In the scientific community, there has been a debate whether the spread of Neanderthal genes was because of an advantage or chance. Recent investigations point out that it is the first one. We know that Neanderthals inherited us a set of diseases like arthritis, but not all they gave us is bad. Neanderthal genes have given us protection against some viruses. That means, that modern humans took the adapted genetic defenses of the Neanderthals rather than waiting for its own.
Reference: Cell, Enard et al. "Evidence that RNA viruses drove adaptive introgression between Neanderthals and modern humans." www.cell.com/cell/fulltext/S0092-8674(18)31095-X , DOI: 10.1016/j.cell.2018.08.034
Read more at: https://phys.org/news/2018-10-modern-humans-inherited-viral-defenses.html#jCp