FEMALE NARRATOR: Just a few thousand years ago, the island of Madagascar was inhabited by giants-- giant lemurs.

These remarkable primates lived only on Madagascar, and they were part of an evolutionary event that continues to this day, a radiation that saw primates adapt to fill the ecological niches that, in other places, were occupied by totally different animals, like sloths, monkeys, and even woodpeckers.

There were the so-called monkey lemurs, named for their skeletal similarities to baboons.

There were three species of koala lemurs, which, of course, they were not koalas, but they specialized in eating leaves and had grasping pincher-like feet that kept their large bodies in the trees.

But maybe the weirdest of these extinct giants were the sloth lemurs.

This family included archaeoindris, the biggest lemur that ever lived.

And most of its members seem to have adaptations for hanging from tree branches, like sloths do today.

What all these strange creatures had in common was their large body size.

They likely ranged from the size of a large terrier to almost as big as an adult male gorilla.

But today, they're all gone.

Their largest living relative is the modestly sized indri.

So what happened here?

How did such a diverse group of primates evolve in the first place, and how did they help shape the unique environments of Madagascar, and how did they get winnowed down, leaving only their smaller relatives behind?

As far as that last question goes, the answer might lie in the arrival of another, different type of primate on the island, us.

Madagascar has been separated from all other land masses since the late Cretaceous period, about 85 million years ago.

And the fossils that date to the time after it split from the Indian subcontinent include some very cool dinosaurs, like Majungasaurus, and weird early mammals like the little cutie, Vintana.

But the fossil record of Madagascar stops abruptly at the beginning of the Cenezoic era, about 66 million years ago.

There are almost no fossils on Madagascar from that whole stretch of time until about 26,000 years ago, which makes the early evolution of the few mammal groups that got to the island kind of mysterious.

But based on genetic studies, we're pretty sure that the ancestors of modern lemurs made it there after it had already become an island.

The most widely accepted estimate says that lemurs arrived on Madagascar between 50 million and 60 million years ago.

So how did those first lemurs get there?

Experts think they probably floated.

That's right.

Some paleontologists have suggested the lemurs rafted over on large mats of vegetation, or maybe inside hollow trees that washed out across the Mozambique channel, a distance today of more than 400 kilometers.

This kind of movement is called sweepstakes dispersal, a rare or chance event where an animal is able to cross a pretty extreme barrier.

And we've talked about this phenomenon before.

Most scientists think rodents got from Africa to South America by way of a similar ocean crossing.

Those seafaring lemur ancestors were probably very small, like modern mouse and dwarf lemurs.

And they might have behaved like them too, sleeping the day away in small groups inside hollow trees.

It's also been suggested that they could have been able to enter a state of torpor or hibernation, again, like modern mouse and dwarf lemurs do.

This would have helped them survive a long-distance trip and emerge ready to colonize Madagascar.

At least, that's the most widely accepted explanation for how lemurs got to the island.

But some experts think that lemurs might have actually taken two trips.

This is based on the study of two fossil species, one from Kenya and one from Egypt, that data to well after the earliest lemurs were supposed to have made it to Madagascar.

And these two fossils bear some resemblance to the aye-aye, the weirdest and earliest branch off the family tree of all living lemurs.

So maybe aye-ayes took a separate trip to Madagascar from all the other lemurs.

We just don't know enough yet to be sure.

However lemurs got to the island, once they landed there, they took over in what's called an adaptive radiation.

Over the last 50 to 60 million years, they diversified into eight different families, five of which still have living members, and they filled a huge variety of ecological niches.

For example, take the aye-aye.

Today, it's the only species left in its family, and it fills the same basic ecological role as a woodpecker.

It uses its extra long, extra creepy third finger to tap on trees to find insect larva, then it chews a hole into the bark with its rodent-like incisors, sticks that skinny finger into the hole, and pulls out grubs.

But in the past, there was a giant aye-aye.

It weighed up to seven times more than the modern aye-aye, and it lived in the dry forests of southwestern Madagascar, where it likely used the same kind of foraging behavior, tapping tree trunks in search of insects, much like a woodpecker.

And while lemurs have managed to fill the many vacant niches on the island, they also shaped its ecosystems.

Living ruffed lemurs specialized in eating fruit, so they play an important ecological role as seed dispersers.

They help plants move their seeds from place to place by eating their fruits and dropping their seeds in new places as they move through the forest.

And ruffed lemurs can swallow seeds that are more than 30 millimeters around, bigger than an American quarter.

But there are trees on Madagascar that produce fruits with even bigger seeds.

And in the past, there was a giant relative of the ruffed lemurs called pachylemur that might have helped disperse those seeds.

We know it was a fruit specialist like its living relatives because of the wear pattern on its teeth.

And at about three or four times the size of living ruffed lemurs, it could have easily taken on those really big seeds.

But there were also less-friendly interactions taking place between Madagascar's plants and animals that have left their mark on the island's plant life to this day.

In southern and southwestern Madagascar, there is an incredibly unique eco-region called the Spiny Forest.

The vast majority of the plants there are only found on the island, and they're adapted to hot temperatures and short rainy seasons.

They're also, as the name suggests, totally covered in sharp, thick thorns, which is strange, because their relatives on the African mainland don't have thorns.

So researchers have hypothesized that the thorns of these tree species are adaptations for defending its leaves from climbing leaf-eating animals that aren't found on the mainland, namely, lemurs.

To test this hypothesis, researchers have compared carbon and nitrogen isotope levels from the bones of extinct lemurs to the levels seen in the plants of the Spiny Forest.

This method is based on the idea that you are what you eat.

The elements found in the food you eat are incorporated into your tissues.

And they found that those isotope levels matched, so it looks like one of the extinct monkey lemurs and one of the extinct sloth lemurs probably ate a lot of the plants of the Spiny Forest.

But since most living lemurs generally don't eat those plants anymore, it seems that those thorns have become an evolutionary anachronism, a trait that co-evolved with species that no longer exist.

So what happened to all the giant primates?

After thriving on Madagascar for millions of years, what put an end to their reign?

Well, in the late Pleistocene epoch, the climate was changing rapidly and becoming more variable.

From around 9,000 to 5,000 years ago, the island's climate swung back and forth between being much wetter than Madagascar is today and also much drier, with droughts sometimes lasting up to 300 years.

Between 4,000 and 2,500 years ago, the climate continued to become drier, changing vegetation and ecosystems throughout Madagascar.

And then, maybe around 2,300 years ago, a new primate evolved on the island that would change everything, humans.

There is some controversy about when exactly that happened, because the early archaeological record of the island is incomplete.

But giant lemurs were still alive when people showed up.

And it seems like we might have hunted them.

There are cut-marked bones of two species of extinct giant lemur from two sites in southwestern Madagascar that seem to be around 2,000 years old.

But those bones were collected in the early 1900s, and we don't have a very good record of their contexts.

So some researchers have argued against this evidence of butchery.

However, some incisors of a giant aye-aye with holes drilled through them were also found in the early 1900s, and were rediscovered in a museum collection in the 1980s.

They can't be radiometric dated, but there is no question that humans modified these teeth.

We just don't know when.

What we do know is that many of the giant lemurs went extinct around 1,000 years ago, along with other megafauna on the island, like the pygmy hippos and the elephant birds.

So it seems the lemurs were able to coexist with the early human inhabitants of Madagascar for at least a while.

This is also when we start to see an increase in charcoal in the sediment record of the island.

That charcoal suggests a greater human impact on the landscape, as people started fires to clear land and promote the growth of grass for cattle to feed on.

The last known remains of a giant lemur, one of the sloth lemurs, date back just 500-ish years ago.

While we can't definitively say that human hunting was responsible for their extinction, it's clear that the extinction was selective.

All the large-body lemurs are gone.

It might be because they were more easily hunted than their smaller relatives.

It might also be because larger animals need more space, so they're more vulnerable to habitat loss and fragmentation.

Or it might be because they tend to reproduce more slowly than smaller species.

It was probably some combination of all of these.

Researchers are still working on figuring out exactly what happened.

They're finding new remains, including some from underwater caves.

They're rediscovering old material from museums and they're examining DNA from both ancient and living lemurs to try to piece together the end of the story.

And while the giants are gone, the ecosystems they shaped and the lemurs that we still find today are reminders of that time when giants ruled Madagascar.