FEMALE NARRATOR: In 2013, a researcher working in the vast collections of the National Museums of Kenya made a surprising discovery.

Tucked away in a cabinet marked "hyenas," he noticed a large fossilized lower jawbone from some kind of carnivore.

And it was big, much bigger than the jawbone of a lion, the largest carnivore in Africa today.

Six years later, it was revealed to the public as an enormous beast that was completely new to science.

It was named simbakubwa kutokaafrika.

And while its name literally means "big lion from Africa" in Swahili, this creature was not a big cat.

And even though it was found in a drawer labeled "hyenas," it was not a hyena either.

It was a hyaenodont, an extinct family of carnivorous mammals that lived from the Paleocene to the Miocene Epoch, and it lived in Africa, Europe, Asia, and North America.

But simbakubwa was not only unique because of its large size.

The fossil jawbone was excavated in the late 1970s at a site called Meswa Bridge in Western Kenya that dates back to about 26 million to 23 million years ago.

It was from this jawbone that researchers were able to estimate a range of potential body sizes for simbakubwa based on the size of its molars and comparisons to living carnivores.

And it looks like, at its smallest, simbakubwa was probably the size of a large lion, and at its largest, it could have been larger than a polar bear.

And it was a hypercarnivore, meaning that it got more than 70% of its calories from meat.

Based on its age and its monstrous size, simbakubwa is the oldest known giant member of its subfamily of hyaenodonts.

These hyaenodonts gave the world some of its largest terrestrial carnivorous mammals ever known.

And while these behemoths were the apex predators of their time, they're not around anymore.

It turns out that becoming the biggest, baddest beast on the landscape can have serious consequences when that landscape suddenly changes.

[MUSIC PLAYING] Hyaenodonts are members of an extinct order of carnivorous mammals called the creodonts.

And even though they were super carnivorous, creodonts evolved independently from the order Carnivora, which includes all modern felids, canids, and other critters.

Creodonts are older and more primitive than Carnivora, and they look different too.

Their skulls were low with small brain cases, and their limbs were generally short and heavy.

Early on in their evolutionary history, about 65 million years ago, the creodonts branched into two lineages, one of which was the hyaenodonts.

But the exact place they arose is still kind of a mystery.

Some experts think they originated in Africa, others say Asia, and some suggest both, with a distinct group of hyaenodonts evolving on each of the two continents.

But the fossils and the phylogenetic evidence we have seem to suggest that they arose somewhere in Eurasia.

And simbakubwa was part of a subfamily called hyainailourinae.

This particular group probably originated in the region known as Afro-Arabia, which includes the whole Arabian Peninsula and Northern Africa, around 48 million years ago.

From there, they spread into Europe, Asia, and North America, where their large size and powerful bite ensured their position at the top of the food chain.

But by the middle Oligocene roughly 30 million years ago, it looks like they went extinct on the northern continents, maybe due to competition from other groups of carnivores that evolved during that time.

But in Afro-Arabia, they persisted, evolving in isolation, separated from the newly-evolved carnivores in Eurasia by a seaway.

And this isolation could have contributed to the rise of giant hyaenodonts, like simbakubwa, because it wasn't until the early Miocene, around 23 million years ago, that evidence of the largest members of this group appeared, approaching the size of modern-day rhinoceros.

What we know about simbakubwa and its close relatives comes from a very fragmentary fossil record, which is mostly made up of dental remains and a few bones from the rest of the skeleton.

This limited fossil record has made it hard to figure out what their family tree or even they themselves looked like.

What we do know from their teeth is that these hypercarnivores had to be able to process a lot of meat, which they did with specialized teeth called carnassials.

These are modified premolars and molars, with self-sharpening edges that pass by each other in a sheering motion, like a pair of scissors.

Now, living carnivores usually only have one set of these scissor-like teeth, but simbakubwa and its relatives had three, turning their jaws into meat-slicing machines.

But making sense of the rest of their anatomy is kind of hard.

In order to sink those teeth into all that meat, they had to catch it first.

So researchers are really interested in figuring out how these giant predators moved around.

And the one clue we have about simbakubwa's locomotion is pretty intriguing.

Many modern carnivores are digitigrade walkers.

This means that they walk on their toes with their heels in the air, making them faster than plantigrade walkers, like us, who walk with the soles of their feet flat on the ground.

Digitigrade carnivores are more common in open grassland environments, where that posture helps them save energy and increases their stealth speed and hunting success.

And simbakubwa's well-preserved heel bone, called the calcaneum, was similar to that of one of its relatives, which has been reconstructed as semi-digitigrade-- not all the way up on its toes, but not totally flat-footed either.

Researchers also think that relative was capable of making powerful leaps.

So based on similarities between the two heel bones, it's possible that simbakubwa was also semi-digitigrade, making it faster and a more agile hunter.

All of these advantages-- a large body, three sets of meat-slicing teeth, and the ability to get around quickly-- probably led to the success and survival of the hyaenodonts through the Miocene Epoch.

They likely hunted really large herbivores, like the relatives of modern elephants and rhinos, occupying a niche that remains vacant today.

But if they were such powerful beasts that were so well-adapted to their ancient landscapes, then why aren't they still around?

Well, the hyaenodonts met their demise by the end of the Miocene, around 5 million years ago, thanks to an even more powerful force-- a rapidly changing environment.

This period of change started back around 23 million years ago, as the Arabian Peninsula approached Eurasia, closing the seaway that had kept simbakubwa and its relatives separated from the carnivores of Eurasia.

From then on, waves of animals were able to migrate back and forth between the two regions.

And among these migrants were hyaenodonts from Afro-Arabia heading north and Carnivorans from Eurasia heading into Africa.

This exchange of animal populations happened at a time when Earth was undergoing major changes, like the formation of the East Africa rift system, where two parts of the African tectonic plate started to move away from each other.

These changes led to dramatic transformations in the Afro-Arabian landscape, with forests becoming drier and turning into more open habitats.

All of these changes spelled disaster for the hyaenodonts, but researchers are still trying to figure out exactly how.

There are currently two hypotheses for the extinction of simbakubwa's group-- changing food webs, as environments changed and large herbivores disappeared, or competition from modern Carnivorans.

The first one suggests that when the environment started to change, it was bad news for the large herbivores that hyaenodonts relied on.

Large herbivores tend to reproduce slowly, which can lead to a rapid decline in numbers if ecosystems change too quickly because they can't adapt fast enough.

Fewer large herbivores meant less food for the large hyaenodonts, which weren't adapted to hunting smaller prey.

This would have made hyaenodonts more vulnerable to extinction than the smaller Carnivorans, which had a broader diet.

The second hypothesis involves competition from members of the order Carnivora, as they moved from Eurasia into Afro-Arabia around 23 million years ago.

Up to that point, hyaenodonts had been dominating the predatory niches in Afro-Arabia.

Fossil evidence suggests that the earliest Afro-Arabian Carnivorans were pretty small mesocarnivores, animals that get at least half of their calories from meat and the rest from other things.

Think like raccoons or coyotes.

And they're usually less dramatically affected by environmental changes than large hypercarnivores are.

So as new Carnivorans dispersed into Africa from Eurasia, they became more diverse and started to compete with the hyaenodonts.

Plus, Carnivorans with complex cooperative behaviors, like living in packs, may have been able to steal prey from lone hyaenodonts.

Between the changing environment, lack of food, and the introduction of smart and sneaky competition, simbakubwa and its relatives might have simply met their match, after succeeding on three continents for at least 15 million years.

The evolution and extinction of these hypercarnivores offers rare insights into a time that saw massive changes in the climate, oceans, tectonic activity, and the dynamics of mini ecosystems.

And as more fossils are found, we may be able to recreate and better understand the African landscape in which they lived, which is also the landscape where our early ancestors evolved.

Ultimately, the giant hyaenodonts of Africa serve as a stark reminder that even the most powerful beasts are no match for a changing world.

[MUSIC PLAYING]