A team led by Ajit Varki of the University of California, San Diego, found new genetic evidence which suggests that a mysterious plague ravaged populations of early humans in Africa, until their number fell to less than 10,000.
The bacteria were exploiting two immune system genes. So the solution was to get rid of the traitorous genes. And those who managed to survive the epidemic, bounced back and started spreading around the world, the NewScientist.Com reported.
For their study, Varki 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 offencive.
They 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? The team 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, which suggests the bug had found a way to dampen the immune response by binding to the two proteins, said the researchers who detailed their work in the 'Proceedings of the National Academy of Sciences'.
"Though it's impossible to discern exactly what happened during evolution, we studied molecular signatures surrounding these genes to hypothesise that predecessors of modern humans grappled with a massive pathogenic menace between 100,000 and 200,000 years ago," Varki said.
"This presumed 'selective sweep' would have devastated their numbers. Only individuals with certain gene mutations survived -- the tiny, emergent population of anatomically modern humans that would result in everyone alive today," he added. (More)
