We humans already had a clouded sexual history before we came to Europe and slept around with Neanderthals.
We also interbred with other species of hominins back in the cradle of Africa, according to new DNA research led by geneticist Michael Hammer of the University of Arizona.
Anthropologists have long pondered how anatomically modern humans - homo sapiens - won the evolutionary battle to survive as the only hominin species, and whether that development involved breeding with other humanlike species known to have existed.
Recent studies that examined DNA extracted from Neanderthal remains concluded that humans did interbreed to some extent with Neanderthals and other extinct species after migrating from Africa into Europe and Asia.
Such DNA samples are not available from African populations of early humans.
Hammer and his colleagues decided to look for evidence of interbreeding in Africa by examining the DNA of living populations. They used mathematical models to predict what that evidence might look like, and then searched nonfunctional areas of the human genome for telltale signs of DNA that had been diminished by generations of recombination.
Results were published Monday in the Proceedings of the National Academy of Science.
The researchers found evidence for interbreeding in three African populations - the Mandenka, the Biaka Pygmies and the San, as recently as 35,000 years ago.
The groups were chosen because their relative isolation made them better candidates for preserving genetic diversity, the study reported.
The authors say the results suggest "relatively recent interbreeding with hominin forms that diverged from the ancestors of modern humans ..."
"Anatomically modern humans were not so unique that they remained separate," Hammer told UA News. "They have always exchanged genes with their more morphologically diverged neighbors. This is quite common in nature, and it turns out we're not so unusual after all."
Hammer's co-authors were August Woerner of the UA's Arizona Research Laboratories; Fernando L. Mendez of the Department of Ecology and Evolutionary Biology; Joseph C. Watkins of the Mathematics Department; and Jeffrey D. Wall of the University of California's Institute for Human Genetics.
Hammer is an interdisciplinary research scientist who holds appointments with Arizona Research Labs, the Department of Ecology and Evolutionary Biology, the School of Anthropology, the BIO5 Institute and the Arizona Cancer Center.
Contact reporter Tom Beal at firstname.lastname@example.org or 573-4158.