Maybe you’ve heard that a huge percentage of your bones are in your feet!

And I'm here to tell you, that’s true! 25% of your bones live in your socks. Which begs the question, why are there so many bones down there?

It seems like overkill. To fully grasp the answer, we have to follow our ancestral footsteps, all the way back into the ocean, hundreds of millions of years ago. [intro music] First, let’s start by  establishing what we even mean by the word “foot.” Sure, it seems like there should be a pretty straightforward definition. But you’d be surprised.

What we put into a shoe consists of 26 bones. Seven tarsals, five metatarsals, and 14 phalanges. You can also divide the foot into three sections: the hindfoot, midfoot, and forefoot.

But while all those bones are solidly confined to what we’d call a foot on us, that’s not true for most other tetrapods, AKA animals with four limbs and a backbone. You may be familiar with those color-coded diagrams of animals’ limbs that highlight the bones that are homologous, meaning they share an evolutionary origin across species. But homologous bones often look really different depending on the animals they’re on, hence the need for color-coded diagrams.

For instance, your femur looks different from a dog’s femur, which is different from a  whale’s femur, et cetera. And that variation means some species have feet with way fewer bones than ours have. Or at least, what we’d call  their feet have less bones.

Like, if I were to put a sneaker on a horse, where would the top of the shoe stop? Here? Down here?

All the way up here? But the bones that are homologous to what goes in our shoes go all the way up here. Half of their leg is made of foot!

So it’s really not that we have any extra bones or anything. It’s that more that compared to some other animals, ours are concentrated down into one small area. And that brings us to our next point.

Regardless of whether or not those bones are considered part of a bone or a leg, not all animals even have the same bones. Some of them, like our good friend the horse, decide to drop some spares. Another example of this is the jerboa, a rodent whose claim to fame is its excellent jumping skills, in addition to a ridiculous ear-to-body ratio.

Not only do these rodents have exceptionally long metatarsals compared to some of their closest relatives, the birch mice, they also have fewer of them. As in, most jerboa species have their metatarsals fused together to make a megatarsal. All this to say that, depending on what direction natural selection pushes them in, volution will run its course and modify the shape and number of bones depending on the need, which creates a lot of diversity.

Of course, natural selection can only work with what has been inherited, and therein lies the real story  of how our many foot bones came to be in the first place. So let’s follow those footsteps all the way back to our ocean-dwelling ancestors, who didn’t even have feet. We’ll start about 560 million years ago, when the ancestors of all the jawed fishes started evolving to have two pairs of fins on their bodies.

Now there’s a pretty big leap to be made from our four fins to four limbs. Our ancient marine ancestors had to hone in on a lot of important details like wrists, ankles, and digits. Letting go of our flimsy fins was a given, since we had to evolve something that could support our weight on land.

After all, we were leaving  the buoyant seas behind us, and the force of gravity  became much less forgiving. To do that, we had to do away with our fin rays, the long bony structures which  support the fin membranes, and replace them with more solid bones. From there, those pairs of fins chock full of rays slowly morphed into four limbs with sets of many-boned digits.

These early tetrapods hadn’t dialed in on how many digits they’d have per limb, and some would have eight digits on each one. But about 350 million years ago, our ancient tetrapod ancestors made the drop down to five, which paved the way for all tetrapods thereafter  to consistently inherit that same number of digits. And this five-fingered and toed pattern is still found across the vast majority of modern tetrapods, even in the ones that have further decreased their digit count, like our horse friends.

You can see this in how they develop as embryos. See, even though they come out with only one full digit per leg at birth, horse embryos briefly start to  develop five digits per limb, before reducing down to one. And hundreds of millions of years have passed since those 5 digits first showed up.

That’s ample time for all the  different land vertebrates to go wild evolving some  pretty extreme modifications of those bones based on how they wanted to get around. Paddles for swimming, wings for flying, hooves for hoofing, you get the picture. And depending on how they  adapt to use their limbs, there ends up being significant tradeoffs between stability versus flexibility.

Stability is key for joints  that act as long stiff levers to support running, for example. In that case, bones can fuse together or are lost over time, which reduces the total number of joints, and cuts out unnecessary bendy bits. On the other hand, or foot, having more joints and bones means you can increase your  flexibility and dexterity, performing lots of different movements with those little mitts.

And for our tree-dwelling primate ancestors, that flexibility was key. Primates popped up around the time that the dinosaurs kicked the bucket, and we took what our tetrapod ancestors gave us and really ran with it. Or more accurately, swung with it.

Non-human primates are known for having not only flexible hands but also hand-like feet, and being able to do a lot with them, like grabbing onto tiny branches. And like I mentioned earlier, the more complex movements you want to make, the more moveable joints you need to have, because of that tradeoff between stability and flexibility. So, it makes sense that arboreal primates that spend a lot of time in trees also have a lot of bones in their feet.

Which, okay, is great for our tree-loving primate cousins, but it still doesn’t answer the question, why do we still have all these bones if we can’t do any of that fun primate stuff with our big, flat feet? Especially since we know other species like horses did away with all their extra bones just fine. Well the thing is, it wasn’t that long ago that we could do all that fancy foot-grabbing stuff.

This SciShow video is supported by Brilliant: the online learning platform with thousands of lessons in computer science, math, and science. They make learning fun with interactive puzzles and quizzes. Brilliant gives you real data to play with, so you get hands-on opportunities to solve problems and learn concepts from the ground up.

Those concepts include data, programming, math, AI, and all sorts of topics that you can use in your day to day life. To give Brilliant a try for free for the first 30 days, go to Brilliant.org/SciShow, the QR code, or the link in the description. That link also gives you 20% off an annual premium Brilliant subscription.

Humans split from our last common ancestor with chimps sometime around the late Miocene, and unfortunately there’s a 7.5 million year gap in the ape-foot fossil record. That means we’re missing some key information on what any ape feet looked like between 4.4 and 11.9 million years ago. That said, fossils from before the split tell us that the feet of our last common ancestor probably looked a lot like today’s chimpanzees.

This means they had a long opposable big toe, or hallux, used from grabbing onto things, and a super flexible ankle for maneuvering around tree trunks and branches. even though we have some blanks to fill in on our handlike-foot-to-actual-foot timeline, a 4.4 million year old fossil from a species called Ardipithecus ramidus does give us a good idea of what that transition may have looked like. This ancient ape still had a long hallux, AKA big toe, sticking out to the side like non-human apes, but their midfoot was less flexible than you’d expect from a strict tree-climber. This tells us they used the flat part of the foot for a pushing-off motion, AKA walking.

So it appears that this particular ape was walking upright on two legs a lot more than its earlier ancestors, but just hadn’t fully given  up on life in trees just yet. While foot fossils of our early ancestors are few and far between, fossils like this help us piece together what that transition would have looked like. Another important step to understand is when our big toe began its slow migration back into line with its buddies.

Remember what I said about  stability versus flexibility? Well, it turns out having a toe that sticks out at a 90 degree angle from your foot is not ideal when you’re walking around. For one, toe stubbings would  increase exponentially, and no one wants that.

Plus more modern studies on human feet show that our big toe is key for helping us push-off with every step, so it’s much more useful where we’ve put it. We know that a couple species like Australopithecus afarensis and Australopithecus africanus that lived around 2 to 4 million years ago were walking around bipedally, for the most part. And based on footprints, we think their big toes were in line with the rest of the foot as early as 3.6 million years ago.

So once we settled on walking on our two feet, we lost the flexibility of our big toes, but kept all the bones. We see this around 1.8 million years ago in foot fossils from two species within our genus Homo, thought to be from Homo habilis and Homo erectus. And we even have some preserved footprints from Homo erectus, too.

Their footprints revealed a long foot arch, just like modern human feet. So by then, we had fully arrived at bipedalism, and the appropriate foot  shape to go along with it! All that said, it was only  over a few million years that our hand-like feet became, well, just feet.

That means our evolutionary path was paved in a relatively short timescale since we came down from the trees. Compare that to horses, whose ancestors never bothered messing with life in the trees to begin with. It was a much more  straightforward evolutionary path.

No long flexible digits to reverse-engineer. But because we share all  the same set of foot bones with our modern great ape cousins, we had to make all those bonus bones work, as best as we could. Humans have evolved the tweaks we needed to make our feet work for us, and until natural selection starts to favor people born with fewer foot bones, all 26 bones per foot are here to stay. [ OUTRO ]