The human population at that time was reduced to small isolated groups in Africa, apparently because of drought, according to an analysis released Thursday.

estimated the number of early humans may have shrunk as low as 2,000 before numbers began to expand again in the early Stone Age.

Previous studies using mitochondrial DNA - which is passed down through mothers - have traced modern humans to a single "mitochondrial Eve," who lived in Africa about 200,000 years ago.