It's been a while since this was posted so I am copying it in:

"The key point is that in both cases the "common ancestor" is the most recent individual from whom all currently living humans are directly descended. The notion that these two are the "first persons ever" is completely falacious, and is something to "get over."

If the current MRCA(Eve) [= Most Recent Common Ancester - female] had more than one daughter, then two separate lines of descent diverge from her. If all the matrilineal descendants of all but one of these daughters die off (fail to produce female descendants) then the daughter whose line continues becomes the new MRCA(Eve).

The same (mathematical) condition applies to the male line. The more recent MRCA(Adam) simply means that the paternal lines of descent of all but one of the males alive at the same time as the female MRCA failed to continue to produce currently surviving male descendants, moving the "title" of MRCA(Adam) to a more recently appearing male."

If the MRCA(Adam) lived 75,000 years ago, wouldn't his mother automatically be the MRCA(Eve)?

I expect that the two numbers were arrived at independently and within the framework that was used are probably accurate but still...