"Archaic" humans interbred with anatomically modern humans in Africa in the last 20,000 to 60,000 years and some of their DNA exists today, researchers said.

Previous studies primarily examined modern human interbreeding with Neanderthals in Europe or other archaic forms in Asia, but a study by University of Arizona geneticist Michael Hammer is the first to definitively suggest interbreeding between separate human forms inside of Africa, a National Science Foundation release said Wednesday.

"It appears some level of interbreeding may have occurred in many parts of the world at different times in human evolution," Hammer, said, adding "Given recent fossil evidence, however, the greatest opportunity for introgression was in Africa" where anatomically modern humans and various archaic forms co-existed for much longer than they did outside of Africa.

Introgression is the movement of genes from one species into the gene pool of another.

"The populations that interbred were similar enough biologically so that they were able to produce fertile offspring, thus allowing genes to flow from one population to the other," he said.

Scientists say they believe contemporary African populations contain a small proportion of genetic material -- about 2 percent -- that moved from a species of archaic humans into the gene pool of anatomically modern humans about 35 thousand years ago.

"This study represents an approach to answering long-standing questions about the contributions of archaic, extinct forms of our genus to the gene pool of our modern human species," said Carolyn Ehardt, program director for biological anthropology at the foundation.