They came into contact with the same alien vehicle that Quaterman did in “Quaterman and the Pit” and destroyed all the Neanderthals...
Neanderthals played softball and humans played baseball.
...leaping and jumping!
some of us at work were discussing neanderthals today ( a little slow at work )and decided since some modern men look like neanderthals still that they were merely a band of hominids that were cut off geologically for a time and inbred for a thosand years or so. the humans that show those traits today have GOT to be inbred.