German neanderthal thigh bone
© Oleg Kuchar/Museum Ulm
German neanderthal thigh bone
It was big news, the announcement in Nature Communications that mitochondrial DNA recovered from a German Neandertal thigh bone believed to be more than 120,000 years old was (1) from a modern Homo sapiens (or almost-modern Homo sapiens), and (2) entered the Neandertal gene pool more than 270,000 years ago.

"This Thigh Bone Could Force Us To Completely Rewrite The History Of Our Species" is the headline on Kristy Hamilton's post at IFLScience. (I see that this long-lived science blog has turned its original attention-getting name into a decorous acronym.)

It was big news because, if confirmed, it means that the early direct ancestors of all 7.5 billion of us anatomically modern humans, the last Homo standing, entered Eurasia much earlier than current theory holds. Current theory being that we emerged from Africa relatively recently, only 60,000 or 70,000 years ago.

But hardly any of the commentary spoke of another major reason it was big news: it calls into question another central thesis of human evolution, which is that we came out of Africa at all.

DNA Complexities

Let's get some complexities out of the way first. Recovering DNA from fossils is a bear, and the talented people who are able to do it deserve our warm gratitude and then some. Often it means not getting DNA from a cell's nucleus at all, although that's the DNA comprising nearly all an organism's genes, the DNA we think of as "the genome."

Instead, fossil DNA studies may yield only DNA from the mitochondria, the cell's energy structures, which contain a tiny amount of DNA (and only a few genes.) The reason for this is simple and numerical. A cell contains only one nucleus, but often hundreds of mitochondria. (More about mtDNA in my recent post about the evolution and domestication of cats.)

The thigh bone, recovered from the Hohlenstein - Stadel (HST) cave in southwest Germany, had been gnawed by a carnivore and has been subject to much contamination from other human DNA since its discovery 80 years ago. The scientists at the Max Planck Institute for the Science of Human History in Jena, Germany, could recover only mtDNA from it. But what they recovered was mtDNA of a particular kind that is common in Neandertal fossils but also very similar to one type of modern human mtDNA. (The researchers are still trying to get nuclear DNA from the bone, according to Rachel Becker at The Verge.)

Neandertal and modern human lineages split between 765,000 and 550,000 years ago, according to GenomeWeb. But the Max Planck researchers believe the modern-humanlike mtDNA from the thigh bone entered the Neandertal line (based on estimated mutation rates) more than 220,000 years ago, possibly as long ago as 460,000 years.

Unlike the DNA in the nucleus, which comes from both mothers and fathers, mtDNA is inherited only from mothers. So that means much Neandertal mtDNA was bequeathed to our kissin' cousins from a modern human woman, or one who was almost nearly a modern human woman.

She mated with a Neandertal man much longer ago than most Neandertal-modern human interbreeding happened - which was significantly later, after the postulated recent migration from Africa.
(And there seems to have been a lot of it, given what both Neandertal and modern human nuclear genomes look like. "Everyone knows Neanderthals gave us genes," first author Cosimo Posth told Aylin Woodward at New Scientist.)

Their hybrid daughter (or daughters?) probably looked a bit un-Neandertalish, but remained among the Neandertals. She passed her mitochondrial genome to her daughters, who passed it to their daughters. (Her mtDNA was inherited by sons too, of course, but sons didn't pass it on.) And so on for many thousands of years. Until it landed in the bones of a Neandertal who lived in what is now Germany more than 120,000 years ago.

Perhaps Not Out Of Africa After All?

So modern humans (or very early modern humans) apparently interbred with Neandertals much earlier than previous estimates. Because Neandertals apparently never got to Africa, that means modern humans etc left Africa much earlier than previous estimates, well before 70,000 years ago.

Dienekes Pontikos and others have for years been arguing for earlier modern human migration. In his commentary on this new paper at Dienekes' Anthropology Blog he goes further, not for the first time. He says that the data favor the idea that the common ancestor of modern humans and Neandertals came, not out of Africa, but out of Eurasia. "It seems to me that this hypothesis, that Modern Humans and Neandertals stem from a non-African ancestor (a non-African population of H. heidelbergensis*, for example), has much to recommend it."

Last year at Gene Expression, commenting on a different Dienekes post that raised this genesis proposal, Razib Khan voted for a sub-Saharan Africa origin of modern humanity on grounds that it was more parsimonious. That's partly because Africans are more diverse genetically than any other humans, a deep structure that dates back at least 200,000 years. But, he acknowledged, "Both views match the data." So a Eurasian origin for Homo sapiens - Khan thinks in the Near East - may be true after all. "Time will tell."

For more on the exceedingly jumbled state of ideas about human evolution, see my recent post on what might be the oldest modern human fossils, dated at more than 300,000 years ago. To add to the jumble, they were found not in the place conventionally viewed as our Garden of Eden, sub-Saharan Africa, but in North Africa.

*Note on H. heidelbergensis: Despite the name (because the first fossil was found more than a century ago near, you guessed it, Heidelberg in Germany), a number of anthropologists and paleontologists believe H. heidelbergensis (Dates: 600,000-100,000 years ago) lived in Africa and western Asia as well. The National Museum of Natural History says this human was the first to control fire, use spears, hunt large animals, build shelters. PBS's Evolution site says these claims are disputed. Of course they are; this is paleoanthropology after all.