Thanks to Curiosity Stream for supporting
PBS Digital Studios. This creature is an extinct relative of horses
and rhinos, known as a chalicothere. And, this creature? It’s also a chalicothere. Even though they look pretty different, both
of these animals lived at the same time, and in the same places, and they did the same
things. They’re both closely related to each other,
and to horses and rhinos. And they both developed highly specialized
adaptations that helped them exploit their main source of food: the leaves at the tops
of trees. The first animal is known as Chalicotherium
, and it wound up having enormous arms and walking around on its knuckles, kind of like
a gorilla. The other is called Tylocephalonyx, and it
turned out to look a little more horse-like, but with a longer neck, and this weird, bony
dome on its skull. So, what happened? How did two of the same kind of animal, living
in the same place, end up looking so different? The answer is parallel evolution. You can’t really talk about the history
of life without talking about natural selection -- the process by which living things can
become more likely to survive, and to have offspring that will also survive, if they
become better adapted to their environments. And chalicotheres are an especially great
example of how this process actually works. Because, in evolutionary terms, “better”
can mean a lot of different things. Success can take many forms, even for organisms
that live in the same environment and face the same challenges. Before they diverged into different types,
chalicotheres first showed up in Asia around 55 million years ago, during the Eocene Epoch
of the Paleogene Period. They soon made it to North America, Europe,
and eventually to Africa. During the Eocene, rainforests were widespread,
and they became home to a new group of herbivores: hooved animals called perissodactyls, which
today include horses, rhinos, and tapirs. “Perissodactyl” means “odd-toed,”
and like other members of this group, early chalicotheres had hooves -- although their
hooves were split in two, which was kind of strange. But as the Eocene ended, chalicotheres’
hooves had been replaced with something new: claws. Now, claws on plant-eaters are pretty rare
– so rare, in fact, that the paleontologists who first found the feet of chalicotheres
thought they must have come from anteaters, or maybe giant pangolins. But we know now that chalicotheres were herbivores,
because their teeth bear the distinct scratches and pits that come from eating leaves, bark
and twigs. So instead of using their claws for digging,
these animals probably used them to get at the tastier leaves that were higher up in
the trees, a type of eating preference called high browsing. The claws may also have helped chalicotheres
stand on their hind legs and brace themselves against tree trunks, making it easier to browse. And they even had specialized neck vertebrae
that let them look up more easily than like a horse or a rhino. So, by about 40 million years ago, chalicotheres
had arrived at a single, pretty successful body plan, optimized for high browsing. But then, things started to get weird. Soon after the ancestral chalicotheres developed
claws, they began to follow different evolutionary paths. They started to evolve in parallel. One group of early chalicotheres got very
good at pulling leaves down to eat. So, over time, those animals with longer limbs
were more successful, until -- about 30 million years ago -- they all had big hulking arms,
long claws on their front legs, and necks much shorter than their ancestors’. And because of this new body plan, these animals
started walking with their front toes curled inward to protect their claws. They began walking on their knuckles! These animals became known as their own subfamily
of chalicotheres, called Chalicotheriinae. But meanwhile, a separate group of ancestral
chalicotheres started to develop its own adaptations for high browsing. Its early members were much better at nibbling
leaves right off the trees, without having to pull them down. Over time, those that were taller and had
longer necks were better at getting food, until eventually, they looked more and more
like long-necked horses. Or maybe, short-necked giraffes. And they still had claws, too. But all their legs were about the same size,
and these animals were bigger and heavier than their long-armed cousins. So, knuckle walking wasn’t an option for
them. Instead, they acquired the ability to retract
their claws. They had specialized tendons that helped them pull
their claws up while they walked, and the tendons were further strengthened by a fusion
of several bones in their toes. And, in many members of this group, domes
also appeared on their heads. No one’s exactly sure what purpose they
served, but the prevailing theories are that the animals used them for head-butting and
as a display to potential mates. As you do. And this group of long-necked, dome-headed
chalicotheres came to be known by the ungainly name of Shizerotheriinae. But, just like the other subfamily, they were
still chalicotheres! So if these two groups of animals lived in
the same area, why did they split at all? Why didn’t they keep evolving as one group,
ending up either as long-armed knuckle-walkers or long-necked horse-like dome head things? Well, that’s what makes parallel evolution
so interesting. It often occurs among closely-related animals,
even ones that share the same range. And in this case, it happened because both
body plans were equally likely to help an animal survive. So, early on, among the original, ancestral
chalicotheres, some variations appeared, which happens all the time in groups of organisms. Some chalicotheres had longer necks, and others
had longer forearms. But both differences helped ancestral chalicotheres
survive, reproduce, and ensure that their long-necked or long-armed traits were passed
on to the next generation. Slowly, over time, chalicotheres drifted apart,
until their differences were so extreme that they were different species - and then, different
subfamilies. But they both remained really good at doing
one thing: eating the tops of trees. Of course, as you know from the lack of them, their success didn’t last forever. Despite their specializations, chalicotheres
aren’t with us today. The last of both families died out in North
America 10 million years ago, and they lingered in Asia and Africa until about one million
years ago. One possible clue to their demise is the rise
of the artiodactyls -- herbivores with four-chambered digestive systems, like deer, cattle, and
hippos. This digestive set-up is much more efficient
than the single-chamber system that chalicotheres had. So chalicotheres would have had to eat a lot
more to get the same nutrition that a giraffe, for example, could get with less food. And changes in climate wouldn’t have helped:
As Africa and large parts of Asia became drier, grasslands started to spread, reducing the
habitat of the tree-loving chalicotheres. Today, horses are rhinos remain their closest
living relatives. So, you can take the story of the chalicotheres
as proof of how versatile natural selection can be -- how it can lead to many different
evolutionary solutions to a problem, all of which are equally successful. But you can also read this story as a lesson
in its limitations. I mean, if anything, chalicotheres turned
out to be too specialized. Each group became too good at high browsing
-- to the point that, when the climate changed and competition pressure was on them, they
couldn’t survive. There are some situations that evolution just
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Is everybody else just Loving the Crap out of EONS?!?
I know I am.
Physical separation. Just like bonobos and chimpanzees became a different genus after getting separated by river Zaire for instance.
I just discovered this show and it's so well done! Keep it up guys :)
Is this saying that these two types of chalicothere lived as one group, continuing to mate whilst evolving separately?
Surely there has to be some form of separation, even if it’s just sexual, or maybe even cultural separation?