Adam and Eve Weren’t Contemporaries
Y-Chromosomal Adam and Mitochondrial Eve weren’t contemporaries—recent calculations suggest they were separated in time by at least 30,000 years, which equates to roughly 100,000 generations.
This seems impossible, but in fact it’s more likely than not, due to differences in reproduction ‘strategies’ for males and females.
In essence, women are limited in the amount of children they can bear within a single lifetime due to the time expended in pregnancy and child-rearing. Men, however, do not have these limitations.
This means fertile women have a chance of bearing a certain number of offspring which more-or-less runs along a smooth bell-curve. The difference is, males don’t have to follow this tendency (since they don't spend nine months gestating and can invest less time and energy in child-rearing), and the numbers of offspring for fertile men is much more variable. This, in turn, means that men have statistically larger numbers of offspring than do women. This is why Y-Chromosomal Adam is a more recent ancestor.
Another interesting difference between these two ancient ancestors is that Mitochondrial Eve’s line will not die out, because mitochondrial DNA is passed down to both male and female offspring. On the other hand, Y-Chromosomal Adam is not always the same person at any given point in history, due to the fact that male lines can die out. When this occurs, a more recent ancestor becomes the new “Adam."