Question: recently anthropologists discovered that neanderthals and pre - historic people lived side by side , i was wondering could this be a reason why there is genetic illness , the genes of these to species where not correctly matched , so genetic codes could not be read properly? please answer , thank you.

Daniel Scully answered on 16 Nov 2011:

In most cases I believe the best thing for a species is to have as much genetic diversity in it as possible.

Having only very few genes makes it much easier for a single illness to wipe out your entire species, but with more diversity there’s a higher chance that someone is able to fight it off.

Repeatedly mixing similar genes can also increase the chance of birth defects… hence why you are not supposed to breed with any of your close relatives.

So if anything, humans inter-breeding with other early human-like species could have contributed to our eventual success.