Most Recent Common Ancestor??

RRR

I need help with the concept of the Most Recent Common Ancestor (MRCA). I gave my father Dawkins's "Ancestors Tale" for Christmas, but before I gave it to him, I read a little bit from it. In one of the earlier chapters, Dawkins explains that according to mathmatical analysis that in 2000 years most people alive today will be the ancestor of everyone alive in 2000 years. This also indicates that the MRCA of the entire world's population today existed around 2000 years ago. How is this possible considering the geographic isolation of some peoples, like some Amazonian or New Guinean tribes or native Australians? Or did I miss something in the concept. Dawkins sited work by Joseph Chang, if that helps.