There were 18,500 humans in the world 1.2 million years ago

A team of geneticists, with the help of the composition of just two human genomes, has computed the size of the human population 1.2 million years ago, from which everyone in the world is descended, as being 18,500 people.

According to a report in the New York Times, the estimate was made by a team of population geneticists at the University of Utah led by Chad D Huff and Lynn B Jorde.

The scientists put the number at 18,500 people, but this refers only to breeding individuals, the "effective" population.

The Utah team based its estimate on the genetic variation present in two complete human genomes, one prepared by the US government's human genome project and the other by J. Craig Venter, the genome sequencing pioneer.

The actual population would have been about three times as large, or 55,500. Comparable estimates for other primates then are 21,000 for chimpanzees and 25,000 for gorillas.

In biological terms, it seems, humans were not a very successful species, and the strategy of investing in larger brains than those of their fellow apes had not yet produced any big payoff.

Human population numbers did not reach high levels until after the advent of agriculture.

Geneticists have long known that the ancestors of modern humans numbered as few as 10,000 at some time in the last 1,00,000 years.

The critically low number suggested that some catastrophe, like disease or climate change induced by a volcano, had brought humans close to the brink of extinction.

If the new estimate is correct, however, human population size has been small and fairly constant throughout most of the last million years, ruling out the need to look for a catastrophe.