In Case You were Confused about the Out of Africa News

Many news outlets are running headlines this week like this one in the Times:

A Single Migration From Africa Populated the World, Studies Find

But others are running headlines like this:

Mysterious Branch of Humanity Possibly Discovered

That story goes on to explain that a "new, ancient" branch of humanity was in Australia before fully modern humans.

Both stores are based on the same group of three articles just published in Nature. Those articles describe studies that analyze large samples of human genomes from indigenous people around the world, 787 in all. They use mutation counts to estimate the date of the common ancestry for all those people. For the most part they come up with dates between 50,000 and 75,000 years ago. Also, they find that all the people outside of Africa are more closely related to each other than they are to most people in Africa, hence, they descend from a single African group.

But wait – what about the skeletons found in Israel, apparently fully modern humans, that have been dated to between 90,000 and 120,000 years ago? What about the tools kits from Saudi Arabia that look like those of fully modern humans and date to 100,000 years ago? Modern human teeth from China dated to 80,000 years ago?

Two of these studies found no evidence for any such people; if they existed, these scientists suggest, they simply went extinct without leaving any descendants. But didn't modern humans interbreed with Neanderthals and Denisovans? Um, yes, they did. And why would they breed with semi-humans they encountered in their travels but not other modern humans?

Which brings us to the third study, based on genomes from Australia and New Guinea:

But on that question, Dr. Metspalu and his colleagues ended up with a somewhat different result. In Papua New Guinea, Dr. Metspalu and his colleagues found, 98 percent of each person’s DNA can be traced to that single migration from Africa. But the other 2 percent seemed to be much older. Dr. Metspalu concluded that all people in Papua New Guinea carry a trace of DNA from an earlier wave of Africans who left the continent as long as 140,000 years ago, and then vanished.

I have some personal history on these questions. When the first genetic arguments for the "one wave out of Africa" were made 15 years ago, I accepted them wholeheartedly. Back then the upper bound for this migration was set at 55,000 to 60,000 years ago, and I began to write and talk about how this date would effect studies of things like language and folklore, given that we now had an actual date for when all the non-African culture of the world diverged. And then came the steady drip of counter-evidence: the Israeli skeletons, the Middle Eastern tool kits, interbreeding with Neanderthals, different genetic calculations that made the date more like 120,000, and so on. That moment of certainty and clarity was, for me, gone. I've never been able to get it back, and these studies don't get me there. It seems quite clear now that all sorts of people and semi-people were wandering around Eurasia 80,000 years ago, leaving traces in Israeli caves and DNA from New Guinea. Plus, my confidence in these genetic dates has been drastically undermined by the work done on the population of the Americas, because it turns out that some geneticists have been calibrating their clocks by asking archaeologists what the date should be and then tweaking their models to get the right date.

So while all of this is very interesting, there is a lot of work yet to be done.