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The purpose of bones might seem obvious. They hold you up, they connect all your muscles together.

But lots of animals get along fine without them. Insects and other inverts don’t have them. Sharks use cartilage.  Even most fish don’t have bones quite like ours.

And no one’s really sure why our kind of bones came to be.  Researchers have had hypotheses, of course, but no real evidence.  That is, until now.  New research published in the journal Science Advances offers some pretty solid support to one idea: that ancient fish started making bones like ours because they stored nutrients.  And the team got that intel using some super cool tech that lets us get a more up-close-and-personal look at fossil fish bones than ever before. More than 400 million years ago in the Silurian Period, ancient jawless fishes evolved bones containing bone cells called osteocytes.  And these eventually became all the rage, All mammals, birds, reptiles, and amphibians, have them. Today, we know osteocytes have a role to play in sensing pressure and other forces, in repairing bones, and in regulating the abundance of minerals like calcium and phosphorus.

But to understand why this modern take on bones evolved, the researchers behind this new paper needed a closer look at early examples of them.  And I mean a much closer look: we’re talking down to a resolution of less than 100 nanometers — and in 3D rather than on a 2D slide. They got that thanks to a technique known as focused ion-beam scanning electron microscopy, which is usually used in materials science for studying things like battery materials and thin films.  In it, an ion beam scrapes away a tiny amount of a sample, which is then scanned in 3D in extremely high resolution by an electron microscope.  This process is then repeated over and over to generate a complete, 3D image of the object. It turns out some of the first bones kind of looked like Swiss cheese.

They were riddled with teeny holes where the osteocytes used to be.  And these cavities were linked by tiny channels a thousand times narrower than a human hair, creating a connective network that’s structurally pretty similar to our own bones. But most importantly, the team noticed that some of the pockets where osteocytes used to be were surrounded by dark, halo-like areas of lower-density bone.  So, basically, bone with less mineral in it. These low-density areas were only found around some of the pockets and weren’t dispersed randomly or evenly.

That led the researchers to think that these regions represent areas where the bone had been leached away through a process called osteolysis, which releases minerals. And it’s something our bones still do today — like, when we don’t get enough calcium in our diets. Now, in the marine environment, calcium isn’t usually lacking.

But fish can and do run low on phosphorus.  And these new-fangled bone cells would have been great at squirreling it away. So their bones could have served as a battery of sorts to recharge their phosphorus supply, which could have allowed them to go on longer journeys or take bigger breaks between meals. And the scientists believe that was probably a good enough reason to keep these fancy new cells in our bones around.

Which is why we have them today! Speaking of things that didn’t evolve the way we always thought, scientists reporting in the journal Current Biology have some surprising news about whales and hippos. Now don't worry!

Hippos are still the closest living relatives of whales, dolphins, and porpoises — the group collectively known as cetaceans.  But since both hippos and cetaceans share some key water-adapted traits — like nearly-hairless bodies and scrotum-less testes — the thinking was that they had a shared ancestor that had all these traits, most likely because it spent much of its time in water. Basically, they assumed that the simplest hypothesis made the most sense.  The new findings, however, show that that’s just not the case.  Instead, hippos and whales evolved from a land-dwelling ancestor, and separately acquired their water-loving features. To uncover this plot twist, the researchers studied the animals’ skin.

First, they found that there were actually some pretty obvious physical differences.  Like, cetaceans’ skin was thicker and full of clusters of fat cells, while hippo skin was much thinner.  Hippos also have specialized sweat glands that produce an orangey liquid known as “blood sweat.”  Yes, BLOOD SWEAT. And while whales, porpoises, and dolphins do have a few whiskers, hippos have both whiskers and some body hair. These little differences hinted that, perhaps, these groups had acquired their adaptations separately after their respective branches of the family tree split off in different directions.

But this was not quite a slam dunk. So, the team looked at the animals’ genes.  They identified 8 skin genes that were inactive in both hippos and cetaceans, meaning that at some point, they’d mutated to the point that they no longer coded for anything.  Now, if this had happened in a shared ancestor, those changes to the DNA should have been the same. But  — drumroll please! — none of them were.

And that suggests that each group turned off these genes separately.  So their shared features didn’t come from a common, water-loving ancestor — they’re convergent evolution! Furthermore, when the researchers dug into the timing of these mutation events, they were able to show that they mostly happened at different times, with whales beating hippos to the punch by 16 million years, on average. It’s fine, though, if your'e a hippo and you're watching this, you're good, you still win.

You have blood sweat, and they don't. Speaking of winners: I’d like to give a big shout out to today’s President of Space, Dave Christopher!  Dave is one of the amazing people who supports SciShow on Patreon. And without him and our other patrons, we wouldn’t be able to make this show happen!

So

Dave: Thank you!  If you want to be like Dave and support what we do here, you now have even more choices than ever, because we have recently launched separate Patreon campaigns for all our channels!  That’s right! SciShow Space, SciShow Psych, and SciShow Kids now all have their own Patreons! So you can pick the channel you most want to support by joining that specific Patreon community. Or join them all, and help us make all kinds of free science content!

They are at Patreon.com/SciShowSpace, Patreon.com/SciShowPsych, and Patreon.comSciShow/Kids. And, as always, you can support this channel at Patreon.com/SciShow.  [♪ OUTRO].