New genetic findings suggest that early humans living about one million years ago were extremely close to extinction.

The genetic evidence suggests that the effective population—an indicator of genetic diversity—of early human species back then, including Homo erectus, H. ergaster and archaic H. sapiens, was about 18,500 individuals (it is thought that modern humans evolved from H. erectus), says Lynn Jorde, a human geneticist at the University of Utah in Salt Lake City. That figure translates into a total population of 55,500 individuals, tops.

One might assume that hominin numbers were expanding at that time as fossil evidence shows that members of our Homo genus were spreading across Africa, Asia and Europe, Jorde says. But the current study by Jorde and his colleagues suggests instead that the population and, thus its genetic diversity, faced a major setback about one million years ago. The finding is detailed in the January 18 issue of Proceedings of the National Academy of Sciences.

To make these estimates, Jorde's group scanned two completely sequenced modern human genomes for a type of mobile element called Alu sequences. Alu sequences are short snippets of DNA that move between regions of the genome, though with such low frequency that their presence in a region suggests it is quite ancient. Because older Alu-containing regions have had time to accumulate more mutations, the team was also able to estimate the age of a region based on its nucleotide diversity. The team then compared the nucleotides in these old regions with the overall diversity in the two genomes to estimate differences in effective population size, and thus genetic diversity between modern and early humans.

"This is an original approach because they show that you can use mobile elements…to flag a region of the genome," says Cédric Feschotte, an evolutionary geneticist at the University of Texas Arlington.

The effective population researchers estimate at about 18,500 reveals that the extent of genetic diversity among hominins living one million years ago was between 1.7 and 2.9 times greater than among humans today. (Other studies have shown that the present-day effective population is around 10,000.) Jorde says the reason the modern effective population is so much smaller than the current number of people (nearly seven billion) is that a population explosion occurred, probably due to the development of agriculture about 10,000 years ago. He does not expect that there would have been such a staggering difference between the effective and actual populations of early humans.

Jorde thinks that the diminished genetic diversity one million years ago suggests human ancestors experienced a catastrophic event at that time as devastating as a purported supervolcano thought to have nearly annihilated humans 70,000 years ago. "We've gone through these cycles where we've had large population size but also where our population has been very, very small," he says.