We Are All Related

Hey, Vsauce, Michael here, and you guys are my family. Scientifically speaking, geneticists tell us that every single person on earth is, at the least, 50th cousins with everybody else on earth. It kind of has to be that way.

Think of it like this—you have two biological parents and four biological grandparents, and eight biological great-grandparents. It keeps doubling. Well, if you follow your family tree all the way back to the time of the ancient Romans, 64 generations ago, that tree would need one quintillion separate individuals, which is a number that is larger than the number of humans who have ever existed. Ever.

So what does that mean? Well, I think Bill Bryson put it best when he said, "You couldn't be here without a little incest— actually, quite a lot of incest." Nothing ridiculous, mind you, but if you go far enough back in your family tree, you're going to find that quite frequently, people who were distantly related had kids.

In fact, if you are currently dating or married to somebody who is from your own country and is of your own ethnicity, there's a one in five chance that the two of you share a common family member fewer than 10 generations ago. Furthermore, because our society is becoming more and more mobile, our most recent common ancestor is becoming more recent.

Mathematical models are now showing us that for everybody, every human on earth right now, our most recent common ancestor probably lived only 2,000 to 4,000 years ago. A recent study—it's a great one, you should read it—found that there was a person alive as early as 300 B.C. who is related to every single person alive on Earth today.

Their study also found that that person most likely lived in Taiwan near a busy port city, so their descendants could quickly spread around the world. Now, all that said, you only receive half of your genetic information from your mother and half from your father, and as you go up your tree, it's quite possible that distant, distant ancestors had literally no contribution to your genome.

So who are you? I've said this before, but because bacteria are so much smaller than the average human body cell, your body contains more bacteria than it contains its own cells. And forget about trying to remember every individual cell and bacteria in your body. They're constantly dying and being replaced.

Even cells like brain cells, which seem to live for a very long time, still take in new nutrients and expel waste, meaning that within about every five years, every single atom that used to make up your body has been exchanged. Do you remember what you were like five years ago? Well, the atoms that make up your body, they sure don't.

This is a little existentially scary, but it leads to a very good question. How many atoms are inside my body to begin with? How much is a billion atoms? Well, here's one way to think about this. If you were to count one number greater every second, it would take you 12 days to count to a million, and if you wanted to count to a billion, that would take you 32 years.

So a billion is a very large number, but not when it comes to atoms. Atoms are so inconceivably small that even if you had a billion, say, sodium atoms, their entire weight would only be 38 femtograms. Each individual cell in your body, too small to be even seen by the eye, contains 100 times more atoms than there are stars in the Milky Way galaxy.

Multiply that by the number of cells that make up what we call ourselves, and each human body contains more atoms than there are stars in the known universe. Numbers that large lead to some pretty mind-blowing truths, for instance, this one mentioned by Bill Bryson—because our bodies contain so many atoms, and because we so vigorously recycle them, statistically speaking, about one billion of the atoms inside your body used to belong to Shakespeare.

A billion of your atoms used to belong to Beethoven. A billion of the atoms in your body right now, as you sit and watch this, used to be inside the body of Buddha. You could also think of it this way—a thousand years from now, when humans first land on a planet outside of our solar system, that first person to step onto the surface of the alien planet—their body will contain a billion atoms that are in you right now.

And as always, thanks for watching.