New DNA evidence could explain what happened to the Neanderthals

One of the great mysteries of early human evolution is what happened to extinct hominin groups like the Neanderthals and Denisovans. These were human groups who lived in Europe and Asia for hundreds of thousands of years before Homo sapiens started streaming out of Africa and taking over the world. Now, an international group of scientists have completed a highly detailed analysis of DNA from what is estimated to be a 50-80,000 year old Denisovan finger bone. And it offers a glimpse at the genes that may have given modern humans an edge over our extinct counterparts. Published today in Science, this study also gives us a more complicated picture of early human relationships than we've ever seen before.

As early as 700,000 years ago, a group of humans left Africa and spread out across Europe and Central Asia. These people became Neanderthals and Denisovans, and the people they left behind in Africa became modern Homo sapiens. These groups were not different species, but rather different human groups who were separated from each other for hundreds of thousands of years. When modern Homo sapiens came trooping out of Africa about 100 to 80 thousand years ago, they would have looked very different from their Neanderthal and Denisovan cousins. Neanderthals were short and barrel chested, pale-skinned with large jaws and sloping foreheads. Denisovans, who lived exclusively in Asia, may have been darker skinned but we don't have enough fossil evidence to say for sure what their bodies would have looked like.

What is certain is that when the dark-skinned, tall Homo sapiens met the Neanderthals, it was probably like two alien groups meeting on Star Trek. By that I mean one group had head lumps in slightly different places than the other — but both were both humanoids, capable of bearing children together.

New DNA evidence could explain what happened to the Neanderthals

Max Planck Institute biologist Svante Pääbo worked with several other researchers on a new DNA sequencing technique that works especially well on old or degraded genetic material — just like what you'd find in a bone that might be as much as 80 thousand years old. So how does a Denisovan finger bone illuminate what happened to Neanderthals? The two groups are genetically close enough that they share a lot of evolutionary history, and so the researchers could use the DNA in this new study to measure the different fates met by Neanderthals, Denisovans, and modern humans.

One thing that stood out immediately to researchers were 23 known areas of the genome that modern humans do not share with either Neanderthals or Denisovans. As Pääbo put it yesterday at a press conference:

It's quite interesting for me to note that eight of them have to do with brain function and brain development, the connectivity in the brain of synapses between nerve cells function, and some of them have to do with genes that, for example, can cause autism when these genes are mutated. So I think that this is perhaps in the long term to me the most fascinating thing about this, what it will tell us in the future about what makes us special in the world relative to the Denisovans and Neandertals.

In other words, some of the genes that make us uniquely human are connected with our brain structure.

Bear in mind that what we call "human" today refers to people whose DNA is a mix of modern Homo sapiens as well as Neanderthal, Denisovan, and probably other early humans too. That means all of us possess some genes that our human cousins did not, and most of us possess some genes that African Homo sapiens did not 100,000 years ago.

New DNA evidence could explain what happened to the Neanderthals

Previous research has revealed that Neanderthal DNA can be found in the genomes of everyone who isn't of African extraction. But, as Pääbo said, "The Denisovans had contributed DNA only to people in Papua New Guinea, Fiji, Australia, and other places in Melanesia." In other words, modern humans entering Asia interbred with Denisovans. But the Denisovan DNA didn't wind up circulating to other areas of the world the way Neanderthal DNA did.

However, looking at the Denisovan genome allowed researchers to discern a greater amount of Neanderthal DNA in Asians and Native Americans than there is in Europeans. This suggests one of two things about how humans spread across the globe. First, it might mean that modern humans coming out of Africa formed families with Neanderthals in Europe, then their children slowly drifted to Asia. There, those mixed children formed families with other groups of Neanderthals in Asia, giving their children a higher percentage of Neanderthal DNA. Eventually, the offspring of these people traveled to the Americas, becoming the founder population for the peoples of North and South America.

A second possibility is that modern humans came to Europe out of Africa in two separate migrations. So they populated Europe, settling down with the Neanderthal locals. But then a new batch of modern humans arrived from Africa. When those new arrivals formed families with the mixed children of Neanderthals and Homo sapiens, it gave Europeans slightly more Homo sapiens DNA than other non-African people on Earth have today.

Regardless of what happened as different human groups came together in the lands beyond Africa, one thing remains certain. All of us share some genetic traits that set us apart from our hominin cousins. And a set of those traits could bear directly on how we think. "It makes a lot of sense to speculate that what had happened is about connectivity in the brain, because . . . Neandertals had just as large brains as modern humans had," Pääbo said. "Relative to body size, they had even bit larger brains [than Homo sapiens]. Yet there is, of course, something special in my mind that happens with modern humans. It's sort of this extremely rapid technological cultural development that comes, large societal systems, and so on." In other words, our brains weren't bigger than other hominins' brains. They were just wired differently.

Read the full paper published today in Science.