How infectious disease may have shaped Human origins

Roughly 100,000 years ago, human evolution reached a mysterious bottleneck: Our ancestors had been reduced to perhaps five to ten thousand individuals living in Africa. In time, "behaviorally modern" humans would emerge from this population, expanding dramatically in both number and range, and replacing all other co-existing evolutionary cousins, such as the Neanderthals. 

Escherichia coli bacteria, like these in a false-color scanning electron micrograph by Thomas Deerinck at UC San Diego’s National Center for Microscopy and Imaging Research, cause a variety of often life-threatening conditions, particularly among the young. Varki and colleagues suggest a genetic change 100,000 or so years ago conferred improved protection from these microbes, and likely altered human evolutionary development [Credit: University of California, San Diego Health Sciences]

The cause of the bottleneck remains unsolved, with proposed answers ranging from gene mutations to cultural developments like language to climate-altering events, among them a massive volcanic eruption. 

Add another possible factor: infectious disease.

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