In this case, they find an embarassing discrepancy with the way things should be among humans, according to their theory. So, they posit an "extinction" event to explain this embarrassing discrepancy, although there is no scientific evidence for it whatever. That's not science. It's what used to be called "saving the appearances." That is to say, desperately thinking up new theories to explain why your old theories don't fit the evidence.
As I understand it, DNA evidence suggests that all the human races probably go back to something that looks suspiciously like a single female ancestor--a woman whom some scientists jokingly refer to as "Eve."
Creationist Myth #244 "There's no proof of speciation."