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Why are there so many different blood types?

It’s a mystery that dates back nearly 20 million years

Do you know your blood type? If you haven’t been in any medical situations where blood type is important, you might not. I certainly don’t, even though I’ve been fascinated with blood since I was a sophomore in high school.

That year, I was diagnosed with Type 1 diabetes. That meant, among many things, that I had to make myself bleed nearly every day as I monitored my blood sugar levels through a finger prick. (I now wear a monitor with an under-skin sensor that continuously checks my blood sugar, saving my fingers from so many pricks.) So when that same year, I learned how to use microscopes in science class, curiosity got the best of me: I took some of my blood and placed it under the microscope. The choreography of red and white blood cells dancing around the slide left me in awe.

But as common as blood is, there’s a lot that we still don’t know about it. We know there are eight main blood groups that make up a majority of the world’s population and dozens of rare types as well. But it turns out that scientists still don’t know why we evolved different blood types.

In the video above, we go over some of the early theories surrounding blood that date back to 200 CE. I also take a test, called the Eldoncard, to figure out my blood type. But as much as I can learn about my own blood, we may never know how or why all the blood types evolved as far back as 20 million years ago. That’s frustrating — but also incredibly fascinating.