When mommies and daddies love each other very much, they make a recipe for a baby. They mix a copy of half of daddy’s recipe with a copy of half of mommy’s recipe to make a baby recipe. The recipe is so long that it takes nine months to make a baby!
And by looking at your recipe later, you can see that you are related to your mommy and daddy because you can see bits of their recipes in you! (Or you’re adopted, but your mommy and daddy still love you!)
And this can go back through the generations. If half of grandpa’s recipe goes into your mom, and half of mom’s recipe goes in you, then one quarter of your recipe comes from grandpa!
Now, if you have first cousins, that means one of your parents was the brother or sister of one of theirs. And those siblings had the same parents… your grandparents. By comparing your recipe to the recipe of your first cousin, you can see that you share a common grandparent. This is called common ancestry. Since recipes get shared in an unbroken chain from ancestor to descendant (that means a baby!), if you have enough information, you can determine whether two recipes have a common ancestor. Fortunately, those recipes are really long, so there is a lot of information.
But sometimes little accidents happen to the recipes. This is really important, but we’ll save that for when you are six. But one particular kind of accident is when you get sick. Sometimes a germ will leave its cooties in your recipe. Ew!
Before, maybe your grandpa had a recipe with a line that said:
Step 146734 Make five itty-bitty toes on the end of each foot.
And afterwards, it might read
Step 146734 Make five itty-bitty toeGERM COOTIESs on the end of each foot.
And now that might be part of your recipe! Because he is your ancestor.
Your friend on the playground might have this in her recipe:
Step 146734 Make five itGERM COOTIESty-bitty toes on the end of each foot.
Do you have a common ancestor with her?
Did you say no? Because the cooties are in the wrong place? Haha, the joke’s on you. The answer is actually yes. All human beings are related. But looking at this one tiny piece of the recipe, we don’t have any evidence that your friend descended from your grandpa. (Don’t ask him about it in front of your grandmother.)
Since grandpa got the cooties in his lifetime, it can only show up in that exact spot in his descendants, or in someone else who coincidentally got the cooties in the same exact place in the recipe. But the recipe is so long this is very unlikely.
But if we look at the whole recipe, you and your friend actually have a lot of recipe cooties in common. Ew! I know. But it’s pretty harmless. Everyone has them. Thousands of them. And because a lot of them are in the same place, we know you share common ancestors. But since a few of them are different (like the one from your grandpa) we know that your common ancestor was further back in generations than your grandpa.
So by comparing the number of shared cooties to the number of unshared cooties, you can see how closely related you are.
And when we compare your cooties to those of a chimpanzee, we find a lot of cooties in different places, but a lot of cooties in the same place! We also have common ancestors, but it wasn’t in your grandpappy’s day or your great great great grandmammy’s day. It was 5 million years ago.
In fact, orthologous cooties fall into a nested hierarchy among primates.