In the 'olden days,' Homo sapiens had sex with Neanderthals.

Technically, it's inter-breeding of sorts - a bit like when a lion had sex with a tiger.

Or a horse having a passionate affair with a zebra.

The human-Neanderthal encounters 40,000 years ago, scientists believe, may have been a crucial factor in causing our allergies today, with researchers suggesting that ancestors of modern humans getting busy with other species has led to things like hay fever.

As The Guardian reports, the "prehistoric couplings" left all non-Africans with one-six per cent Neanderthal DNA.

According to US genetics company 23andme, a group of humans, the more pioneering among them, left Africa to explore and met Neanderthals and Denisovans who had spent 200,000 years adapting to diseases and viruses in Eurasia.

Janet Kelso, a researcher at the Max Planck Institute, who worked on the study, said:

Interbreeding with archaic humans does indeed have functional implications for modern humans. The most obvious consequences have been in shaping our adaptation to our environment - improving how we resist pathogens and metabolise novel foods.

As a result, many modern humans carry three genes that boosts the immune system - giving people a stronger resilience to the cruelties of the world.

However, the DNA came at a price in the form of allergies, which cause us to sneeze, itch, and react to certain environmental factors.