Tuesday, May 15, 2012

The sixth day

Anthropologists pretty much agree that all modern humans had their ancestral origins in Africa, and spread from there to the other continents in a great exodus beginning about 60,000 years ago.

Which is not to say that there were not already other members of the human family in Europe, Asia, and in what is now the island of Indonesia, the descendants, presumably, of earlier migrants from Africa. The origin of these pre-modern humans -- Neanderthals, Denisovans, Homo erectus, and Homo floresiensis -- is hazy, but it now seems certain that our species (or subspecies) -- Homo sapiens -- interacted with the older residents, eventually driving them to extinction. But not, it seems, without some interbreeding. It turns out we carry snatches of Neanderthal DNA in our modern genome.

Writing in Scientific American, Michael Shermer notes: "I always suspected that Neandertals and anatomically modern humans interbred, based on a simple observation: humans are the most sexual of all the primates, willing and able to do it just about anywhere, anytime, with anyone (and even with other species if the Kinsey report is to be believed)."

In any case, it's a grand adventure story, taking Homo sapiens out of Africa, eventually to Australia and across the Bering Strait to the southern tip of South America. It is a story closely tied to climate change and the rise and fall of sea level during the most recent Ice Age.

I'd love to be around 100 years from now when we know the story in more detail. Piecing it together requires shrewd detective work, relying on bits of bone, flakes of stone, and the occasional artifact. And -- evidence that we couldn't have imagined even a generation ago -- DNA. The whole winding adventure is recorded in our genome.

This much is clear: We are the ultimate invasive species. We have entered every pristine environment on Earth, displacing native species, even alien members of our own species. We are unstoppable, prodigious, voracious, the quintessential weed. Like it or not, there is now a single habitat -- ours -- that we share with bacteria and great blue whales. A pale blue dot. Entrusted, by happenstance, to our care.