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Y-chromosomal Adam is the patrilineal most recent ancestor from whom all living men are descended. Like Mitochondrial Eve, this Adam is not an actual fossil specimen, but a theoretical male who can provide some interesting insights into human evolution.
Despite the similarity of the origins these ancestors’ names, this Adam and Eve did not, in fact, live at the same time. Mitochondrial Eve (the most recent matrilineal ancestor) lived around 140,000 years ago, while Y-Chromosomal Adam lived much more recently, around 60,000 years ago. Both Adam and Eve lived in Africa, and even though they did not live at the same time, their descendants were certainly familiar with one another.
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Tracing the Y-Chromosome Line
Human cells each contain 23 chromosomal pairs, including 22 pairs of autosomes, and one pair of sex chromosomes. In females, the sex chromosomes are in the form of a pair of X chromosomes. Males have one X chromosome (obtained from the mother) and one Y chromosome (obtained from the father).
Apart from a tiny region of the Y chromosome which can recombine with an X chromosome, recombination between X and Y pairs does not occur. That means Y chromosomes are passed down along the male line in an unbroken form. Apart from random mutations which occur from time to time, Y chromosomal DNA is handed down the male line virtually unchanged.
This means that as with females and mitochondrial DNA, all male humans who are alive today have Y chromosomal DNA which can be traced back to one common ancestor, who has been dubbed the Y-chromosomal Adam (note that this doesn’t mean “Adam" was the only male alive at the time—it simply means he is the only male from his time period with a direct male line which extends to the present day).
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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."