Author James Solheim
Then 32, then more than 60, then more than 120,
then more than 200, more than 500,
then more than a thousand
great great great great great great great great great grandmas
about 200 years (two centuries) ago.
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You have more than a thousand grandmas just ten generations back (red dots in the outer ring).
YOU
When we get back to about five thousand years ago or a little more, all people belonged to two groups: ancestors of everybody alive today and ancestors of nobody alive today.
Your number of grandmas doubles in each generation. Doubling for just twenty generations is over a million.
In this picture, you and your recent grandmas are way, way, down in the middle.
Each dot in the big circle's edge stands for twenty thousand grandmas. That's how hard it is to draw all the grandmas from the long-ago time when we all share the same ancestors.
Doubling makes numbers go up very fast. That's why everyone on earth comes from the exact same grandmas who lived only about five thousand years ago or a little more. Every living person today comes from all of them.
If just one of those millions of grandmas had not been born, none of the billions of people today would have ever lived.
How can that be true? At first, doubling raises numbers slowly. Two leads to four, four leads to eight, and so on. But soon the numbers get huge. If it weren't for branches that join other branches, a family tree holding a thousand ancient grandmas at one level would have two thousand grandmas just one level before that.
In about 1300 to 1400 A.D., your grandma total was over a million. Just one generation before that, the number was over two million.
In fact, if the same grandmas didn’t start appearing over and over in different parts of the family tree, you would have needed more than four billion great-great-great-great-etc. grandparents in your tree as recently as the year 1000—more than ten times the number of people in the world at that time!
Mathematician J.T. Chang and computer scientist Douglas L.T. Rohde showed how it happened. Even if we include the most isolated people on earth, our search back in time quickly reaches a point where everyone who is an ancestor to any living person is an ancestor to every living person.
And the most recent individual grandparent to everybody on earth might have lived as recently as two thousand years ago!
Sometimes people claim that Genghis Khan has more descendants than anyone else who ever lived. That's not true. His mom and grandma are ancestors of all those same people—plus they are ancestors to multitudes in other family branches outside of Genghis's. So they have way more ancestors than he does!
Everyone on earth comes from emperors. Everyone on earth comes from slaves. You come from the Iraqi inventors of writing and the African creators of the oldest known math tool.
And in only a few thousand years, you could be a
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grandparent to everyone on earth.
Everyone on earth will be alive because of you.