Was humanity born in the mother of all plagues?
Date: 10.9.2012
New Scientist 04 June 2012: Around 100,000 years ago, the human race was on the brink of extinction. Confined to Africa, our population had fallen to less than 10,000. Yet within a few tens of thousands of years, we began spreading around the world.
New genetic evidence suggests that one factor contributing to the population bottleneck was a massive epidemic of bacterial disease. The bacteria were exploiting two immune system genes, turning them against us. So the solution was simple: get rid of the traitorous genes.
Ajit Varki of the University of California, San Diego and colleagues looked at two genes called Siglec-13 and Siglec-17. Both code for proteins that are involved in controlling the immune system, helping to decide whether immune cells should go on the offensive.
Varki found that both genes are active in chimpanzees, but not in humans. Siglec-13 has been entirely deleted from the human genome, while Siglec-17 is non-functional as a result of losing one letter from its code.
Why would we have got rid of two useful immune genes? Varki reconstructed the lost proteins and found that two dangerous bacteria, Group B Streptococcus and Escherichia coli K1, could bind to them.
Wondering if the bacteria could exploit the proteins, he expressed each protein in some human immune cells. The modified cells had a weaker response to the bacteria than immune cells without the proteins. That suggests the bacteria had found a way to dampen the immune response by binding to the two proteins.
Varki thinks that early humans were confronted with a massive epidemic of bacterial infection. The two bacteria he studied are particularly dangerous to newborn babies, who often die after being infected. That could explain why the human population fell so precipitously, and why we got rid of the Siglec genes that made us so vulnerable...
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