As an avid family history buff, I found Roots of human family tree are shallow to be an interesting article.
That means everybody on Earth descends from somebody who was around as recently as the reign of Tutankhamen, maybe even during the Golden Age of ancient Greece. There's even a chance that our last shared ancestor lived at the time of Christ.
"It's a mathematical certainty that that person existed," said Steve Olson, whose 2002 book "Mapping Human History" traces the history of the species since its origins in Africa more than 100,000 years ago.
The article doesn't say this common ancestor was the only person living then. Just that everyone living today can trace at least one line of their ancestry back to this one individual.
Every person has two parents, four grandparents and eight great-grandparents. Keep doubling back through the generations — 16, 32, 64, 128 — and within a few hundred years you have thousands of ancestors.
It's nothing more than exponential growth combined with the facts of life. By the 15th century you've got a million ancestors. By the 13th you've got a billion. Sometime around the 9th century — just 40 generations ago — the number tops a trillion.
But wait. How could anybody — much less everybody — alive today have had a trillion ancestors living during the 9th century?
The answer is, they didn't. Imagine there was a man living 1,200 years ago whose daughter was your mother's 36th great-grandmother, and whose son was your father's 36th great-grandfather. That would put him on two branches on your family tree, one on your mother's side and one on your father's.
In fact, most of the people who lived 1,200 years ago appear not twice, but thousands of times on our family trees, because there were only 200 million people on Earth back then. Simple division — a trillion divided by 200 million — shows that on average each person back then would appear 5,000 times on the family tree of every single individual living today.
Our family can trace four lines back to four of five children born to George Morton (1585-1624) and Juliana Carpenter Morton (1584-1665) of Plymouth, MA. That is only 12-14 generations back. I also know that Melissa and I are 9th cousins once removed. I had never really stopped to think about this mathematics in quite this way, but it makes sense.
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