May 31st, 2013

Before African Eve

You probably remember the African Eve hypothesis, which claimed that based on mitochondrial DNA all humans were descended from a single small and comparatively recent population. Studies of the Y-chromosome found similar results--very disturbing because it seemed to imply that a single population had managed to displace all previously existing human, with no survivors and no intermixing, potentially a huge genocide at the heart of the spread of modern humans.

The African Eve theory took a hit when Neanderthal and Denosovian (an Asian variant of early humans) DNA became available and it became apparent that both types of Archaic humans had contributed single-digit amounts of DNA to parts of humanity.

A few months ago, that theory  took an even heavier hit. An African American man submitted his DNA to Family Tree DNA, a company that uses DNA to trace family trees. The result: His Y chromosome DNA diverged from standard DNA lines of 300,000 years ago, considerably (a hundred thousand years) before anatomically modern humans developed. To quote the article:

“About 300,000 years ago falls around the time the Neanderthals are believed to have split from the ancestral human lineage. It was not until more than 100,000 years later that anatomically modern humans appear in the fossil record. They differ from the more archaic forms by a more lightly built skeleton, a smaller face tucked under a high forehead, the absence of a cranial ridge and smaller chins.

Hammer said the newly discovered Y chromosome variation is extremely rare. Through large database searches, his team eventually was able to find a similar chromosome in the Mbo, a population living in a tiny area of western Cameroon in sub-Saharan Africa.”

It sounds as though our type of humans did push out of Africa and replace most of the existing populations, but our ancestors did intermingle with existing humans as they did it. The latest discovery also points out that Africa then (and now to some extent) was very diverse genetically. A subset of African humans pushed out into the rest of the world. That same subset pushed into other areas of Africa, but didn't completely displace or genetically swamp previously existing African populations.

There is quite a bit of controversy over whether Neanderthals and other archaic humans were separate species. The fact that their genes are in our DNA seems to say that they were subspecies rather than separate species, but the traditional definition of a species--no fertile offspring with other species--is actually a bit misleading. Polar bears and brown bears can interbreed and produce fertile offspring, as can coyotes and wolves. A more accurate definition of how we apply the term species is "a group of animals that don't normally produce fertile off-spring." That can be because they can't interbreed or because ecological or geographic barriers keep them apart.

In terms of human ancestors, humans spread over the parts of Africa, Asia and Europe that they could spread to well over a million years ago. The first wave of humans that spread into Europe may have gone extinct in one of the nastier ice ages, but the continent got repopulated by the ancestors of the Neanderthals at least half a million years ago.

At that point, humans probably fell into a pattern that continued nearly to modern times. Our ancestors were spread over a huge area, with widely different ecologies. The continents reshaped their people, with dark skin in the tropics, light skin where it was vital to get enough vitamin D in winter. It reshaped Neanderthals into temperate forest hunters and our ancestors into open country endurance athletes. Asia went its own poorly understood way. We know some of what was going on in China before modern humans arrived there, but essentially nothing about pre-modern humans in India. At the same time, DNA flowed from one population to another, though not completely freely. There were geographic barriers and ecological ones.  Archaic humans were far more different from each other and from us than any race or culture that exists today--probably six to ten times the genetic distance.

Then, fifty to sixty thousand years ago, our ancestors pushed out from their ancestral lands in northwestern Africa into the rest of the world, ending the other types of humans as separate populations, though a few pockets of them lingered until tens of thousands of years later.  Once our ancestors spread, the same forces that had changed archaic humans into separate types of humans began to work on them--ecology reshaping us while gene flow kept us together. Only a subset of African genetic diversity spread to the rest of the world. Genetically, if you had to classify modern humans the logical divisions would be several different types of Africans with deep branches, and then the rest of the world's ethnic groups as twigs off of one of the African branches, with little depth to our differences.