There are two very distant ancestors in my family tree.
To introduce them, I want you to imagine all the living people in the World, all seven billion of them. Call them Generation-0.
Now imagine all the men who were fathers of people in Generation-0. Call them Generation-1. There will obviously be far fewer people in Generation-1 than Generation-0, perhaps a little over a billion
Then Generation-2 is all men who were fathers of men in Generation-1. There cannot be more men in Generation-2 than in Generation-1, that would require someone to have 2 fathers[see note re chimeras below]. There could be the same number, but most likely there will be fewer, as some men will have fathered more than one son.
Then Generation-3 is all men who were fathers of men in Generation-2, Generation-4 all men who were fathers of men in Generation-3, and so on.
As the size of each preceding Generation tends to fall, eventually (perhaps at about Generation-10000) we get a Generation with just 1 man.
And that man is very special. Not because he was particularly different from his contemporaries. He is special because he is my paternal ancestor.
He is my many, many times great grandfather in the paternal line. As he is for every person now living, or will ever live in the future, or any historical figure you can think of. His Y-chromosome has passed on from father to son for many thousands of years, to reach me, you or your father.
We will never know his name, or even if he had a name, but he is usually referred to as Y-chromosomal Adam.
He wasn't the only man at the time, or the only man having children. But the Y-chromosomes of all other men have failed to reach us, as every line of their patrilineal descent ended at a man who had no sons.
Similarly, I, you, and all other living people, descend in the maternal line from one woman, referred to as Mitochodrial Eve (because the mitochondrial DNA passes from mother to child).
I don't actually like those names, as they imply that Y-chromosomal Adam and Mitochodrial Eve were the first humans (they weren't), or that they were a couple (highly unlikely). But I can't think of better.
Y-chromosomal Adam lived in Africa. It is difficult to date him, but it is estimated he lived sometime between 160,000 and 300,000 years ago. Mitochondrial Eve also lived in Africa, about 150,000 years ago.
What I find most important about Y-chromosomal Adam and Mitochodrial Eve is that their existence means that every living person is a distant cousin. I find that very moving.
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