Panama Canal fossils reveal ancient collision of worlds
It was the biggest event in our planet's history since the extinction of the dinosaurs.
Three million years ago, the Americas collided.
The creation of the Panama Isthmus - the narrow land bridge that joins the two continents - wreaked havoc on land, sea and air. It triggered extinctions, diverted ocean currents and transformed climate.
Now a multi-billion dollar project to widen the Panama Canal is set to reveal new secrets about the event that changed the world.
Panama is a tiny country, but in a perfect location.
Positioned just north of the equator in the Caribbean, its famous canal is the strategic hub of the global shipping industry.
The 80km (50-mile) -long Panama Canal, completed in 1914, connects the Atlantic and Pacific oceans. Its existence means that ships can avoid - at a price - the treacherous 8,000 mile journey round Cape Horn, the southernmost tip of South America.
Three years ago work began to widen the Panama Canal for the first time in its history. Authorities hope that this will increase revenue from shipping.
However, the massive excavations have also proved to be a "gold mine" for scientists, trying to uncover Panama's hidden past.
Dr Camilo Montes, a geologist at the Smithsonian Tropical Research Institute (STRI) in Panama, explained: "What this represents is a once-in-a-century chance to find out what really happened when the Americas collided".
As entire hillsides are being blasted away to expand the canal, amazing fossils are emerging that shed light on this key event. However, scientists only have a short window to collect the fossils before they are re-buried beneath concrete.
"Basically what we are doing is a rescue effort", said Dr Carlos Jaramillo, another geologist at STRI. "It's a race against time. When a new fossil site is discovered we have two, maybe three months. That's all!"
"We work all day, every day, in the canal. Whether there is blazing sun or pouring rain, we always have a team out searching for the fossils."
One of those fossil hunters is Aldo Rincon, a young student with sharp eyes. Last year, he stumbled upon one of the most important discoveries so far: the jaws and bones of horses, rhinos, and camels.
He told the BBC News of the moment of discovery: "I spotted a row of teeth sticking out of the mud. The rest of the team had scattered. For a moment, I was alone with these beautiful fossils, it was so exciting."
Clash of civilisations
What Mr Rincon has found helps us better understand an extraordinary event that scientists call the "Great American Interchange".
Dr Bruce MacFadden, an expert on fossil mammals at the University of Florida, US, explained: "When the Americas collided about three million years, it caused of a kind of land rush".
"Animals that were native to North America - sabre-toothed cats, horses, camels and elephants - surged south across the land bridge. Animals from South America such as giant sloths and armadillos, moved north".
In an ecological experiment on a scale never before seen, the animals of two continents freely mixed. Unable to compete with the waves of invaders, some species on both continents went extinct.
The event helped shape the ecology of the Americas to this day.
However, Aldo Rincon's new discovery muddies the water. The animals that he has found were all natives of North America, but 17 million years old - dating from long before the Great American Interchange.
They show that the Panama Isthmus may have started to form much earlier than previously thought, allowing some migrants into Central America.
Other fossil discoveries have also hinted at this possibility.
Dr MacFadden said: "Giant predatory birds dubbed 'terror birds' seem to have migrated from between the Americas as early as five million years ago."
"It is possible that long before the seaway finally closed, a chain of islands spanned the gap. Perhaps Terror Birds and other animals were the original 'island-hoppers', migrating from one island to the next."
The formation of the Panama Isthmus, however, did not only affect the Americas. It also transformed global climate, and might even be responsible for the UK's dismal and damp summers.
Dr Pierre Sepulchre, a climate scientist at the Pierre-Simon Laplace Institute, France, told BBC News: "When the Panama Isthmus formed, ocean currents got re-routed.
"Warm Caribbean waters that had once flowed through the gap between the Americas were now forced northwest towards Europe, creating the Gulf Stream."
Without the Gulf Stream, the UK would have a freezing climate like that of Newfoundland on the east coast of Canada.
But there were other even more dramatic effects as well.
"Its controversial," said Dr Sepulchre, "but some scientists think that the formation of the Gulf Stream transported extra moisture into the Arctic atmosphere. This fell as snow, triggering the build up of the Greenland Ice Sheet."
In turn, this may have kick-started the Ice Age.
So, the formation of a tiny land bridge in one remote part of the tropics seems to have triggered a 'domino effect' that influenced the whole world.
Sex in the Caribbean
Back in Panama, the land bridge also allowed coral reefs to thrive for the first time.
Today, the Caribbean boasts some of the world's finest coral reefs, the mainstay of its tourist industry. But this wasn't always the case.
Dr Aaron O'Dea, a palaeontologist at the Smithsonian Tropical Research Institute, spends his time exploring wild stretches of Panama's coast in a small inflatable boat.
"I've been in some terrible scrapes," he told the BBC News. "I've been hit huge waves, half drowned and battered against rocks!"
What keeps him going is his quest for fossil shells.
He collects bulky samples from rocks of different ages and laboriously counts every species present. In this way he builds a picture of how marine life has changed in the Caribbean over millions of years.
His findings are startling. Before the Panama Isthmus formed, there were few coral reefs. Afterwards corals abound.
Dr O'Dea explained: "At first, nutrient-rich Pacific waters flowed into the Caribbean creating a kind of pea soup, full of plankton, quite unsuitable for corals. However, when the Isthmus formed, Pacific currents were cut off, waters became crystal clear, and coral reefs boomed."
Intriguingly, at the same time, marine animals changed from their normal habit of cloning themselves and started to reproduce sexually.
"It all about food," said Dr O'Dea. "When it's in short supply, animals tend to switch to sexual reproduction because it gives them the edge over their competitors."
Worryingly, what Dr O'Dea is learning about the past also may provide a dire warning for the future.
"Compared to the ancient coral reefs that I study, the modern reefs are in an awful state," he said.
"What's happening is that fertiliser from agriculture is being flushed into the Caribbean. The fertiliser is feeding plankton, returning the sea to the pea soup state that existed before the Isthmus formed. This, in turn, is killing the corals."
So, as scientist learn more about the formation of the Panama Isthmus - the event that changed the world - it is just possible that their findings may help safeguard the future of the Caribbean's fragile ecosystems.
"That's my hope," said Dr O'Dea, "but it will take a lot to reverse the damage that's already been done."