Hank Green: Ever since a convenient rock got most of the dinosaurs out of the way 66 million years ago, mammals have evolved a remarkable diversity of forms; running, digging, flying, swinging through the trees. What is it that made their success possible? Their diet, the giant ecological hole left behind by the dinosaurs? Maybe, but we also have to consider the humble shoulder.

And to do that considering we're gonna let our guest take it away.
So please, welcome our very good friend, science correspondent Emily Graslie.

Emily Graslie: So, to start off, let's look at the group of animals known as tetrapods, which includes everything with four limbs.

A lot of them, not all, but a lot, have limbs that look pretty similar. Think about an alligator, a lizard, a turtle and a salamander. The all have that same squat posture; kind of splayed, with their limbs swinging out to the sides.  But mammals have something else going on.

There are a few, like echidnas and other monotremes, that still have that lizardy gait. But others, the group we know as therian mammals, generally keep their limbs close to the body. They swing their limbs straight back and forth, under and in line with the body, in what researchers call ''a parasagittal posture''.

And in these mammals, the limbs have become super diverse, in terms of what they can do. Think about the movements of your arms and shoulders and the vast differences compared to the limbs of a horse or a bat or a whale or a mole. Scientists think the way our shoulders are put together may be the reason why this diversity is possible.

For one thing, we have a ball-and-socket joint, connecting our arms to our shoulders. But it turns out, our whole shoulder is set up to be very mobile. Consider the human arm; it seems pretty solidly stuck there, onto the rest of your body, but it turns out your shoulder blades don't actually attach to your rib cage anywhere, nor does your humerus, the upper arm bone.

In fact, touch your collarbone, at the base of your neck. That's the only true joint; the kind of bone-to-bone connection between your whole arm and your chest. Everthing else is held up by muscle.

In other tetrapods, like lizards or frogs, these bones are more rigidly connected across the sides of the body, functioning more like a single unit and less independently from one another. And this affects how mobile the whole limb can be. For example, in one study from Harvard, scientists looked at the shoulders of three types of animals:  a tegu, which is a kind of lizard, an echidna, a non therian mammal and an opossum, which has the upright posture and back-and-forth gait of a typical therian.

The echidna was important, because it's somewhere in-between; it's a mammal, but has more sticky, outy limbs. It's not parasagittal. The researchers used scans of these animals' shoulder bones and the muscles attached to them, as well as computer models, to see how flexible each animal's shoulders were.

They found that the way the bones and muscles all fit together gave the opossum the highest range of motion. The tegu actually wasn't that far behind. But the opossum was a little more flexible, or at least could move within that range of motion more easily.

Whereas, it was more complicated for the tegu to move within its range. That could constrain how much it can actually move in real life, and maybe how much it can be modified by evolution for other purposes.  Prior research had also shown that the tegu's gait put a twisting force on its bones, while the opossum's gait was more about compression. Bone is better at dealing with compression than twisting, so the difference in gait may mean less bone stress, lighter bones, and less muscle effort.

So, the opossums' shoulders are more flexible, make it simpler for it to accommodate its movements and those movements are easier on the bones. In short, it's a shoulder that seems ready to adapt and diversify into the fantastic range we see in therian mammals. Meanwhile, echidnas were a useful in-between point for comparing the two.

Though they weren't perfectly in-between and in some ways were more limited than either the tegu or the opossum. Of course, these are just three unique, modern animals, with their own niches and lifestyles. So, while we can look at them to learn what makes therian shoulders special, they don't tell us how the shoulder evolved.  Luckily, we can turn to the fossil record and it turns out the emergence of therian shoulders is more wacky than straightforward.

Researchers from The Field Museum in Chicago, a city, by the way, whose nicknames include ''The city of big shoulders'', were looking at the shapes of limb bones in the fossil record. You might assume that the fossils followed a pretty linear trend, from reptile-like to mammal-like, but no! Mammal ancestors evolved a huge variety of forms over their evolutionary history, including ones we don't see anymore today.

They didn't just hit on mammalian shoulders all at once. They diversified from a reptilian starting point into all kinds of forms.  A group known as pelycosaurs, which includes the famous dimetrodon, were relatively early members of the group of animals that would eventually include mammals. They had shoulders that were not very mammal-like and which all looked fairly similar to one another.

But as time went on, the ancestors and ancient cousins of mammals diversify. Some did indeed become more therian-like, but not all. Others took different routes and got weirder, like kembawacela, which was an ancient digger, but whose limbs were really unique.

Something like a mole has big hands and powerful elbows and kembawacela does too, but its limbs started from a very different point to achieve the same function. Biomechanically speaking, they're really different. It's a good reminder that evolution doesn't happen in a straight line.

Each animals evolves to fit the world it lived in.  Mammals aren't the only four-limbed animals in the world. And crocs, and lizards and other critters like them are also well adapted to do what they do. But it's interesting to look at the sheer diversity of how mammals get around in the world and ask what made this possible.

And through careful study of both living animals and the fossil record, scientists now think our modern, adaptable shoulders are part of the answer.

Thanks for watching everyone! I'm so glad that I could join you today. Back to you, Hank.

Hank Green: Thank you, Emily, and thanks to the team of researchers who helped make this video possible. If you enjoyed it and want to get more involved with SciShow's community, you can check out patreon.com/scishow.