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April 29, 2009

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Multiple out-of-Africa migrations seen for early humans

March 23, 2009
World Science staff

Fos­sil ev­i­dence sug­gests that ear­ly, an­a­tom­ic­ally “mod­ern” hu­mans may have split in­to many iso­lat­ed popula­t­ions be­fore leav­ing Af­ri­ca in a se­ries of migra­t­ions, sci­en­tists re­port.

Sci­en­tists gen­er­ally be­lieve hu­mans evolved in Af­ri­ca and from there spread out to oth­er re­gions, start­ing over 60,000 years ago.

In new re­search, Ger­hard We­ber of the Uni­ver­s­ity of Vi­en­na and col­leagues used ge­o­met­ric pat­terns of fos­sil­ized skulls found in var­i­ous parts of Af­ri­ca to com­pare the di­vers­ity among early rep­re­sen­ta­tives of the ge­nus Ho­mo, the ev­o­lu­tion­ary group to which hu­mans be­long.

The re­search­ers con­clud­ed that, rath­er than a sin­gle out-of-Af­ri­ca dis­per­sal, their ev­i­dence shows early mod­ern hu­mans in Africa were di­vid­ed in­to dif­fer­ent popula­t­ions by the Pleis­to­cene, the era from about two mil­lion to 11,000 years ago.

Skulls from early mod­ern hu­mans showed the great­est varia­t­ion in shape over the last 1.8 mil­lion years, in­di­cat­ing that early mod­ern hu­mans had split in­to mul­ti­ple, tem­po­rarily iso­lat­ed popula­t­ions, the re­search­ers ex­plained.

Af­ter that break­up there seems to have fol­lowed a com­plex migra­t­ion pat­tern in which dif­fer­ent popula­t­ions left the con­ti­nent left at dif­fer­ent times, they added. They re­port that the skull shapes of early mod­ern hu­mans most closely re­sem­bled those found in lat­er hu­mans out­side of Af­ri­ca, pro­vid­ing a link be­tween some iso­lat­ed Af­ri­can popula­t­ions with lat­er migra­t­ion.

Un­der­stand­ing the di­vers­ity of an­a­tom­ic­ally mod­ern hu­mans in Af­ri­ca be­fore the migra­t­ions is cru­cial to any anal­y­sis of mod­ern hu­man ori­gins, the re­search­ers wrote in a pa­per de­tail­ing their find­ings. “The Af­ri­can con­ti­nent de­serves more at­ten­tion in the mod­ern hu­man ori­gins de­bate,” they added in the pa­per, to ap­pear in this week’s early on­line edi­tion of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

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Fossil evidence suggests that early, anatomically “modern” humans may have split into many isolated populations before leaving Africa in a series of migrations, scientists report. Scientists generally believe humans evolved in Africa and from there spread out to other regions, starting over 60,000 years ago. In new research, Gerhard Weber of the University of Vienna and colleagues used geometric patterns of fossilized skulls found in various parts of Africa to compare the diversity among early representatives of the genus Homo, the evolutionary group to which humans belong. The researchers concluded that, rather than a single out-of-Africa dispersal, their evidence shows early modern humans were already divided into different populations in Pleistocene Africa, the period from about two million to 11,000 years ago. Skulls from early modern humans showed the greatest variation in shape over the last 1.8 million years, indicating that early modern humans had split into multiple, temporarily isolated populations, the researchers explained. After that breakup there seems to have followed a complex migration pattern in which different populations left the continent left at different times, they added. They report that the skull shapes of early modern humans most closely resembled those found in later humans outside of Africa, providing a link between some isolated African populations with later migration. Understanding the diversity of anatomically modern humans in Africa before the migrations is crucial to any analysis of modern human origins, the researchers wrote in a paper detailing their findings. “The African continent deserves more attention in the modern human origins debate,” they added in the paper, to appear in this week’s early online edition of the research journal pnas.