Living in the Universe

Hanging by a Thread

May 3, 2008 on 8:00 am | In Life on Earth | Comments Off

 First Week of May 2008

When astrobiologists think about life on Earth or life in the universe, they tend to assume life will persist. It’s lasted for four billion years on the Earth. They also think that intelligence or the advancement of life through evolution is a more or less steady process. But recent studies show that human beings—the pinnacle of intelligent life on Earth—had a brush with total extinction just seventy thousand years ago.

The human population at that time was apparently reduced to small, isolated groups in Africa because of drought. A large new analysis and a separate study by researchers at Stanford shows that a number of early humans may have shrunk as low as two thousand, before the numbers began to expand again in the early Stone Age. Let’s hear from Spencer Wells who works at the National Geographic Society and has the magnificent title “Explorer in Residence,” which makes you wonder why is he in residence when he should be out and about exploring:  “This study illustrates the extraordinary power of genetics to reveal insights into some of the key events in our species’ history. Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world. Truly an epic drama written in our DNA.” Wells is Director of the Genographic Project, which was launched in 2005 to study anthropology using genetics.

The report I’m talking about was published in the American Journal of Human Genetics, and it uses mitochondrial DNA, which is passed down through mothers. DNA is a wonderful tool for tracing life’s history on Earth. Mitochondrial DNA is remarkable, a remnant of a strange evolutionary event: the merger of an ancient bacterium with the cell ancestral to all plant and animal life. It also carries the imprint of more recent evolution. In many species, humans included, it passes only from mother to child. No paternal genes get mixed in to it. That makes it easy to see when particular genetic mutations happen and thus to construct a family human tree. Various branches of that tree are now well studied.

Humans began in Africa, spread to Asia around sixty thousand years ago, then to Australia fifty thousand years ago, Europe thirty-five thousand years ago, and the Americas fifteen thousand years ago. What hasn’t been so well examined, though, are the tree’s African roots. The genetic diversity of Africans probably exceeds that of the rest of the world put together, but the way that diversity evolved is unclear.

This new study has shed light on this issue using the mitochondrial DNA of more than six hundred living Africans to show how genetic diversity developed in Africa, and in doing so, they’ve shed light on how modern man spread around his home continent long before he took his first tentative steps into the bigger, wider world. The team paid particular attention to samples from the Khoi and San people of southern Africa. These people, known colloquially as Bushmen, make their living hunting and gathering. Indeed, their way of life is thought by anthropologists to resemble quite closely that of the pre-agricultural people throughout the world.  Comparing Khoi and San DNA with other Africans shows that the first big split in Homo sapiens happened shortly after the species emerged two hundred thousand years ago. Most people now alive are on one side of the split. Most Bushmen are on the other.

The consortium’s analysis of which DNA “matrilines” are found where suggest that for much of its history the species was divided into two isolated populations, one in Eastern Africa and one in the south of the continent. The two groups were defined by this split. However, few other matrilineal lines from the first hundred thousand years of the species history have survived to the present day. This suggests that the early human population was tiny and reinforces the idea that Homo sapiens may indeed become close to extinction. Indeed, there may have been one point as few as two thousand people left to carry humanity forward. This shrinkage coincides with a period of prolonged drought in Eastern Africa and was probably caused by it. The end of the drought was followed by the appearance of many new matrilines that survive to the present day.

The researchers estimate that by sixty to seventy thousand years ago, the period when the exodus that populated the rest of the world began, as many as forty such groups were flourishing in Africa, though the migration only involved two of those groups. The African matrilines seem to have remained isolated from each other for tens of thousands of years after the exodus. It wasn’t until forty thousand years ago that they began to reestablish conjugal relations, quite possibly as a result of the technological revolution of the late Stone Age, which yielded new, finely crafted tools. Only the Bushmen seem to have missed out on this party. They were left alone until a few hundred years ago, when their homelands were invaded from the north by other Africans and from the south by Europeans—not a particularly happy event for the Bushmen.

So we have this extraordinary story of humanity in early Africa, shrinking at one point through environmental diversity almost to the point of extinction. Look at us now, 6.6 billion people strong. We dominate the world (and also mess it up). Our technology, space travel, computers, and genetic engineering are marvels. But back then we were simple hunter-gatherers, no more successful and far less abundant than the apes of our earlier lineage, reduced to two thousand strong, the size of a village. We were maybe bad winter away from total extinction. The success in this world, and in this universe, of intelligent life is not guaranteed. For all our power and intelligence, there was a time when humanity was hanging by a thread.