"Australian Aborigines, Papuans and Melanesians were relatively isolated after the early dispersal along the southern route," study lead author Hugo Reyes-Centeno, of the University of Tübingen, said in a statement. Other Asian populations apparently descended from members of the later northern wave of migration, the researchers said.
The delay between these waves of migration could be due to ancient environmental factors, "specifically climatic conditions that might have impeded the crossing of the Arabian Peninsula, such as desert conditions," Harvati said.
Ancient environmental factors might not only have prevented migrations, but also spurred them, Havarti said.
"For example, the documentation of severe droughts throughout eastern Africa between about 75,000 to 135,000 years ago could have encouraged a dispersal into other parts of Africa as well as outside of the continent," Harvati said. "More favorable conditions within Africa could have limited migrations out of the continent between 75,000 to 50,000 years ago."
The researchers cautioned that interbreeding between modern humans and other lineages of humans might influence the results of this new study. For example, instances of interbreeding with the now-extinct Denisovan lineage might have introduced ancient genes into certain modern human groups, perhaps making them look as if they left Africa earlier than they actually did. [Denisovan Gallery: Tracing the Genetics of Human Ancestors]
"Our study did not specifically test for hybridization with archaic humans, and, of course, it is possible that such admixture could contribute to our results," Harvati said. "We feel, however, that the very low levels of admixture that have been proposed are not sufficient to drive our findings."
The researchers said continued fieldwork and genetic advancements might help confirm this model of multiple, relatively isolated waves of migration.
"The story of human evolution tends to be simplified," Harvati said. "However, more complex models, such as multiple dispersals versus a single dispersal out of Africa, gain strength as more data and new methods become available."
"Further fieldwork in the region of the southern route — for example, the Arabian Peninsula, southeast Asia, Melanesia — is essential in order to further understand the timing and route of early modern human dispersals," Harvati said. "Of course, this is a vast geographical space that has been largely understudied, but it is crucial in developing our knowledge of the first Eurasians."
The scientists detailed their findings online April 21 in the journal Proceedings of the National Academy of Sciences.
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