This is making a few basic assumptions, I'm sure JiK will correct me if needs be:

1. Male common ancestry is worked out by looking at variations in the Y Chromosome, and arriving at a date by estimating rate of change over time. This can only look at the male line.

2. Female common ancestry is looked at using mitochondrial DNA, which is carried in the egg, and therefore from the Mother. Again, date comes from looking at variation within modern population, and estimating a rate of drift.

This means the Y chromosome can only be traced along a line where every father had a son. He could have 20 children, but if they're all daughters, then his Y chromosome will not be passed on. 'Adam' is the most recent person we can get back to in this way. Similarly for a Woman, if she has no daughters, her mitochondrial DNA line is broken.

The most recent common ancestor is a bad name, it's the most recent common traceable ancestor we have here.



As the X chromosome can be inherited from either the Mother or Father, you can't use it to trace a population back in the same way as either the Y, or the mitochondria