The Human Spine's Evolution
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00:00If you look at the spine from the front or side, it looks pretty straightforward.
00:04You've got this stack of bones that runs from your neck to your tailbone,
00:08and they're all connected together by muscles and ligaments.
00:11But if you were to split the spine down the middle,
00:14you'd find that it's made up of a lot more than just bones.
00:17It's actually made up of a bunch of different tissues,
00:20including nerves, blood vessels, spinal fluid, and even some cartilage.
00:25And it's the way all these different tissues come together in this one structure
00:29that makes the spine such an amazing piece of anatomy.
00:33In order to really appreciate how complex the spine is,
00:36let's take a journey back in time and see how it evolved over the course of human history.
00:41The evolution of the spine is an interesting one.
00:45As far back as 365 million years,
00:48we can see ancestors of vertebrates like sea squirts and lancelets
00:52swimming around with flexible rod-like spines.
00:56These early vertebrates were likely more like elongated tadpoles than anything else,
01:00and their spines probably helped them with swimming.
01:03Fast forward a few million years,
01:05and we start to see vertebrates that are more like fish.
01:08Fish are the first vertebrates that had true backbones,
01:11which are made up of lots of small bones called vertebrae
01:15that are stacked on top of each other.
01:17The vertebrae in a fish's spine are connected together
01:20by these tough bands of tissue called ligaments.
01:23And the whole thing is encased in a bony armor called the ribcage.
01:28This armor protects the delicate inner structures of the spine and gives the fish its shape.
01:33It also allows for some flexibility so that the fish can bend and twist as it swims.
01:39But it's still pretty rigid overall, so it doesn't allow for much movement.
01:44As far as we can tell, the transition from water to land was a pretty difficult one for vertebrates.
01:49Even though the first vertebrates to crawl up on land were likely very different from modern-day reptiles and mammals,
01:57they probably faced many of the same challenges.
02:00One of the biggest challenges of moving from water to land is getting around.
02:04Water provides a lot of support and buoyancy, so it's pretty easy to move around in.
02:09But on land, there's no such thing as an upright posture.
02:13If you don't have a strong spine to support your body, you'll just collapse on the ground.
02:18Some early tetrapods, the four-footed vertebrates that were ancestors to both reptiles and mammals,
02:24tried to get around this problem by basically laying on their sides.
02:28They weren't crawling in the traditional sense because they weren't using their limbs to drag themselves along.
02:34Instead, they were using their spines to support their bodies while they moved sideways.
02:38This method of moving around is still used by some animals today, like snakes.
02:45It's not exactly efficient, though, so most vertebrates ended up evolving different ways of moving around.
02:51These new ways of moving around required new adaptations in the spine.
02:56Many of the vertebrates that tried to stand upright on two legs or move around in this way needed a stronger spine.
03:02For example, some early dinosaurs evolved longer, thicker vertebrae that were fused together to form a solid support structure.
03:10This gave them the ability to walk upright on two legs without collapsing.
03:15Other dinosaurs took this idea to the extreme.
03:18Some species, like Stegosaurus, had very long spines with even longer, more robust vertebrae.
03:25In these animals, the spine was essentially a second skeleton that supported the entire body.
03:30Over time, different groups of vertebrates evolved different kinds of spines that worked for their particular way of moving.
03:37For example, birds need to be able to fly so their spines have evolved to be lighter and more flexible than the spines of other vertebrates.
03:46This allows them to maneuver through the air more easily.
03:50Mammals, on the other hand, tend to have stiffer spines than birds, but they're generally less flexible than dinosaurs.
03:56One thing that all mammal spines have in common is a little piece of connective tissue called the meniscus.
04:04The meniscus is a small, tough band of fibrous tissue that sits between the bones of the spine.
04:10It's only about half an inch wide, and it's shaped a little bit like a crescent moon.
04:16These little discs are what allow the bones of the spine to move and bend without breaking.
04:21They also help to protect the spinal cord from being damaged.
04:25You may be wondering why the spine needs these little discs if it's made out of bone.
04:30After all, bone is already a hard, rigid tissue.
04:34That's true, but bone isn't the only tissue in the spine.
04:37There's also a lot of soft tissue, too.
04:40The spinal cord itself is made up of soft, nerve tissue,
04:43and there are also a lot of blood vessels that run through the spine.
04:47All of these soft tissues need to be protected from the hard, bony skeleton,
04:52and that's where the meniscus come in.
04:54Interestingly, the meniscus aren't the only structures in the spine that help to protect the spinal cord.
05:00The vertebrae themselves have these little notches on the backside that help to hold the spinal cord in place.
05:06If the spinal cord gets jostled around too much,
05:09it can actually tear itself away from the brain and spinal nerves, which can cause serious damage.
05:14These notches help to keep the spinal cord firmly attached to the spine.
05:19Even with all these protective structures, though, the spine is still vulnerable to injury.
05:25If someone falls and lands on their tailbone, for example,
05:28they could crack a vertebra and damage the spinal cord inside.
05:32This is why it's so important to protect the spine when you're moving heavy objects
05:36or participating in other high-risk activities.
05:40Speaking of the tailbone, let's talk about the coccyx.
05:44The coccyx is the last part of the spine, and it's made up of four fused vertebrae.
05:50In humans, the coccyx is pretty small, but in other animals it can be much larger.
05:55For example, the coccyx of a gorilla is about twice the length of a human coccyx.
06:01The coccyx serves a couple different functions.
06:04First, it helps to support the weight of the body.
06:07Second, it acts as an attachment point for various muscles.
06:10These muscles help to control the position of the trunk and pelvis,
06:14and they also help with things like standing and walking.
06:19Now let's move up the spine and talk about the thoracic spine.
06:23The thoracic spine is the part of the spine that's attached to the ribcage.
06:27It's made up of 12 vertebrae, and each vertebra is connected to a pair of ribs.
06:32The thoracic spine is pretty stiff because it's surrounded by all these bony plates that make up the ribcage.
06:40This stiffness helps to protect the vital organs in the chest cavity, like the heart and lungs.
06:45The ribs themselves are also pretty unique.
06:48They're curved pieces of bone that connect to the vertebrae in the back and to the sternum in the front.
06:54The ribs form a protective cage around the chest cavity, and they also help with breathing.
07:00When the diaphragm and other breathing muscles contract, they pull on the ribs, causing them to expand.
07:06This expansion of the ribcage increases the volume of the chest cavity, which allows the lungs to expand and fill with air.
07:14Finally, let's talk about the cervical spine.
07:17The cervical spine is the part of the spine that supports the head.
07:21It's made up of seven vertebrae, and it's the most flexible part of the spine.
07:26This flexibility allows us to turn our heads from side to side, nod up and down, and tilt our heads back, behind our necks.
07:35The cervical spine is pretty vulnerable to injury because it's right up here at the top of the spine.
07:40If someone gets tackled or falls down hard, for example, they could easily crack a cervical vertebra and damage the spinal cord.
07:48This can cause paralysis and even death.
07:52That is why it's so important to protect your head when you're playing sports or engaging in other high-contact activities.
07:58The cervical spine is also unique in that it supports our head, which is really heavy.
08:04The average human head weighs about 10 pounds, and that's not counting the weight of the neck.
08:09To support this weight, the cervical vertebrae are much larger and stronger than the vertebrae in the rest of the spine.
08:17They also have these little bony bumps on the backside called spinous processes.
08:21These spinous processes help to reinforce the spine and prevent it from bending forward.
08:27So that's what the spine is all about.
08:30It's a complex structure that allows us to move around and do all the things we need to do throughout the day.
08:36Without it, we wouldn't be able to walk, run, jump, or even breathe.
08:42So next time you feel the urge to bend over backwards, just remember to take it easy.
08:47Our spines are pretty amazing, but they're not invincible.
08:51Hey there everyone, I hope you enjoyed learning about the spine.
08:55If you want to learn more about the human body, check out some of our other videos.
09:00We've got everything from an introduction to anatomy and physiology to an overview of the cardiovascular system.
09:08Thanks for watching, and I'll see you next time.