Though it may seem difficult to believe in a world with almost seven billion people, humans were once thin on the ground, and at times teetered on the brink of extinction.

For most of the history of our species, there weren't many of us, probably in the "tens of thousands, comparable to modern populations of gorillas and chimpanzees," says Richard Durbin, a genome scientist with the Wellcome Trust Sanger Institute in Cambridge, England.

Durbin and Heng Li, a colleague at the Broad Institute of Harvard and Massachusetts Institute of Technology have a paper in this week's edition of the journal Nature in which they present a new method they have developed to look at a single person's DNA sequences and using them as a representative of all their ancestors -- and thus map out the history of the species.

They applied it to seven people: a Chinese man, a Korean man, three Europeans and two Yoruba men from west Africa. What they found was interesting.

Anatomically modern humans originated in southern Africa. It's generally believed that some of them began to move northwards out of Africa between 50,000 to 60,000 years ago.

However the researchers' evidence shows that there was at least some differentiation between African and non-African human populations as early as 100,000 and 120,000 years ago.

In addition, once the European and Asian groups had left the continent, it wasn't as if they never intermingled again. Genetic exchange between the groups continued until around 20,000 years ago, their research found. That doesn't necessary mean one person every year, "but maybe every few hundred another bunch left," says Durbin.

Those populations who left lived through very hard times. The people who became modern Europeans and Asians underwent a severe population bottleneck sometime between 100,000 and 30,000 years ago, getting down to as few as 1,000 people who were reproducing, it appears.

"It also looks like the Africans also decreased at the same time, though not as much," Durbin says.

What's known about that era is that it was one of extreme changes in climate, with ice ages coming and then receding. This was especially a problem in more northern latitudes and perhaps less so in Africa. The latest ice age only finished about 10,000 years ago, after which agriculture began to flourish.

Today's human population represents an enormous shift from those early, few and threatened primates. "The fact that we're pervasive in the world now is sort of an anomaly in the history of great ape populations," says Durbin.