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.
Researchers have long known that modern humans lack the genetic variation found in other living primates, such as chimpanzees or gorillas, even though our current population size is so much larger. One explanation for this lack of variation is that our species underwent recent bottlenecks–events where a significant percentage were killed or otherwise prevented from reproducing. Some researchers proposed that the lack of variation in our maternally inherited mitochondrial DNA suggested these bottlenecks took place as our ancestors spread out of Africa relatively recently.