The Leg Bone’s Connected To The Ankle Bone—But Why?

16:58 minutes

Line engraving of skeleton seen from behind, with left arm extended
Credit: Wellcome Collection/CC BY 4.0

A muskox, an armadillo, and a human are about as different as can be. But underneath that skin, fur, and armor, we’re all more alike than we think: one skull, two arms, two legs, a spine. Over 500 million years of evolution has resulted in the same bony framework underlying all mammal species today.

But why is the leg bone connected to the ankle bone, as the song goes? And what can the skeletons of our ancestors tell us about how humans became the walking, talking bag o’ bones we are today? Science writer Riley Black, author of the new book Skeleton Keys, joins Ira to explain why our skeletons evolved to look the way they do.

Read an excerpt from Skeleton Keys.

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Segment Guests

Riley Black

Riley Black is a science writer and the author of several books, including The Last Days of the Dinosaurs, Skeleton Keys and My Beloved Brontosaurus: On the Road with Old Bones, New Science, and Our Favorite Dinosaurs.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. A musk ox, an armadillo, and your uncle Earl. They’re about as different as three mammals can be. But underneath that fur, armor, and skin lies something that connects us to a common ancestor– our skeletons. One skull, two arms, two legs, and a spine. Musk ox, armadillo, uncle Earl– we all have that. That’s the bony framework underlying all mammal species today.

But how do we end up this way? That is, why is the leg bone connected to the hip bone? I almost sang it as the song goes. Give us a call. Our number is 844-734-8255, if you want to talk about that. We’re going to be talking about the skeleton. Or tweet us at @SciFri. We’re going to find out. Joining us now to take us through the evolution of our skeleton and the mystery of our bones is Brian Switek, author of the new book, Skeleton Keys, out now. Welcome back to Science Friday.

BRIAN SWITEK: Thanks for having me back on.

IRA FLATOW: You’re welcome. I think the most interesting thing about the human skeleton to me is that our bones– I think people don’t realize they are alive. They have blood vessels. They do more than just hold up a lot of weight, right?

BRIAN SWITEK: Absolutely. Our skeletons are incredibly active. I mean, even as you and I are talking, as our listeners are hearing us, our bones are being incredibly busy. Their bone cells are laying down new bone material. Their cells, called osteoclasts, that are eating up old bone material, that our bones are constantly shifting, responding to everything from gravity to the hormones in our systems, that they are not this dead static tissue, but they’re incredibly dynamic.

IRA FLATOW: And the marrow puts out products, right?

BRIAN SWITEK: Absolutely, as we were just talking about, too. Blood cells come from our bone marrow held inside of our bones.

IRA FLATOW: So what keeps our bones alive? Is our circulation connected to our bones?

BRIAN SWITEK: So as you mentioned in our lead-up, that bones are fed by blood vessels. And you can see some of these. Our bones are porous. It’s not just this cement-type structure that if you were to look into it in the science called the histology, that you just see something that’s flat, that there are all sorts of nerves and blood vessels that are commonly feeding into the bone, that there’s a give and take between our skeletons and the rest of our bodies.

IRA FLATOW: So take us back to the time before bones. When did we first see the first signs of something bone-like in our history of the earth?

BRIAN SWITEK: So we might think that skeletons and bones would be synonymous, that they would go together, because that’s the way it is in our own bodies. But we know that this isn’t entirely true, because we have things like sharks and rays, even alive today, that have skeletons, but those are made of cartilage, not bone tissue. So the skeleton came before bone. The first thing that bone basically looked like when it showed up was about 455 million years ago as this thing called aspidin, and it didn’t have cells in it yet. It wasn’t as reactive as our current bone tissue. Is it was much more like teeth. But it was the precursor to real bone tissue that evolved as armor outside these fish. They really looked like a biological Roomba, almost, these roundish fossil fish with these tails sticking out the back.

And that was basically the beginning of mineralization of this external skeleton. Once that happened, the internal skeleton that already existed, that was made of a more flexible material, could start to be mineralized, or ossified. So bone started out on the outside, then it was able to form on the inside. And eventually, when fishes, when our ancestors lost that outside armor, we were still left with the internal bony skeleton inside.

IRA FLATOW: Why did that change? Why did it go from outside to inside?

BRIAN SWITEK: So bone initially evolved as a kind of armor protection against many of the invertebrates that were the dominant form of life in the oceans. I mean, they still are on our planet. But the things that were catching and eating are our ancestors– they needed some form of protection against all those rasping and gnashing mouth parts.

Eventually, as vertebrates started to come into their own, when vertebrates evolved their own jaws, when they were able to swim a bit faster thanks in part to the push and pull of muscles against that hardened internal skeleton, that armor on the outside– it ended up weighing them down. They were investing a lot of energy basically into protection. So it’s almost like a trade-off in these great evolutionary arms races between, are you going to invest in a lot of armor and be relatively slow, or are you going to lose all that weight and be relatively quick and nimble and live in a different way? So that probably has something to do why that external skeleton was eventually lost.

IRA FLATOW: At some point reading in your book, ancient fish developed this new structure, a jaw bone.

BRIAN SWITEK: That’s right.

IRA FLATOW: But you said there’s some debate over how they actually did that.

BRIAN SWITEK: That’s right. So the earliest jaws that we know of date back to around 420 million years ago. It’s this really basic anatomical hinge. Prior to this, you had these armored fish, as basically just this hole that was at the front of their skulls, and they didn’t really have very much control about what went into their mouths. They also couldn’t really breathe very efficiently, because a jaw, as it opens and closes, helps pump water back then over the gills and make them more efficient breathers. But how that jaw formed is still debated.

One of the front-running hypotheses is that the gill arches, the bony of struts that underlied the gills in these fish– that they were made of two parts, and it basically had the precursor to that hinge, so that the front-most gill arch ended up becoming the support that eventually became the jaw. Now, there are other methods by which this could have happened, perhaps through a developmental route. But how exactly we got the jaw is still being discussed in debate amongst paleontologists. But we know at least from the fossil record that it does show up about 420 million years ago.

IRA FLATOW: So if you have a jaw, that means you’ve got to have teeth, right? Why would you have a jaw if you don’t get teeth? So do we find evidence of teeth?

BRIAN SWITEK: That’s right. Teeth show up at about the same time. So teeth, and the precursor of bone that we mentioned before, aspidin, show up around the same time as a kind of external armor. And teeth basically got carried along. It’s not as if these fish evolved a jaw, and then there is a sudden need for teeth or anything like that. That’s not how evolution works.

Instead, the teeth or the precursors of teeth were already on the outside of the body, around the area of the mouth, as a form of armor, as a form of protection. So when that jaw evolved, now there’s something extra to grip with. There’s something that literally gives a little bit more bite to when these fish would chomp down on something. And that variation allowed teeth to become locked in as these new sorts of structures that we still have.

IRA FLATOW: How different are teeth from bone?

BRIAN SWITEK: So bone is made of, principally, two different components, a mineral component called hydroxyapatite, and a protein component, called collagen. And these are basically the hard parts and the flexible parts of bone that make them so useful to us, that bone has a bit of give to it. It can bend and flex. If it was just the mineral part, it would shatter very easily.

Teeth have an outer coating of enamel. And that’s a very hard substance. And it doesn’t respond the way bone does, that basically you get your two sets of teeth, you have your milk teeth when you’re little, and then you get your adult set of teeth. But that’s it. Your teeth aren’t continuing to grow on the outside or change. You can wear them down, but they don’t repair themselves the way that bone does. So it’s almost much more a cement-like structure, but it’s also a lot harder, and that’s what makes it so great for biting and just constantly literally chewing through our lives.

IRA FLATOW: Let’s go to San Antonio to Lupe. Hi. Welcome to Science Friday.

LUPE: Hi, there. I have a two-part question. This is mostly for the older person. Everybody has bones. Why don’t we have an efficient and safe way to strengthen our bones, and what is your recommendation for strengthening our bones if you can’t do weight-bearing exercise or strength training? Thank you very much.

IRA FLATOW: All right. Well, I don’t know if you qualify to answer those, Brian, but give it a shot.

BRIAN SWITEK: Well I’m not going to answer the medical question, just because I’m not a doctor. But I will say, the point to that, that exercise and what we go through– it strengthens our bones. It’s important to the health of our bones. For example, people who go up into space, in zero-G environments for months at a time, lose about 1% of their bone mass per month, because there’s not that push and pull on that exercise that there normally is down here on Earth to keep bones strong. So they lose some of their bone density. Their bone become more fragile.

Why we don’t have a system that keeps our bones healthy as we age– I mean, this is something where we might be living longer than we ever expected to, or that evolution might have not the evolution plans, but that it’s something that just hasn’t been an adaptation yet, that our bones actually do a lot of work repairing themselves and constantly putting down new tissue. And things like osteoporosis, loss of bone density that many of us unfortunately experience– that has to do with, the bone cells that are laying down new bone material that aren’t doing so as well, or the bone cells that are eating old bone tissue being a little bit too active, or possibly both.

So it’s really a matter of the basic maintenance crew that’s always acting in our bones starting to get a little bit out of whack. and how that might change– that’s more of a medical question, but it has to do with just the basic day-to-day life of our skeletons.

IRA FLATOW: And it’s strange to think that we could have anything other than two arms and two legs. But how do you wind up with a number like this? How did we get the two arms and two legs? You write it was a weird genetic quirk in ancient fish?

BRIAN SWITEK: That’s right. So I mean, we could have wound up, all other things being equal, with just two limbs, or we could have wound up with extra limbs. This is just really a historical accident. So the beginnings of limbs were pectoral fins– so basically, fins on fish that are in the equivalent of their chest area. And it seems to be this genetic mutation, this duplication event, that created another set of limbs. Something happened, a mutation occurred that during development, fish started to get the second set of limbs.

And we know that this happened– aside from tracing back through DNA comparisons, matching those against the fossil record, but the fact that if you think about your arms and you think about your legs, they’re laid out in the exact same way, that there’s a single upper arm bone, just like there’s a single upper leg bone, your femur. There are two lower arm bones, just so you have your fibula and tibia in your lower leg. And then the mess of all those hand and foot bones and your finger bones and your toe bones– they’re laid out in exactly the same pattern. That’s what researchers call homologous, that these are basically the same structures, just expressed slightly differently. So if that mutation hadn’t happened, if that duplication event never happened, vertebrates might have very different body plans, and something that is humanoid may never have evolved at all.

IRA FLATOW: That’s why when we think of aliens, they can have any amount of limbs, right?

BRIAN SWITEK: Right. And that’s an easy way to make something look a little bit sci-fi, is just stick an extra pair of limbs on there, and you get an alien.

IRA FLATOW: Speaking of aliens, let’s go to Cincinnati. No, that was a joke. Let’s go to Adriana in Cincinnati. Hi. Welcome to Science Friday.

ADRIANA: Well, thank you so much. Thanks for taking my call. I was just wondering– as someone who has a lot of neck troubles, I was told that that top vertebrae, the atlas, I believe, is key to the passing of the information to the brain. And from a design standpoint, it seems really flawed. I’m wondering if that’s common in other animals and this skeleton structure. And I’ll take my answer off the [INAUDIBLE].

IRA FLATOW: Design flaw up there?

BRIAN SWITEK: I mean, really, human skeletons are really unusual. They’re really kind of weird. There’s no other animal that walks like we do or stands upright like we do. And a lot of these balancing issues, like the skull being balanced on the atlas, or a lot of the back problems that we have, or the fact that our shoulders are relatively easy to dislocate, comes from our human ancestry of basically moving out of the trees around 4 million years ago or so and starting to spend more and more time on the ground in this upright posture, that we didn’t end up knuckle-walking like chimpanzees and gorillas do, and having this more horizontal posture where there’s not as much pressure and not as much weight being stacked up on our spines in some of these ways. But we had a different way of moving.

And to go back, for example, to our shoulders dislocating, part of the reason that that can happen is because our shoulders aren’t really connected to the rest of our body very firmly. You might think that it has this solid connection, because we use our arms for just about everything. But if you follow your shoulder blades, just floating over the back your rib cage, it connects to your upper arm bone, your humerus, and that all connects by way of your collarbone to the top of your rib cage, this is really this tiny connection for this critical appendage that’s really just held in the soft tissue. So of course, if you whack that too hard, or you do something and pull it, it’s going to get pulled out, because there’s nothing really anchoring it there other than the muscles in the soft tissues.

So this is all basically– we can thank our ancestors of millions and millions of years ago as they moved onto the ground for some of these problems that we now experience.

IRA FLATOW: This is Science Friday from WNYC Studios. I’m Ira Flatow, talking with Brian Switek, author of Skeleton Keys, a great book about bones and things you never really realized. And our spines– we were just talking about our shoulders and up there, our spine. What changed about our spine that made it possible for our ancestors to stand up?

BRIAN SWITEK: So there are a couple of changes. And if you look at Lucy, the famous skeleton of Australopithecus afarensis that lived about 3.5 million years ago, a prehistoric human, the skeleton of Lucy– in terms of the arms, [INAUDIBLE] are relatively long, the fingers are relatively curved. This is still a human that’s spending a lot of time climbing around in the trees. But the spine and the lower body are all about upright movement on the ground.

And in terms of the spine, you see a phenomenon called lumbar lordosis. So basically, the lumbar section of the back, low down in your back, the vertebrae become wedge-shaped, where they’re a little bit narrower towards one side than the other. And that’s because of the way that these things stack. Basically, that S curve of our spine, that it’s not a straight rod, but there’s a curve to it where it’s out at your shoulders, and then it goes in towards your stomach and then back out again towards your hip. And this is part of the balancing act for being able to stand upright.

Changes in the hip were also related to this, where we have bowl-shaped hips that basically hold the weight of our viscera and our internal organs in our upright posture. Where if you look at the skeletons of, for example, a gorilla or a chimpanzee, that they have these hips that flare out in a different way but aren’t really holding in the organs in the same way, because they spent a lot more time on all fours. So these are some of the changes that just came with that balancing act.

IRA FLATOW: Adam on Twitter shouts out, hey, the hyoid bone. What’s the story there?

BRIAN SWITEK: Yeah. So this is a bone that we often forget about, because it’s another one that’s not anchored directly to the skeleton. So the hyoid bone is hidden behind your lower jaw. It’s a bone in your throat, and it’s an anchor point for the musculature of your tongue. And it’s thought to be very important to the evolution of speech.

So sometimes when paleoanthropologists in particular want to figure out whether a prehistoric human had basically the mobility to make certain sounds, they’ll look for the hyoid bone to see what its shape is and if it relates to what we see in ourselves to get an idea of what sort of sounds may they have been capable of making. So this is one of the secret bones in our skeletons that isn’t always obvious, but it’s certainly critical to our day-to-day life.

IRA FLATOW: Last question for you. You say if things had gone a bit differently, we might still have an eye bone. What is an eye bone?

BRIAN SWITEK: So our distant ancestors, our proto-mammal ancestors going back to things like Dimetrodon, so that sail-backed lizard-looking creature that looks kind of like a dinosaur but is much more closely related to us– they had bones in their eyes called scleral rings. And you can see this today in reptiles and some fish and some other organisms. Dinosaurs had them as well. And no one entirely knows what the function of these are. It’s thought to be supporting the eye shape systems, specifically in creatures that might have eyes that aren’t totally round, or creatures that live in high-pressure environments, like the deep sea, like ichthyosaurs in the past.

So we’re not entirely sure what it does. But around 200 million years ago or so, when our weasel-like proto-mammal ancestors these things called cynodonts, started to get smaller, when dinosaurs were getting big and mammals were getting small, something changed, and we lost those eye bones. And I, for one, am thankful for that. I’m glad that I’ve never had to go to the emergency room with a broken eye bone. That would probably be terrible.


IRA FLATOW: It’s a great book. Brian Switek is author of Skeleton Keys. He’s also a writer for a Laelaps blog for Scientific American. You answered a lot of questions in the book. Thank you. Thank you for taking time to be with us today.

BRIAN SWITEK: It’s been a pleasure. Thank you so much.

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