By Ann Gibbons
ScienceNOW Daily News
19 January 2010

With 6.8 billion people alive today, it’s hard to fathom that humans were ever imperiled. But 1.2 million years ago, only 18,500 early humans were breeding on the planet–evidence that there was a real risk of extinction for our early ancestors, according to a new study. That number is smaller than current figures for the effective population size (or number of breeding individuals) for endangered species such as chimpanzees (21,000) and gorillas (25,000). In fact, our toehold on the planet wasn’t secure for a long time–at least 1 million years, because our ancestral stock was winnowed with the emergence of our species, Homo sapiens, 160,000 years ago or so and, again, with the migration of modern humans out of Africa. “There’s this history of a precarious existence not just for our species but for our ancestors,” says co-author Lynn Jorde, a human geneticist at the University of Utah in Salt Lake City.

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