Why do scientists say that the MRCA of all living humans can't have lived alone?

Mitochondrial Eve is likely to be a closely related group of women who shared the original marker set, rather than a single woman. All other living women in that time had descendents that have not passed down their mitochondrial genetic markers to living descendents. They were part of a viable gene pool but over time their mitochondrial markers became less frequent and passed out of the gene pool.

https://docs.google.com/viewer?a=v&q=cache:qVYAqf2...

The most recent common mitochondrial ancestor is the women, out of an ancient population, who shared the variations of mitochondrial genome we can track to *all living* women we have sequenced to compare. This history of mitochondrial genomic change stays the same but as today's population dies off the matrilineal MRCA actually changes, coming forward.

Think of a family tree whose living descendants can be all be traced back to one women X. With the death of one distant cousin that branch is no longer part of the unbroken chain to the woman X but rather all living members can be traced back to woman Y, a few generations younger. Woman X was MRCA but now woman Y is.

http://en.wikipedia.org/wiki/Mitochondrial_Eve

Now try to track markers for all 23 chromosomes back to a MRCA with all of them and this ancestor is only around 1,500 BC in eastern Asia.

http://www.nature.com/news/2004/040927/full/news04...

http://familypedia.wikia.com/wiki/Most_recent_comm...

The whole thing is complicated because chromosomes assort independently, they do not remain as liked sets of 23 passed down. Further they recombine so markers mix and match and change over time in meiotic cross over. This is why they track the little Y and the mitochondria chromosomes with a clear path of inheritance through the generations and no crossover recombination.