We now know Caucasions have at least 4% Neanderthal DNA in them (I'm surprised it's not more, since the difference between them and other people in terms of physical features is Neanderthalic - not accidental that they look like a caricature of whites).
In fact, all modern humans except for Sub Saharan Africans (THE Homo Sapiens Sapiens) have some Neanderthal admixture in them, including Native Americans.
In Asia extending to Melanesians all the way to Australia, DNA from the recently discovered Denisovians has also been found.
I just read a few weeks ago that auto immune diseases may be a legacy of this hybridization. Spreading to lands whose diseases they had no previous exposure to, many Homo Sapiens acquired resistance via genes contributed by Neanderthals with such pairings. While conferring protection, they are alien enough that modern bodies sometimes attack them, thus resulting in the auto immune disorders.