New research from two papers suggests that mating between two human species led to modern humans having the ability to resist infection. However, this might have had a downside: our vulnerability to allergies could have also increased. Both reports are published in the American Journal of Human Genetics.
The human immune system is extremely powerful, able to resist attacks from potent pathogens. How did it evolve into such a complex organisation? According to the new research, much of it is due to what happened centuries ago when modern humans left Africa, driven away by environmental difficulties, from new foods, pathogens, climate change, and the existence of other human species.
The interactions among modern humans and other ancient human species is said to account for 1 to 6 % of modern Eurasian genomes – these might have come from ancient hominins like Neanderthals. The modern human genome was subjected to the addition of archaic alleles which have apparently caused the innate immune system to evolve into a defensive organisation against infection.
How did this interbreeding trigger the evolution of the immune system? It is to be noted that one’s genes determine, to a great extent, one’s susceptibility to an infectious disease, and therefore, one’s recovery as well. Therefore, as per natural selection, the individuals with the genes that best protect them would survive the pressure imposed by pathogens. This would, in turn, influence the variations in the human genome pertaining to genes conferring immunity.
When the first team from France, led by Lluis Quintana-Murci, analysed modern data gathered from the 1,000 Genomes Project together with that from ancient hominins, they worked on a group of 1,500 genes related to the innate immune system. They found that some innate-immunity genes had undergone very few changes over time while others seemed to have been substituted by new forms that might have been ‘preferred by nature’ in the aftermath of disease and environmental change.
These adaptations are mostly dated 6,000 to 13,000 years ago – a time marked by the shift among humans from hunting to farming.
Also, the innate-immunity genes are more likely than other genes to have been inherited from Neanderthals.
Therefore, the team concludes that the the exchange of genes among human species might have been important in the evolution of the innate immune system.