Sneezing and tearing up are some of the body's most common allergic reactions.
Although these are natural responses, their degree vary for each person.
But, in recent studies carried out by two scientific teams, the researchers discovered that allergic reactions of people may be linked to the DNA they inherited from their Neanderthal ancestors, NPR reported.
According to the researchers, like in modern humans, the allergic reactions of Neanderthals served as their body's main defense mechanism that can immediately react to pathogens. Then, when early humans arrived and met the Neanderthals, some members from both groups mated with one another.
As a result, some of the Neanderthals' genetic traits, including allergic reactions, were then pass on to their offspring with early humans.
"Neanderthals, for example, had lived in Europe and western Asia for around 200,000 years before the arrival of modern humans," Janet Kelso of Germany's Max Planck Institute for Evolutionary Anthropology and member of one of the research teams said according to Discovery News.
"They were likely well-adapted to the local climate, foods and pathogens," she added. "By interbreeding with these archaic humans, we modern humans gained these advantageous adaptations."
The researchers noted that due to interbreeding, about 2.5 percent of many Europeans and Asians' DNA came from Neanderthals. They were able to obtain this data by going through the samples collected through the 1000 Genomes Project.
However, the scientists mentioned that although traces of Neanderthal DNA has helped in the development of modern humans' immunity system, these also led to various consequences, which, in this particular case, are the allergic reactions to non-threatening pathogens.
The exact link between these two factors is still unclear but for the researchers, it's proof that shows how an evolutionary trait that protected early ancestors before may have transformed into something that's not so beneficial for humans today.
"It's a bit speculative, but perhaps this is some kind of trade-off," Kelso said according to the Seattle Times. "Increased resistance to bacterial infection was advantageous, but may have resulted in some increase sensitivity to non-pathogenic allergens."
The two studies, titled "Introgression of Neanderthal and Denisoval-like Haplotypes Contributes to Adaptive Variation in Human Toll-like Receptors," and "Genomic Signatures of Selective Pressures and Introgression from Archaic Hominins at Human Innate Immunity Genes," were both published on January 7 in the American Journal of Human Genetics.