Eons

In 2009, researchers found single block of rock in Russia that contained fifteen skeletons of a little mammal-relative dating back to the late Permia Period, about 260 million years ago.

The fossils belonged t Suminia, which before this was only known from its skull.

And the skeletons revealed tha Suminia was very different from what scientists had expected, based on what its relatives looked like.

Its hands had long fingers curved claws, and an opposable thumb.

And it had a long, flexibl tail that may have been capable of grasping, like some monkeys.

These features led researchers to conclude that Suminia was the first tetrapod or four-limbed vertebrate to be highly adapted for life in the trees.

Which doesn't mean that it was the first vertebrate in the trees.

So when and how did tetrapods make this move in the first place?

And why?

Turns out, the story of climbing may have less to do with trees and more to do with their leaves.

Trees- well, tall woody plants that we would call trees- first appeared in the middle Devonian period, about 390 million years ago.

Tree is really more of a shape category than an evolutionary one.

Before this, life on the plane was prettyflat.

No plants were more than a meter tall and n animals could reach higher than that, either.

And by the beginning of th Carboniferous Period, about 25 million years later, tree-like plants were well-established in forests all across the globe.

These forests were dominated b giant relatives of clubmosses, plants that, today, stay pretty low to the ground.

And when these swampy forests first evolved, nothing was living in the trees.

All land animals were still hanging out on the ground.

But eventually, arthropod found their way into the trees.

The oldest insects with wings show up in the mid-Carboniferous, and from then onwards, the fossil record is filled with flying insects.

And we know they set up shop in the trees because we have fossilized leaves with evidence of bug bites on them.

But it wouldnt be until nea the end of the Carboniferous roughly 80 million years after trees evolved that the first tree-climbing amniotes would appear.

This is the group that contains all modern mammals and reptiles, plus their extinct relatives.

And we arent sure why it took them so long, but since most Carboniferous tetrapods were predators, mayb they just didnt expand upwards until there was something to eat in the trees following the flying insects.

These first climbing amniotes were the early reptiles Anthracodromeus and Cephalerpeton, both found in the midwestern United States.

Anthracodromeus was only abou 15 to 20 centimeters in length, with relatively long forelimbs, as well as long hands and feet.

And those limbs ended i strongly curved claws that had a hook at the end.

These are features we see i many climbing lizards today and theyre good for clinging onto th sides of objects like trees.

As a close relative Cephalerpeton also had similar limb proportions, which means it may have been climbing around in the trees at about the same time, too.

But while these two reptile could have climbed trees, they dont seem to have been spending most of their time in them.

See, the bones of the fingers and the toes- the phalanges- are usually longer towards the tips of th digits in animals that live in trees.

But in Anthracodromeus and Cephalerpeton, theyre all around the same length Which means that they were scansorial- adapted for climbing but not arboreal adapted for spending the majority of their lives up in the trees.

The difference may seem subtle but its important because not every climbing animal is a tree-dweller, an not every tree-dwelling animal is a climber.

So these early reptiles were probably just visitors to the tree tops, chasing insects up the trunks o climbing up to escape predators but they would have still spent a lot of time foraging, sleeping, and mating on the ground.

But it wouldnt take long for tetrapods to evolve that did spend more time in the trees: and these were the varanopids.

Now, varanopids are an enigmatic group of animals.

Paleontologists still arent sure if theyre o the mammal side of the amniote family tree or the reptile side.

Either way, while most varanopids were ground-dwellers, two of them became climbers in the Lat Carboniferous and Early Permian Periods.

These were Eoscansor, from Ne Mexico, and Ascendonanus, from Germany.

These two had those same climbing adaptations in their hands and feet that we saw before the lon forelimbs, the long digits, and strongly curved claws.

But they took these traits one step further, with more flexible, inward-curling digits an longer phalanges at the tips of their digits.

This allowed them to grab onto the tree trunks and branches more effectively.

Ascendonanus was even found in a petrified forest its skeletons were uncovered next to petrifie tree trunks, where they had all been buried by a Permian volcanic eruption.

These climbers probably spen even more of their time in the trees than the earlier reptiles did but not their entire lives.

The first reptiles to do that would be the weigeltisaurids.

Theyre another group of ancient reptiles, known from the Late Permian in western Europe, Russia, and Madagascar.

And theyre the earlies tetrapods in the fossil record that could glide!

They had these weird, rod-lik bones growing out of the sides of their torso.

And we dont even know what the bones are.

It's been suggested that they could be extensions of the so-called belly ribs, the gastralia, but others thin these are separate structures.

These bones would have bee covered by skin, much like the wings of modern gliding lizards.

This is an example of convergent evolution and it indicates that the weigeltisaurids were also gliders.

Now, every gliding animal around today is arboreal.

Gliding only works if you start from a high position.

And a reliable way to get hig above the ground is to live in trees.

But their wings arent the only adaptations for tree-dwelling that these little guys had.

In addition to the hand and foot adaptations we mentioned before, their torsos were also flattened, like living gliding lizards, and they probably had a series of stiffening muscles down the torso and in the tail.

We see these muscles in moder gliding lizards, and they help keep the animal balanced and control pitching when climbing in the trees or gliding.

And one thing all of these early tree-climbers had in common is that they kept the ancestra amniote body plan, which looks verylizard-y.

So their climbing adaptations were mostly in the hands and feet.

But around the same time as the weigeltisaurids, a tree-climber evolved that was not lizard-y at all.

It was Suminia, which appeared on the scene 260 million years ago with radical new adaptations: opposable digits and maybe even a prehensile tail.

This ancient mammal-relativ also had a shorter torso, with limbs that were significantly longer, and thinner bones that were more lightweight.

And Suminia was unusual both for its new adaptations to being arboreal and because it closest relatives look nothing like it.

Suminia was an anomodont, a group of mammal-relatives that was almost entirely made up of ground-dwellers.

Some of these anomodonts, like Tiarajudens, had relatively stocky limbs and could get to the size of a capybara not very good candidates for tree-climbing.

So how and why did Suminia end up in the trees?

Well, researchers noticed that its skull had interlocking, leaf-shaped teeth with large serrations.

This type of tooth evolves in herbivorous amniotes pretty often, and shows that Suminia ate leaves.

All the previous tree-climbers were carnivores and insectivores, with teeth that were either pointy an needle-like or large and curved backwards -- both good for catching prey.

Before Suminia, nothing had evolved to eat tree leaves except for the insects, which had them all to themselves for tens of millions of years.

Anomodonts had experimented with many ways to process plants from toothless beaks to molar-like teeth and Suminia added to this diversity.

It had bigger cropping teeth at the front of its mouth, and teeth that sheared against each other furthe back, which is a very effective setup for slicing through tough, fibrous plants, especially leaves.

And as anomodonts were tryin different things ecologically, Suminias ancestors probably followed food from the ground onto fallen logs, and then further and further up into the treeslike the reptiles before them.

As they kept going up, selective pressures acted on Suminias skull and body to make leaf-eating and moving about in the trees more efficient.

Since evolution had to start with a body plan that was adapted for life on the ground and not very lizard-y at all Suminia developed completely new adaptations, adjusting to life in the trees in very different ways.

Like, evolution had to branch out and find new solutions to help Suminia keep its balance, which meant longer limbs and a shorter torso, helping center its mass.

And while it had somewhat claw-like fingertips, it also had opposable digits fo effectively grabbing branches.

Suminia also developed a flexible, possibly prehensile tail to stabilize itself, as it didn start out with the musculature needed to support a long, stiff tail for counterbalancing.

But why did it take so long for a tree-climbing herbivore to appear?

It could be because Suminia was one of the first herbivores with teeth that could chew leaves in the canopy effectively.

Or it could be because forest canopies as we know them now didnt really exist until the early to middle Permian.

The dominant Carboniferou trees didnt have branches that grew as far outwards as those of conifers and Permian trees like Glossopteris.

Its easier to get from one tree to the next when the canopy is denser and branches are closer together.

A tree-climbing herbivore could jump, climb, or swing to the next tree instead of going back to the ground.

So the spread of denser canopies in the Permian encouraged amniotes to leap from being simply scansorial to fully arboreal by the end of the Permian.

And with them came features like gliding and opposable digits.

The End-Permian extinction wiped out both the weigeltisaurids and Suminia.

But extinction often leads t opportunity for the survivors, and in the evolutionary sandbox of the Triassic, new groups of tree-dwelling amniotes evolved.

And the same anatomica features that first appeared in the Permian would convergentl evolve again and again whenever a group of amniotes starte living in the trees and this includes our own primate ancestors.