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You may have heard the fun fact that humans and giraffes have the same number of neck bones, which is both true and wild. We have exactly seven, and so do pretty much all the mammals alive today, regardless of how long their necks are. But the reason why is bizarre, and involves millions of years of evolution, your diaphragm, and even cancer. Here's the 4-1-1 on our seven special neck bones. [♪ INTRO] This video is about vertebrae, the bones of your spine. Humans have four types.
The cervical vertebrae are in your neck, the thoracic vertebrae are in your upper to mid back, your lumbars are in your lower back, and the sacral vertebrae are all fused together into a wedge thing in your pelvis. Humans have seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, and five sacral vertebrae. Technically there's also your coccyx which is a vestigial bit of the tail that our ancestors once had, but we can ignore her. Now that we've got that out of the way, let's talk about other animals. Specifically the vertebrates, because, you know, vertebrae.
It's in the name. The main categories of vertebrates are reptiles, mammals, amphibians, and fishes. And if you think I forgot birds, no I didn't, because they're reptiles, too. And even though we all have bony backs, there's a lot of variation in what that looks like for different animal groups. Some have a lot more vertebrae variation than others. A 2011 analysis of vertebral anatomy in squamates, the reptile group containing snakes and lizards, found that the total number of vertebrae changed most dramatically when there was
more selective pressure to make big changes to an animal's body shape, not just its size. For instance, the species with the fewest vertebrae was a dwarf chameleon, which had just 14 vertebrae, while the snakes that they looked at had anywhere from 136 to more than 290 vertebrae. Snakes are odd too, because their vertebrae are de-regionalized, meaning that after the first few, they don't really have the features you expect for something that's a lumbar vertebra versus a thoracic. And if that wasn't enough variation for you, let's talk amphibians.
They're all fairly good at playing with the number of vertebrae they have, but some are better than others. One study suggests that salamanders that are full-time aquatic through all of their life stages have more variation in vertebral numbers than the ones that spend some life stages on land, and some in water. Some populations of salamanders can even have different numbers of vertebrae based on the temperature that their egg was while they were developing. Yes that's real, and I don't know why. So for most vertebrate groups, it's pretty easy to play around with how many bones you've got in your spine. And then, there's the mammals.
Mammals pretty much always have between 26 and 27 vertebra. 7 cervical, and between 19 and 20 thoracic and lumbars. Exceptions to this rule are rare and often weird, like the hero shrew. Their vertebrae are interlocked in a way that keeps them stuck together like puzzle pieces, and some species have as many as 10-12 lumbar vertebrae, where most other mammals have like, five. But every species has their mutants, so you'll still find oddballs with more or fewer vertebrae than you'd expect. And yeah, that does include us. Like, the singer Adele has a sixth lumbar vertebra, so there's your pop culture trivia fact of the day.
You're welcome! Anyway. For a species to change their vertebral formula, it starts with an individual that has a unique mutation. If that mutation isn't actively harmful to the animal's survival, it gets passed down, and members of the next generation might have it, too. Give it a few more generations and you've got a permanently altered spinal column. When a mammal species has a different vertebral count than its relatives, the most common change is for a single vertebra to change types, rather than adding or subtracting bones. For instance, humans and chimps have the same number of vertebrae. But instead of our 12 thoracic vertebrae, chimps have 13, and instead of five lumbars, they only have four.
Same total number of vertebrae, just different types. And variation within species is relatively rare too, but it's most common in species that are using their spines for stability, not flexibility. Think sloths, not cheetahs. And yeah, that weird hero shrew. This may be due to the fact that most of the changes we see in mutant individuals start off asymmetrically, meaning only one side of the vertebra is different. So it would only have one rib, or only be half-fused to the rest of the sacrum. An animal that relies on a lot of spinal movement to get around would probably struggle if their
spine was asymmetrical, which may be why they don't really riff on their vertebral formula much. But even though mammals do have some wiggle room in the lower vertebrae, virtually every mammal species, from humans to hero shrews, giraffes to gray wolves, all have exactly seven cervical vertebrae. There are three groups of mammals that differ from this pattern, out of the thousands alive today. One is the manatee, which usually has six cervical vertebrae. The others are the two-toed and three-toed sloths, which split on the evolutionary tree between 23 and 40 million years ago.
The two-toed variety can have as few as five vertebrae and the three-toed can have as many as 10. That's just three times that mammal evolution led to changes in the number of neck bones a species has. Three. Vertebrates only evolved powered flight three times, and that feels way more complicated than adding or subtracting a neck bone or two. And we know that it isn't just that natural selection hasn't pressured any other mammals to stick their necks out, or squish them down. Giraffes are the obvious example of a species that could use a few extra bones to stretch their necks out, but camels and llamas also have long necks that might benefit from a few bonus bones.
There are definitely times where having a shorter neck would help, too. Manatees are just giant round blobs without a defined neck region, which is probably why they didn't need that seventh cervical vertebra anyway. Meanwhile, their close relatives, the dugongs, also have pretty short necks, but they accomplish this by basically just having pancakes for vertebrae. So what gives? The answer is in our genes. Like, way in there. If you think back to your biology classes, you might remember learning about Hox genes.
They're a group of super-important genes that give us the blueprint for all the earliest steps of embryonic development. These genes basically tell that undifferentiated ball of cells how to make all the subunits of the body that you need. So to change how many vertebrae you have, or what kind they are, you need to make tweaks in those Hox genes. But based on all the vertebrate variation in non-mammals, it seems like most animals can alter their Hox genes, no problem. So why can things like birds and salamanders do it and we can't?
The answer is cancer! In 1999, a researcher named Frietson Galis published a paper looking at why we all have seven neck bones. As part of this work, she looked for groups of people who were more likely to have altered spinal vertebrae. Specifically, she wanted to identify populations who had something called a cervical rib, a small rib-like bone coming from their seventh cervical vertebra. Having one of these means that person has a mutation in Hox gene expression for forming their neck bones. And Galis did find evidence in the literature that there's a group of people more likely to have cervical ribs than the rest of the population.
It was people with cancer. People with certain childhood cancers and cancers that developed in utero were more likely to have cervical ribs, and so were babies that were stillborn for undiagnosed reasons. An earlier study of 1000 children with tumors and 200 children without tumors found that 21.8% of the children with malignant tumors had at least one rib abnormality, compared to only 5.5% of children without tumors. And not all tumors were equal; the rate was 33% for neuroblastoma, and around 27% for both leukemia and brain tumors. In a later study, Galis and a team of researchers looked at the frequency of
cervical ribs among miscarried fetuses and stillborn infants from a single hospital, some of which had other congenital abnormalities. And what they found was shocking. Around 30% of the fetuses and stillborn infants that had no other visible abnormalities had at least one cervical rib. And for those with multiple other major abnormalities, the rate was over 60%. They also did a literature review looking for papers reporting on cervical ribs in adults. While these papers varied in sample size, the rate of cervical ribs in adults ranged from just over 1% down to 0.05%.
The largest study looked at over 4 million x-rays and found that only 0.2% of those people had cervical ribs, so this mutation is super rare among people that make it to adulthood. Basically, this strongly suggests that mutations that change the number of cervical vertebrae we have are really strongly associated with a lot of bad things, which means those changes are far less likely to be passed down to the next generation. All that said, there are also some non-cancer reasons to worry about messing with your cervical spine. Having cervical ribs is also linked with something called thoracic outlet syndrome.
It's a degenerative condition where the nerves in your neck and shoulder are compressed, which can severely limit your ability to use your arms. So it's not cancer, but it's still pretty bad. Another less cancer-y hypothesis about why we're stuck with our lucky seven cervical vertebrae has to do with our diaphragms. The diaphragm is the muscle that sits below our lungs, and when it contracts, the change in pressure causes our lungs to inflate. Then when the muscle relaxes, our lungs constrict back down, releasing the air.
In early fetal development, the cells that turn into the diaphragm originate at the cervical spine and then migrate down to where your diaphragm will go. And the nerve that makes the diaphragm contract comes out of our cervical spine, too. Because our diaphragm is so connected to our necks, it's been suggested that messing with cervical vertebrae also puts you at risk of compressing that nerve, meaning that you might not be able to breathe. Which would be bad. But nerves aren't the only issue here. The diaphragm's position in our chest is super important, and I'll show you why. Put your hands on the bottom of your ribcage, like this, and take a deep breath in.
You should feel that the bottom of your ribcage expanded outwards. Your diaphragm is connected to those ribs, which is why they move when you breathe. If your diaphragm was only attached to the ribs higher up, where they don't move, it would be much harder to take deep breaths. So in order to get the deepest breaths that we can, the location of the diaphragm in your chest really matters. That's why for most animals, if you breathe with a diaphragm, that muscle will be attached right around the border between the moveable and immovable ribs. There are a few animals that have unusually long ribcages and so, their diaphragms aren't in the exact spot we're talking about.
And two of them happen to be some animals we talked about before: Two-toed sloths, and manatees, both of which have fewer than seven cervical vertebrae. So it seems like when the two-toed sloths and manatees reduced their cervical vertebrae, it may have messed with the location of their diaphragm too, but they just made it work. This hypothesis also lines up nicely with what we know about the rest of the vertebrates, because all the other vertebrate groups don't have diaphragms. Birds use air sacs to breathe, other reptiles have muscles that manually expand their ribs, and don't even get me started on amphibians.
Not having a diaphragm to worry about would also help explain why those guys can mess with their vertebral counts and not, like, suffocate. Okay, but what about the cancer thing? Why don't the other animals get more cancer when they change their neck vertebrae around like we do? We're still not totally sure. It's been suggested that sloths and manatees have lower metabolic rates than other mammals.
See, when your cells metabolize energy, they also produce byproducts called free radicals, or oxidants. These are usually counteracted by anti-oxidants, but if your metabolism starts raging out of nowhere, your body might not be able to keep up. If you're exposed to too many free radicals, they can cause DNA damage, which could lead to cancer or other diseases. So in theory an animal with a lower metabolic rate would experience less cancer than one with a faster metabolism, although we don't know that for sure. It also seems like birds and reptiles have lower rates of cancer in general.
It could be that they have a protective mechanism against cancer that we don't, so messing with Hox genes is no big deal. The answer to the question of why we have the same number of neck bones as a giraffe is long and complicated, and spans hundreds of millions of years. Until some species of mammal decides to stick their neck out and try something new, we're all stuck with seven. [♪ OUTRO]
