Humans may have come close to extinction about 70,000 years ago, according to the latest genetic research.
By Dr David Whitehouse
BBC News Online science editor
The study suggests that at one point there may have been only 2,000 individuals alive as our species teetered on the brink.
From just a few, six billion sprang
This means that, for a while, humanity was in a perilous state, vulnerable to disease, environmental disasters and conflict. If any of these factors had turned against us, we would not be here.
The research also suggests that humans (Homo sapiens sapiens) made their first journey out of Africa as recently as 70,000 years ago.
Unlike our close genetic relatives - chimps - all humans have virtually identical DNA. In fact, one group of chimps can have more genetic diversity than all of the six billion humans alive today.
It is thought we spilt from a common ancestor with chimps 5-6 million years ago, more than enough time for substantial genetic differences to develop.
The absence of those differences suggests to some researchers that the human gene pool was reduced to a small size in the recent past, thereby wiping out genetic variation between current populations.
Evidence for that view is published in the American Journal of Human Genetics.
Because all humans have virtually identical DNA, geneticists look for subtle differences between populations.
One method involves looking at so-called microsatellites - short, repetitive segments of DNA that differ between populations.
These microsatellites have a high mutation, or error, rate as they are passed from generation to generation, making them a useful tool to study when two populations diverged.
Researchers from Stanford University, US, and the Russian Academy of Sciences compared 377 microsatellite markers in DNA collected from 52 regions around the world.
Analysis revealed a close genetic kinship between two hunter-gatherer populations in sub-Saharan Africa - the Mbuti pygmies of the Congo Basin and the Khosian bushmen of Botswana.
The researchers believe that they are "the oldest branch of modern humans studied here".
The data also reveals that the separation between the hunter-gatherer populations and farmers in Africa occurred between 70,000 and 140,000 years ago. Modern man's migration out of Africa would have occurred after this.
An earlier genetic study - involving the Y chromosomes of more than 1,000 men from 21 populations - concluded that the first human migration from Africa may have occurred about 66,000 years ago.
The small genetic diversity of modern humans indicates that at some stage during the last 100,000 years, the human population dwindled to a very low level.
It was out of this small population, with its consequent limited genetic diversity, that today's humans descended.
Estimates of how small the human population became vary but 2,000 is the figure suggested in the latest research.
"This estimate does not preclude the presence of other populations of Homo sapiens sapiens (modern man) in Africa, although it suggests that they were probably isolated from each other genetically," they say.
The authors of the study believe that contemporary worldwide populations descended from one or very few of these populations.
If this is the case, humanity came very close to extinction.