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Go to Brilliant.org/SciShow to level up your STEM skills this year. [♪ INTRO]. The marrow in our bones does so much for us.  In addition to storing fats and making sure our bones stay healthy and strong, it’s responsible for pumping out hundreds of billions of blood cells every day!

It is a little weird though, like when you first learn this, you’re like: “Oh—are—like, I thought our bones were just for, like, standing up, not for manufacturing blood!”  And it turns out it’s also weird when you compare us to other animals. While birds are pretty similar, fish make blood cells in their kidneys. And in frogs, production tends to start in the liver or kidneys, then move to the bones as the cells grow up.  Plus, when you think about it, since blood connects to everything in your body, blood cells could come from anywhere.  Which makes you wonder why our lineage settled on bones .

Well, though the answer isn’t 100% certain, scientists have a strong idea: it’s because the space inside our bones is dark. Now, no one fully understands why blood is made in different places, but the areas do seem to have something in common: they protect blood-generating cells from damage. Of special concern are the hematopoietic stem cells, which produce every type of blood cell, as well as hematopoietic progenitor cells, which are similar but can’t renew themselves quite like stem cells can.

Damage to these blood-making cells can create mutations in their genomes.  And mutations can kill the cells or lead to functional issues in the blood cells they generate.  Luckily, the right conditions can help limit mutations. And that’s where bones come in, probably because one of the things that causes mutations is sunlight! Now this idea isn’t exactly new.

Over 40 years ago, a researcher hypothesized that blood production migrated into bones because vertebrates migrated onto land.  See, the Sun’s rays get scattered by water, so the light is more diffuse under the water.  On land, light is more direct, so animals would be more vulnerable to UV damage.  The only problem was, there wasn’t much to back this hypothesis up. Then, in a 2018 Nature paper, researchers published evidence that the blood-producing cells of fish also seek sun protection!  The researchers were examining zebrafish when they discovered little umbrellas of melanocytes covering their kidneys.   Melanocytes are pigment-producing cells, and in these fish, they form an opaque layer over the kidneys, the home of their hematopoietic stem and progenitor cells.  Now, that alone doesn’t prove the goal is Sun protection, so the research team engineered fish that couldn’t make the umbrellas to see what would happen.  Lo and behold, the blood-making cells in those fish ended up with much more UV damage. These shade-making parasols were found in a number of other fish species, from catfish to lungfish to lampreys.

And the researchers noted similar pigmentation patterns occur in the livers and kidneys of different species of tadpoles, wherever the animals produce their blood.  Thing is, once tadpoles grow legs, blood production moves into their bones, which presumably readies them for a bright future on land! And that’s basically what experts now think happened long ago in the first land vertebrates.  In some of them, their blood-making cells wandered a bit—something these cells do from time to time, until they happened to find themselves inside a bone.  And if that meant their blood stayed healthier for longer, it could have given them just enough of an edge to help them win out, leading to an entire lineage of animals with blood factories in their bones.  This episode goes out to Kris, for asking about bones and blood! And if you want to keep learning more about the science behind topics like this, you might want to check out Brilliant’s Physics of the Everyday course.

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