Infectious Disease May Have Given Birth To Modern Humans

Infectious disease may have played a role in weeding out our human ancestors until only around five to ten thousand remained, who then became the pool from which "behaviorally modern" humans emerged.

Around 100,000 years ago, our population was reduced to somewhere between five and ten thousand. Scientists have speculated that gene mutations, climate change, and even the development of differentiating behaviors such as language might have been the cause of this.

New research, however, indicates that infectious disease might have been a factor in the emergence of the modern human.It seems that the inactivation of two genes in our immune system may have rendered our ancestors better capable of warding of E.Coli and Streptococci, two leading causes of sepsis and meningitis in human fetuses 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, M.D."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."

To prove that modern humans might have evolved from the ability to "eliminate the target proteins," the team of researchers looked at the absence of two receptors, Siglec-13, and Siglec-17.

Siglec-13 is found in our relatives, chimpanzees, but is no longer a part of our modern human genome.

Siglec-17  is still in our genome, although it is a shorter version which makes it inactive and of no use to invading disease-causing agents.

The scientists are looking at the two deactivated receptors as signs that around 100,000 to 200,000 years ago, ancestors of the modern human had to deal with disease of epic proportions. These changes in the genome may reflect the genetic makeup of the survivors who went on to propogate our current species.

"Speciation is driven by many things. We think infectious agents are one of them," said Varki.