Humans interbred with a now-extinct lineage of humanity before leaving Africa as per the Proceedings of the National Academy of Sciences report.

Modern humans are now the only surviving lineage of humanity. Others, who once roamed the Earth, made their way out of Africa before humans did. This includes the familiar Neanderthals in West Asia and Europe and the newfound Denisovans in East Asia.

Genetic analysis of the fossils of these extinct lineages has revealed they once interbred with modern humans, forming unions that may have endowed our lineage with mutations that protected them as they began expanding across the world about 65,000 years ago.

Researchers analyzing the human genome find evidence that our species hybridized with a hitherto unknown human lineage even before leaving Africa, with approximately 2 percent of contemporary African DNA perhaps coming from this lineage. In comparison, recent estimates suggest that Neanderthal DNA makes up 1 percent to 4 percent of modern Eurasian genomes and Denisovan DNA makes up 4 percent to 6 percent of modern Melanesian genomes.

Michael Hammer, a population geneticist at the University of Arizona in Tucson, and his colleagues gathered DNA samples from the Center for the Study of Human Polymorphisms in Paris and sequenced about 60 regions of the human genome that apparently have no function.

The investigators focused on three populations that presented a good sample of the geographic and cultural diversity of sub-Saharan Africa. They were Mandenka farmers in western Africa, Biaka Pygmies in west-central Africa and the San Bushmen of southern Africa. Researchers looked for unusual patterns that suggested ancient interbreeding with other lineages. This included a search for long haplotypes, or sets of DNA sequences, not seen in other modern human groups, the idea being that while short haplotypes could potentially be explained by a few chance mutations within these modern human populations, comparatively long haplotypes were instead likely inherited from a significantly different lineage.

The researchers were able to discover particularly strong evidence for genetic mixing in the Biaka and San, in the form of a trio of unusual haplotypes. By comparing these sets of genes with those from comparable modern human ones, the investigators estimated that the unusual genes may have come from a lineage that first diverged from the ancestors of modern humans about 700,000 years ago.

In the future, Hammer's team wants to look at the entire genome sequences of several modern human groups in Africa to get a better picture of how interbreeding might have occurred.