March 7, 2002. Copyright 2002. Graphic News. All rights reserved.

Humans emerged Òout of africaÓ again and again

LONDON, March 7, Graphic News: Modern humans emerged out of Africa in at least two major migrations and bred with populations of pre-humans they encountered, according to a leading evolutionary biologist.

New genetic analysis of human gene trees from different populations showed that, rather than evolving simultaneously in several parts of the globe as some scientists believe, humans migrated out of Africa and did not replace existing populations but mixed with them.

Dr. Alan Templeton and Professor Charles Rebstock of Washington University in St. Louis, Missouri, used a computer program called GEODIS to analyse human genetic trees for mitochondrial DNA -- genetic material which passes unchanged from mother to child -- Y-chromosome DNA inherited from the father, and DNA from eight other regions of the human genome, to reconstruct the movement and history of humans.

The evidence shows an exodus out of Africa between 420,000 and 840,000 years ago and a more recent migration between 80,000 and 150,000 years ago. Africa has played a dominant role in shaping the modern gene pool through successive population expansions, writes Templeton in the March 7 issue of the journal Nature.

ÒHumans expanded again and again out of Africa but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world,Ó Templeton said.

Although fossils have yet to be found, scientists believe the human and ape lines diverged around 6 million to 4 million years ago. The secrets of our earliest ancestors are preserved in fossilized bones -- a 3.2-million-year-old partial skeleton nicknamed ÒLucyÓ (Australopithecus afarensis), found in Ethiopia in the 1970s; and a 3.5 million-year-old skull (Kenyanthropus platyops) found last year in Kenya. 

Meave Leakey, a member of the famed fossil-hunting Leakey family, said either of these species could be the early direct ancestor of humans.

In 1997 came the dramatic discovery of a hominid that could be the evolutionary Òmissing linkÓ between these early species and the first humans. Fossil evidence shows that Australopithecus garhi, also found in Ethiopia, may have used stone tools and eaten meat.

Many scientists believe that it was Homo erectus that became the direct ancestor of our own species. H. erectus lived between 1.7 million and 250,000 years ago, may have used fire, and was the first hominid to emigrate from Africa, spreading all the way to China and Indonesia.

Templeton is convinced that these wanderers bred with the people they encountered, rather than replace them, in a Òmake-love-not-warÓ scenario -- particularly during the most recent migration because older gene signatures had not been wiped out.

His findings are somewhere between the two dominant theories of human evolution. The multi-regionalist school believes humans evolved around the world at about the same time, while the Òout of AfricaÓ theory suggests modern humans emerged from Africa and replaced earlier humans.

ÒHumans expanded again and again out of Africa,Ó Templeton concludes, Òbut these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world.Ó

/ENDS
Source: Nature, Scientific American, Time