In the 1860s, an American paleontologist was  studying specimens found near the town of Pebas   in the Peruvian Amazon that were collected in  a rich – and unusual – bed of mollusc fossils.

The fossils didn’t seem that old, but they were  pretty unlike the region’s existing critters.

And they seemed to even include some animals  that usually preferred saltier water.

That was strange, since Pebas was more than 2,000   kilometers up the Amazon  River, far from the ocean.

And scientists immediately  questioned the discovery,   trying to figure out how it was possible.

What they didn’t know was that,  just 13 million years ago,   this area wasn’t the upland  jungle it is today – instead,   it was a massive series of lakes and  wetlands, which we now call the Pebas System.

Back then, the ancient sun beat down  on brackish lakes and river channels   that stretched on for hundreds of kilometers.

And in the water were some of  the most diverse and strange   crocodilians ever seen on Earth  – including possibly the closest   a croc ever came to becoming a blue whale  – at least, in terms of feeding strategy.

And this incredible wetland was fed  by… the Amazon River.

Well, kind of.

Because while there would one day  be an Amazon, the waterway that   fed the Pebas actually flowed in the opposite  direction, westward, away from the Atlantic.

So what did life look like when the  Amazon watershed flowed backwards?

How did its direction shape the  evolution of life around it?

And what force could have possibly  been strong enough to up-end one of   the world’s mightiest rivers between then and now?

It turns out evolution can take some  unique paths when a river gets…pirated.

It is truly an incredible thing  just how big the Amazon River is.

Its drainage basin covers nearly  the entire width of the continent.

So, rain that falls in Peru, for example,  only 160 kilometers from the Pacific,   will instead flow over 4,000 kilometers to  the Atlantic… But this wasn’t always the case.

When South America first started  breaking away from Africa,   the highlands under eastern Brazil were  actually the uplands of the continent.

This means that, during the Eocene, roughly 55  to 34 million years ago, when our story begins,   the mighty rivers of South America  ran westwards, not eastwards.

Back then, those rivers probably  drained somewhere near the Gulf   of Guayaquil on the border of Peru and Ecuador.

In fact, if you took a time  machine to Peru during this period,   it would look significantly different  from the Amazon rainforest today.

While part of the continent to the east was  covered in rainforest, the western coast of   South America probably wasn’t dominated  by a tropical jungle or sky-high peaks.

Instead, it was more like a savanna,   with forests growing up around rivers between  scrubbier trees and open fields of grass.

That’s because, by the end of the Eocene,   the Earth was going through  a bit of a cold, dry spell.

The Atlantic Ocean, which loads up the westwards  winds with evaporated water, was much narrower   than it is today, because the rift between South  America and Africa was still relatively new.

That means there was less moisture  blowing across South America.

And without the modern Andes Mountains  as a barrier, much of that moisture was   just carried out over the Pacific Ocean,  instead of being forced to fall on land.

And, yes, you heard me correctly,  I said “without the Andes.” Because, although there were some  mountains to be found along the   western coast of South America, they  weren’t the giants that we see today.

And the animals that inhabited this region  were also not like what we see there today.

The Cenozoic fossil record of the Amazon, all the  way from 66 million years ago up until recently,   is pretty poor – partly because it’s  really hard to look for fossils there.

But we can still get some hints  about what life was like at the time.

Fossils unearthed along riverbeds near the town  of Santa Rosa in east-central Peru, for example,   paint a picture of grasslands interspersed with  rivers with trees growing along their banks.

These were populated by small,  arboreal creatures like rodents,   as well as the marsupials Wamradolops  and the shrew opossum Perulestes who   lived among the riverside trees above  crocodilians lounging in the water.

In the grasslands nearby grazed herbivorous,  sheep-sized, hoofed mammals called notoungulates.

But things were changing.

Ever since breaking  away from Africa back in the Mesozoic,   South America was steadily pushing westwards.

This was causing the continent to collide with  the oceanic plates to its west.

And slowly,   bit by bit, the Andes were starting to form.

It would take a long time for them to  reach their present range and height.

But by about 23 million years ago,   the land in Peru and Colombia had risen enough  to cause a problem: rivers don’t flow uphill.

Plus, as the Andes grew, the  increasing weight caused the   planet’s crust to flex down next  to the mountains, creating a basin… Like if you placed something heavy in the  middle of a shelf, making it sag downwards.

Suddenly, you had a continent’s worth of water,   now completely blocked by  a nascent mountain range.

And that changed everything.

So… what happens if you stop  a continent’s worth of water?

Tucked up against the newly risen  Andes, the water became trapped.

Unable to cross the mountains,   but also unable to drain eastwards towards  the Atlantic, the water pooled and pooled.

Ultimately it swamped the continent’s interior and   turned much of South America’s heartland into  an expansive mish-mash of lakes and wetlands.

This ecosystem would come to be known  as the Pebas System and it was gigantic.

At its height, this ecosystem covered  as much as a million square kilometers,   an area the size of the modern country of Egypt.

By this time, around 23 million years ago  in the Miocene epoch, the Earth had once   again warmed from the chill of the previous epoch,  and you would have definitely felt it there.

It was hot and humid, a land of palm swamps, with  rainforests popping up on the drier bits of land.

For many of the creatures of the savanna,   this change would likely have been a  disaster, dooming them to extinction.

Even for those who didn’t go extinct, the Pebas  created a maze of barriers and connections.

Land-based species may have struggled to  cross the many big, open bodies of water,   isolating groups from palm trees to mammals.

And aquatic organisms also struggled to adjust,   like the ancestors of today’s poison dart frogs,  who had to deal with changing water conditions.

But for life in the Pebas, change  brought new opportunities, too.

Like, the sudden appearance of a land bridge  across a swamp or a new waterway could have   united long-separated populations  or created shortcuts to new areas.

And it turns out that this back and  forth dance of isolation and connection   promoted intense diversification in  the species living there at the time.

Using techniques like molecular phylogeny,  which uses estimated rates of genetic change   to figure out how recently two  species diverged from each other,   we can see radiations for all kinds of critters,  like reptiles, invertebrates, and especially fish.

The species diversity of these groups abruptly  increased as populations constantly found   themselves in new environments, only to get  trapped there, isolated, and forced to adapt.

Indeed, by about 13 million years ago,  we can see that the Pebas had become   home to an incredibly diverse  and abundant array of animals.

Opossums and carnivorous marsupial-relatives  called sparassodonts moved along the ground,   while primates like capuchin monkeys and  marmosets jumped around in the treetops.

In the water, the bizarre, rhino-like  Astrapotherium lazed around,   half-submerged in the water like a hippo.

It also  would have been joined by manatees and dolphins.

But the most impressive thing was probably  the crocodilians.

A whopping seven different   crocodilians all seem to have been  living here together at the same time.

Some of them were even really big,  like the gigantic Purussaurus.

Adults of this species could reach 10  meters long – longer than saltwater   crocodiles today – making them the largest  continental predator of the Cenozoic.

They may have had a bite stronger than  any living animal, which they used to   dine on some pretty large prey, like  meter-long turtles or ground sloths.

Or take Mourasuchus.

With a wide, flat  head, huge gullet, and tiny teeth,   scientists are still debating how,  exactly, this strange animal fed.

Some have suggested that it gulped huge  amounts of water into a throat pouch,   then strained out small fish and arthropods  to eat, kind of like a baleen whale.

Meanwhile, other, smaller,  mollusc-eating crocs were there too,   using crushing teeth to eat the clams and other  invertebrates that thrived in the lake beds.

These little guys, which include  critters like Gnatosuchus,   may have actually been the  dominant crocs in some areas.

And if you’re wondering where all  of Pebas’ water did eventually go,   it seems like it turned northward,  draining toward the Caribbean instead.

There even seem to have been episodes  where the salty water from the Caribbean   actually flowed back into the area,  turning the Pebas System brackish,   explaining those salt-loving molluscs that  early paleontologists found so fascinating.

This connection is what brought coastal life like   dolphins and manatees into  the heart of the Amazon.

Of course, today, the Pebas is  long gone and the Amazon reigns.

Which raises the question, what happened?

What on Earth could have flipped   pretty much a whole continent's watershed?

The answer is, once again, the Andes.

While their uplift may have helped create   the Pebas System in the first place  by blocking the exit to the Pacific,   their upward trajectory didn’t stop there.

As they kept rising, lifting the land higher,  more and more sediment washed down from their   upper reaches during rainstorms, blanketing  the Pebas in silt and eventually filling it in.

By about 10.5 million years ago, the  land had become so tilted that the   waters that fed the Pebas began to  run eastwards towards the Atlantic.

And around 500,000 years later, we start  to see sediments from the Andes showing up   at the mouth of the Amazon, a sign that,  by then, water was flowing all that way.

This phenomenon, when rivers  abruptly get rerouted,   is known as “river capture” or,  wait for it, “river piracy”.

Over time, more and more water  that had once flowed toward the   Pebas reversed course and started  running down toward the Atlantic.

This may represent the largest  series of river piracy events   in Earth’s history and continued for  a long time, even up to the present.

Today, parts of the Rio Orinoco are in the  process of being pirated away by the Rio Negro.

And this was, well, not great news for some of the   organisms that had been living in  the Pebas, like many of the crocs.

The big crocs disappeared completely,  and, like, thank goodness for that.

And while those little, clam-eating  crocs stuck around for a while longer,   even they eventually disappeared  – along with their prey.

And the new drainage, along with other  factors like climate change drying things out,   may have contributed to the  extinction of other critters… Like those carnivorous sparassodonts,  who probably struggled once the thick   rainforest turned back into an open canopy,  and they left no modern descendants.

So this might seem bleak.

But, actually,   alongside these extinctions was a new  flourishing of speciation and diversification.

Because while many once-separate watersheds  became combined into one continuous corridor,   there were also new cycles of  opening and closing waterways,   similar to what happened  during the Pebas System’s time.

Today, the area that had once been the Pebas is   divided into the modern Magdalena,  Orinoco, and Amazon river systems.

This spawned yet more diversification,   especially among the fish that swam  in the rivers, and among the monkeys.

Capuchin monkeys, for instance,  may have taken advantage of the   new river-side highway to spread from  the Atlantic-facing forests westwards.

And a 2021 analysis of the ranges of  modern animals found evidence of this   intra-continental interchange in the  current distribution of stingrays,   electric fishes, birds, and mammals, for example.

Over time, things would settle down into  the flow that we’re more familiar with.

But we can still see the effects of the Pebas and  those river captures in what lives there today.

The connection to the Caribbean that brought   critters like dolphins and manatees,  mean they still patrol the rivers.

And patches of specially enriched soil throughout  the region that help support high plant diversity,   may be the remnants of marine nutrients  that washed in during the Pebas.

Overall, the flip-flopping of South America  was a dramatic tectonic and evolutionary story,   one that set up the continent for the  rich species diversity that we see today.

The constant closing and opening of pathways  repeatedly isolated and re-united species,   driving them to evolve in spectacular ways.

And towering above it all are the Andes,  the longest continental mountain chain in   the world and the force that flipped a continent  around and forever shook up life in the process.