How Intermingled Are Our Family Trees?

By Trace Overby

Whether you realize it or not, we’re related. If you go back into the deep branches of your family tree, somewhere down the line, we will have an ancestor that is the same. It’s crazy to think about, but it is true.

Mathematically speaking, there are not enough ancestors for everyone to have their own set. There aren’t even enough for every branch in your family tree to be separate. That means somewhere in everyone’s family tree there are ancestors who show up more than once. And that means incest.

Could these Cobblers be more than friends? Photo by Kiani Krantz.
Could these Cobblers be more than friends? Photo by Kiani Krantz.

Every 25 years you go back in your family tree, the number of ancestors you have doubles. So if you were to go back 1,000 years, you would have 500 billion ancestors! That isn’t right, though. There have only been seven billion people to ever walk this earth, which means that your ancestors are showing up in more than one spot in your family tree.

A demographer named Kenneth Wechtel estimates that if there was a boy born in England in 1947, and you traced his family tree back to the time America was discovered, he would have 60,000 ancestors. Ninety-five percent of the people in his family tree were different individuals. Five percent would be ancestors that pop up in multiple places in his family three. At the time of the Bubonic plague, he would have about 3.5 million ancestors, and only 30 percent of those ancestors fill only one place on the family tree, which means . . . 70 percent fill more than one spot!

In royal families it was quite common that individuals were married off to a close relative. With the family of Charles II of Spain, it got so crazy that he would have been less inbred if a brother and sister would have been his parents. Needless to say, the branches in his family tree were very tangled.

So while it seem gross to think about, this incest is what makes us today. Some scientists believe that we are all at least 50th cousins. That seems weird (it could mean we’re related to Hitler), but in a way it is really cool.