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]