Neandertal/Denisovan Ancestors Interbred With Unknown Hominin

As if the tangle of early archaic Homo sapiens relationships couldn't get any more confusing, evidence has now surfaced that the ancestors of both the Neandertals and Denisovans interbred with a hominin only known from its DNA signature.  From the University of Utah, through Science Daily:

For three years, anthropologist Alan Rogers has attempted to solve an evolutionary puzzle. His research untangles millions of years of human evolution by analyzing DNA strands from ancient human species known as hominins. Like many evolutionary geneticists, Rogers compares hominin genomes looking for genetic patterns such as mutations and shared genes. He develops statistical methods that infer the history of ancient human populations.

According to the article, Rogers performed a study that argued that Neandertals and Denisovans separated earlier than has previously been suggested but that his evidence for this was thin.

The new study has solved that puzzle and in doing so, it has documented the earliest known interbreeding event between ancient human populations -- a group known as the "super-archaics" in Eurasia interbred with a Neanderthal-Denisovan ancestor about 700,000 years ago. The event was between two populations that were more distantly related than any other recorded. The authors also proposed a revised timeline for human migration out of Africa and into Eurasia. The method for analyzing ancient DNA provides a new way to look farther back into the human lineage than ever before.

"We've never known about this episode of interbreeding and we've never been able to estimate the size of the super-archaic population," said Rogers, lead author of the study. "We're just shedding light on an interval on human evolutionary history that was previously completely dark."

According the Rogers, the DNA evidence puts the final nail in the coffin of the complete Out-of-Africa replacement model of modern human origins:

The researchers also proposed there were three waves of human migration into Eurasia. The first was two million years ago when the super-archaics migrated into Eurasia and expanded into a large population. Then 700,000 years ago, Neanderthal-Denisovan ancestors migrated into Eurasia and quickly interbred with the descendants of the super-archaics. Finally, modern humans expanded to Eurasia 50,000 years ago where we know they interbred with other ancient humans, including with the Neanderthals.

This was likely something like Homo antecessor.  As is also true with the Chinese evidence, this evidence suggests that throughout human evolutionary history, there has never/rarely been a time when these groups of archaic and early modern Homo sapiens could not/did not interbreed. As J. Lawrence Angel once said “When two groups of people meet, they may fight, but they will always mate.”

The Science Advances article is open access. 

http://dx.doi.org/10.1126/sciadv.aay5483