One of the more startling discoveries arising from genomic sequencing of ancient hominin DNA is the realization that all humans outside Africa have traces of DNA in their genomes that do not belong to our own species.

The approximately six billion people on Earth whose recent ancestry is not from Africa will have inherited between 1 percent and 2 percent of their genome from our closest but now extinct relatives: the Neanderthals. East Asians and Oceanians have also inherited a small amount of ancestry from the Denisovans, another close relative of Homo Sapiens.

Now a new study, published in Science Advances, suggests that early humans living inside Africa may also have interbred with archaic hominins. These are extinct species that are related to Homo sapiens.

The interbreeding outside Africa happened after our Homo sapiens ancestors expanded out of Africa into new environments. It was there they had sex with Neanderthals and the related Denisovans.

This led to new discoveries. Early genetic studies of people from across the globe had previously suggested that our current distribution was the result of a single expansion out of Africa around 100,000 years ago. But the identification of Neanderthal and Denisovan ancestry in modern Eurasians complicated things.