Ever wonder what – or who – is to blame for your peanut allergy or inability to walk through a garden in spring without sneezing?
You can thank the Neanderthals for that.
Human's distant and extinct cousins, along with another long-gone species of human, Denisovans, have seemingly passed on a gene that has had a significant impact on homosapiens' immune system. That gene is believed to be responsible for the some people's sensitivities to things such as pollen, peanuts and eggs.
But, on the flip side, those same genes now help humans in the fight against some diseases related to bacteria, fungus and parasites.
Two new studies from US and European scientists examined changes in the immune system of early humans to discover when changes to our "genetic blueprint" were made. It was published in the American Journal of Human Genetics.
Researchers say that humans bred with the archaic humans in Europe thousands of years ago, which resulted in dramatic changes to a cluster of genes that continue to be felt today.
This "inheritance" from Neanderthals and Denisovans, the former of which became extinct 40,000 years ago, means the variation in those genes gives humans the ability to react to pathogens, which are biological agents responsible for causing diseases.
This sensitivity is what allows humans to respond against some diseases, but could also make many humans more susceptible to allergies.
The reason for that, Janet Kelso, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said, was because Neanderthals lived in Europe and western Asia for about 200,000 before humans did.
"They were likely well adapted to the local climate, foods, and pathogens. By interbreeding with these archaic humans, we modern humans gained these advantageous adaptations."
The genes are Toll-like receptor (TLR) genes – TLR1, TLR6, and TLR10. Some of those genes are most similar to the Neanderthal genome and the third is most similar to the Denisovan genome.
Previous studies have revealed that up to 6 per cent of genomes in European and Asian people were inherited from Neanderthals, Denisovans and other now long-gone relatives.
The findings highlighted the importance of interbreeding in human evolution, Dr Kelso said.
"What has emerged from our study, as well as from other work on introgression, is that interbreeding with archaic humans does indeed have functional implications for modern humans, and that the most obvious consequences have been in shaping our adaptation to our environment – improving how we resist pathogens and metabolise novel foods."