SSS 2010-01-21（在线收听）

At one time or another, you’ve probably said, “It’s a small world.” Well, it used to be much, much smaller. Because according to scientists from the University of Utah, about a million years ago our ancestors numbered fewer than 20,000. The estimate appears in the Proceedings of the National Academy of Sciences.

The scientists based this ancestral census on a reading of modern human genomes. In particular, they tracked the positions of transposable elements. These small pieces of DNA are basically genomic parasites. Over time, they’ve spread through our genomes and though they’re still capable of hopping from one place to another, they do so quite rarely. One particular transposon first appeared about a million years ago. And by seeing where these sequences now sit, scientists can get a sense of the size of the breeding population back then.

Their estimate of 18,500 individuals is pretty sparse considering that these early humans were spread across the Old World. By comparison, some 25,000 gorillas and 21,000 chimps were concentrated in Africa alone. So a million years ago, we were essentially an endangered species. Here’s hoping our recovery is permanent.