How Infectious Disease May Have Shaped Human Origins
Inactivation of two genes may have allowed escape from bacterial pathogens, researchers say

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.

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.

In a paper published in the June 4, 2012 online Early Edition of The Proceedings of the National Academy of Sciences, an international team of researchers, led by scientists at the University of California, San Diego School of Medicine, suggest that inactivation of two specific genes related to the immune system may have conferred selected ancestors of modern humans with improved protection from some pathogenic bacterial strains, such as Escherichia coli K1 and Group B Streptococci, the leading causes of sepsis and meningitis in human fetuses, newborns and infants.  

“In a small, restricted population, a single mutation can have a big effect, a rare allele can get to high frequency,” said senior author Ajit Varki, MD, professor of medicine and cellular and molecular medicine and co-director of the Center for Academic Research and Training in Anthropogeny at UC San Diego. “We’ve found two genes that are non-functional in humans, but not in related primates, which could have been targets for bacterial pathogens particularly lethal to newborns and infants. Killing the very young can have a major impact upon reproductive fitness. Species survival can then depend upon either resisting the pathogen or on eliminating the target proteins it uses to gain the upper hand.” More here

In the above photo, 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.

Human evolution is really cool, and who wouldn’t want to know how we got to where we are? The survivors of this bottleneck event, infectious disease or otherwise, gave us our Mitochondrial Eve and Y-Chromosomal Adam. The work being done on both of these is fascinating, especially the maps!