The isthmus that changed the world

Panama: The Atlantic Ocean, the Gulf stream, ice ages, seasons, even the evolution of humans from our ape-like ancestor owe their existence to Panama and the most important geological journey on Earth.

Once, the world was much more simple: there was just the one ocean (Panthalassa), a supercontinent, and a broad and stable band of tropical weather around the equator. By 65 million years ago, the planet had sorted itself out into pretty much what we see now – but with a crucial difference. The continents of first North America and then South America had drifted over from parts of Africa, but separating them was a deep ocean channel.

In a series of volcanic eruptions, an archipelago formed and was carried on the Caribbean plate from the west up into the gap. By 3 million years ago, the land bridge of Panama connected the two continents. It changed our world.

The isthmus divided the vast sea into the Pacific and Atlantic Oceans, completely changing the global climate. Warm equatorial Atlantic currents are deflected north, creating the Gulf Stream, and because these waters are no longer diluted by the Pacific, the Atlantic is saltier and warmer than the Pacific. The imbalance produced the first in a series of (23 so far) ice ages that helped shape life on earth, changed rainfall patterns, created seasons (through mobilising the shifting inter-tropical convergence zone) and led to Africa’s vast savannahs – which is credited with attracting our primate ancestors down from the trees, leading to us. Wow, quite a lot for a little spit of land.

Locally, it was a crazy time. For the first time, the weird and wonderful creatures that had evolved in South America – from porcupines and sloths (which evolved from African cousins), marsupials (whose ancestors crossed the onceuponatime land bridge that connected Australia to ice-free-Antarctica to South America) to anteaters and armadillos, whose emergence remains a mystery – were able to continue their explorations beyond Colombia to North America.

And all the North American animals – from camelids to pigs, to dogs to horses and elephants – were able to make the journey south. Their migration was far more successful: while much of the native South American fauna was wiped out with the arrival of better-equipped, large predators like the sabre-toothed tiger, North American animals can still be found everywhere from Patagonia to Alaska.

Every land animal that has made the transcontinental journey has had to traverse Panama, crossing an isthmus that is just 80 km wide in parts. It’s a palaeontologist’s dream come true and there are certainly plenty of them beavering away in the mud here. Now is the perfect time for such investigations because the Panama Canal Authority is midway through a widening project that’s exposing new banks rich with calcified lives. But time’s running out – the palaeo researchers have got about a year left before canal water buries the past forever.

We visit the fossil sites with researchers from the Smithsonian Tropical Research Institute, based here. At one site, evidently a former sea bed, we cannot take a step without stamping on 15-19 million year old shellfish. There are fragments of turtle shell (black and patterned), enormous ancient oysters and clams. They are so abundant and so fresh-looking with shimmering mother-of-pearl detail that if it wasn’t for the fact that we’re several kilometres from the coast and high up, I’d suspect they were recent wash-ups.

The next site, Las Cascadas, is 20 million years old and more interesting because it was a terrestrial forested site. Nick and I help with the dig and we find a tiny fossil tooth and lots of fragments of fossilised wood – pretty exciting. So far at this site, Carlos Jaramillo and his team have discovered two species of teeny tiny horses (20cm high), a bear-dog (yup, how it sounds) and a rhino – all North American species.

But, what’s got Carlos’s knickers properly twisted is the find at the site a few months ago of South American species, including a 15-metre-long boa dubbed Titanaconda, a massive caiman and a freshwater turtle. Geologists at STRI now think (from geochemical, geomagnetism and thermogeo data) it is likely that the continents crashed into each other 23 million rather than 3 million years ago.

There is little biological evidence to support the much earlier date. Ice ages started 3 MYA, the first sloth poo and bones appear in the north 3 MYA. Marine palaeontologists find a massive crash in biodiversity on the Caribbean coast 3 MYA, once they were deprived of the nutrient-rich waters of the Pacific. Around the same time, corals changed with those reefs favouring clear warm waters (symbiotic algae hosts) increasing, and those that prefer churned up nutrients declining. Aaron O’Dea, who’s been studying the ancient coral fossils, actually found a time lag of 1-2 million years between the isthmus forming and the coral extinction. He now thinks that a mass die-off occurred at the time of the continental join but that small populations persisted in niches in the Caribbean, throwing off the extinction data.

“There were twice as many coral species in the Caribbean before the isthmus formed. Caribbean biodiversity is now remarkably poor for a tropical ocean,” Aaron says. “What we’re seeing, I think, are the effects of that mass extinction. The Caribbean is still in a slow recovery.” Sadly, it’s a recovery that’s unlikely to continue under the full force of rapid global warming.

So why does the geological evidence point to the continents’ collision happening so much earlier. Are the geologists simply wrong? Carlos believes that the continents joined below the ocean 23 MYA, but there was still a gap between the two land masses that was broad enough to prevent land animals from crossing, but narrow enough that freshwater species like snakes, crocs and turtles could survive the swim. And how big was that channel? “I think between 100km and 200km,” he estimates.