In 2010 researchers concluded that as result of sexual liaisons between Neanderthals and Homo sapiens some 80,000 years ago, many of us carry a trace of Neanderthal DNA in our genomes (among Europeans and Asians, somewhere between 1 and 4 percent). But, says Stanford magazine (November/December 2011), the evolutionary impact of the phenomenon was unclear until late last year, when Stanford immunologist Peter Parham found that “these genetic exchanges significantly strengthened modern human immune systems.” Specifically, the Neanderthal DNA fortifies a class of immune genes called human leukocyte antigens (HLA), which play a vital role in protecting people against viruses and infections.

It makes sense that an injection of outsider DNA would enhance human immunity, since diversity in the genetic code almost always produces a stronger constitution. In the case of Neanderthals, the HLA genes they conferred come in thousands of versions, granting an exceptional level of viral defense, which likely helped ancient humans survive mass migrations when they were exposed to regional pathogens.

A mix of immune genes is so essential to our species’ survival, explains Stanford, “that people are attracted to the scents of prospective sexual partners with disparate HLA types.” This might explain why Neanderthals and Homo sapiens hooked up in the first place.