers Pacific water to the Indian Ocean. Subduction permits Australasia to move gradually northwards, thereby narrowing the gateway and also shifting it relative to the major currents in the tropical Pacific. Mark Cane and Peter Molnar of Columbia University and MIT have analysed the recent evolution of the Indonesian gateway (Cane, M.A. and Molnar, P. 2001. Closing of the Indonesian seaway as a precursor to east African aridification around 3-4 million years ago. Nature, v. 411, p. 157-162). Their findings suggest that the main flow switched from warm, South Pacific surface waters to cooler waters that originate in the North Pacific at about 4 Ma. Cooling of surface waters in the Indian Ocean would have reduced the amount of water vapour transferred to the air masses that are involved in the East African monsoons. The reduction in seasonal rainfall would have dried that area substantially. Though providing a plausible cause for regional climate change, the coincident transformations of two major ocean gateways adds greater complexity to the Plio-Pleistocene climate system. In terms of modern climate, the Indonesian gateway provides a means of understanding the teleconnection that seems to exist from correlation between drought-flood cycles in East Africa and the El Niño – Southern Oscillation in the tropical Pacific.
See also: Wright, J,D. 2001. The Indonesian valve. Nature, v. 411, p. 142-143.
Multiregionalists nailed by Y chromosome?
One of the big problems in using genetic material from living people to chart relatedness, and perhaps evolutionary origins, is simply getting the material. For the mitochondrial DNA studies that first hinted at a common African origin for all modern humans, the best material is placental tissue. A focus on male lineage using Y chromosomes is not so difficult; it can be done using blood samples. Nonetheless, a survey based on 12,127 samples from 163 population is a monumental achievement (Ke, Y. and 23 others 2001. African origin of modern humans in East Asia: a tale of 12,000 Y chromosomes. Science, v. 292, p. 1151-1153).
The significance of this study by a large team from China, the USA, Indonesia and Britain is that it focuses on the region most favoured by multiregionalists for the hypothetically separate descent of modern humans from ancient ancestors of Homo erectus stock in different parts of the Old World. The male chromosomes all carry evidence of mutations to a Y-chromosome marker that originated in Africa, abetween 35 to 89 ka ago. The huge mass of data from the whole of East Asia do not support even minimal contribution from any source other than one that originated in Africa around the time it is thought that fully modern humans began to leave in significant numbers.