Two new studies undermine “over-simplistic models of human evolution”

by Mar 18, 2016 1:03am GMT

The relationship between early human groups was incredibly complicated.

You’ve probably heard the story about how Neanderthals were living in Europe for hundreds of thousands of years, when suddenly a bunch of Homo sapiens came pouring out of Africa about 70 thousand years ago. Thirty thousand years later, pretty much all the Neanderthals were dead. Many anthropologists believe that Homo sapiens killed off our large-browed cousins in a quest to dominate the Eurasian continent. But over the past 10 years, that view has changed radically thanks to new techniques for sequencing ancient DNA.

Now, two new studies make it even less likely that modern humans killed off the Neanderthals. Instead, we interbred with them at least three separate times, and our ancestors were likely sharing tools with them half a million years ago.

A mysterious common ancestor

Writing in Nature, a team of scientists recount how they carefully sequenced the DNA from a mysterious group of 430-thousand-year-old humans found in Sima de los Huesos, a cavern in Spain’s Atapuerca mountains. Thanks to careful preservation of the remains, they were able to extract both mitochondrial and nuclear DNA, allowing them to analyze genetic contributions from the group’s mothers and fathers. What they discovered has upended the classic story of how Neanderthals got to Europe, and when. The Sima hominins are clearly early Neanderthals, living in Spain far earlier than expected.

Paleolithic sexytimes reveal that Homo sapiens made it out of Africa earlier than we thought.
Previously, scientists believed the last common ancestor of Neanderthals and modern humans was Homo heidelbergensis, an early human who lived about 700 to 200 thousand years ago. But the Sima hominins reveal that humans and Neanderthals must have diverged between 550 thousand and 765 thousand years ago, a timeframe that eliminates H. heidelbergensis as a possible progenitor. Paleoanthropologist Maria Martinón-Torres told Nature News that the common ancestor of Neanderthals and modern humans probably lived about 900 to 700 thousand years ago. One possibility is Homo antecessor, whose 900-thousand-year-old remains have been found in Spain.
Making things more confusing, the Sima humans have mitochondrial DNA that appears to be from Denisovans, another group of early humans that settled in Europe hundreds of thousands of years before modern humans did. Writing in Nature News, Ewan Callaway explains that Matthias Meyer, an author on the new study, “now favors the hypothesis that an as yet unknown species from Africa migrated to Eurasia and bred with Neanderthals, replacing the mitochondrial DNA lineages. (Supporting this idea, stone-tool technologies spread from Africa to Eurasia around half a million years ago, and again 250,000 years ago).” In other words, these early humans leaving Africa weren’t killing Neanderthals; they were sharing tools and families with them.

More Neanderthal sex than ever

Even tiny groups of humans would have had the tech to out-compete Neanderthals.
Meyer’s findings fit nicely with other recent studies, which show that even when modern humans left Africa, they weren’t killing their distant human cousins. Instead, they had children with Neanderthals and Denisovans, in several distinct events. If the meeting between humans and Neanderthals had been genocidal, it’s unlikely we would see patterns that show Neanderthal and Denisovan DNA enter the modern human genome multiple times. A paper that has just come out in Science explains how modern human DNA contains clear evidence that our ancestors got busy with Neanderthals and Denisovans at least three different times in the past 100 thousand years.
The researchers analyzed DNA taken from 1,523 people spread across the globe, looking for traces of Neanderthal and Denisovan DNA sequences. What they found was that different populations of people had distinct, different collections of DNA from Neanderthals. They write:

Collectively, these data suggest Neanderthal admixture occurred at least three distinct times in modern human history. Although most South Asian populations show shared histories of archaic admixture, we find significant evidence of differential Neanderthal admixture between some European and East Asian populations.
In other words, modern humans didn’t sweep out of Africa, killing everything in their paths. They settled down with the locals, many different times. Evolutionary biologist Carles Laleuza-Fox, who was not involved in the study, told The New York Times’ Carl Zimmer, “This is yet another genetic nail in the coffin of our over-simplistic models of human evolution.” These papers also testify to how long different groups of humans have been intermingling, sharing ideas and hearths. Even though humans are notorious for hating and killing strangers, there’s no denying that migration is written into our DNA, as well as a history of embracing people who are different.

Nature, 2016. DOI: 10.1038/nature17405

Science, 2016. DOI: 10.1126/science.aad9416

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