However....it seems even the usual details of this idea has been challenged by creationists who do contorted calculations to 'prove' that the Most Recent Common Ancestor (MRCA) of extant humans was only 6500 years ago. (the usual calculations say about 150,000-200,000 years)

Here is an article about one such claim made on "the Answers in Genesis website"

http://www.evolutionpages.com/Mitochondrial%20Eve.htm