DNA studies have already proved that ancestors of modern humans got intimate with Neanderthals in Europe and another Asian relative called the Denisovans.
Now, researchers found that our earliest ancestors had sex with a hitherto unknown human lineage and it may date to beyond the point when anatomically modern humans first emerged 200,000 years ago.
The species who may have contributed to the modern gene pool include Homo erectus, the upright walking man, and the "tool-using man" Homo habilis, the researchers said.
Lead researcher Michael Hammer, from the University of Arizona, said it looks like our lineage has always exchanged genes with their more morphologically diverged neighbours.
"We think there were probably thousands of interbreeding events. It happened relatively extensively and regularly," Hammer was quoted as saying by LiveScience.
For their research, published in the journal Proceedings of the National Academy of Sciences, Hammer and his colleagues sequenced about 60 regions of the human genome that apparently have no function.
These genes are less subject than functional DNA to change as a result of recent evolutionary pressures driving the survival of the fittest; in such a way, they can give a clearer view of how populations might have mixed or not in the past.
The team focused on three populations that presented a good sample of the geographic and cultural diversity of sub-Saharan Africa -- Mandenka farmers in western Africa, Biaka Pygmies in westcentral Africa, and San Bushmen of southern Africa -- looking for unusual patterns that suggested ancient interbreeding with other lineages.
They discovered strong evidence of genetic mixing in the Biaka and San. By comparing these sets of genes with those from comparable modern human ones, they estimated the unusual genes may have come from a lineage that first diverged from the ancestors of modern humans about 700,000 years ago.
For context, the Neanderthal lineage diverged from ours within the past 500,000 years, while the first signs of anatomically modern human features appeared only about 200,000 years ago.
"The populations that interbred in Africa were on a similar scale of divergence as the expanding modern population and Neanderthals were outside of Africa," Hammer said.
"They were similar enough biologically so that they were able to produce fertile offspring, thus allowing genes to flow from one population to the other."
The length of exotic haplotypes (a set of DNA sequences) from this extinct lineage suggests interbreeding might still have occurred until as recently as 35,000 years ago, he added.
