The slender strip of land known as the Isthmus of Panama has transformed the world's biology like few other locations. Geologists have been locked in debate about this vital connection's age, with estimates ranging from 3 million to 23 million years old. New research, carrying important implications for our understanding of biological history, suggests the youngest estimates are correct.

For tens of millions of years after the break-up of Laurasia and Gondwana, the North and South American continents existed in isolation from the each other. Species evolved independently, while the Atlantic and Pacific Oceans were joined, allowing interchange for marine species.

Suddenly, this all changed. Panama became simultaneously a bridge for land dwellers and a barrier for marine species. Big cats, bears, and rodents poured across from North America, extinguishing most of the southern continent's marsupials. A few creatures, most notably the opossum, went the other way.

To understand each continent's evolution, knowing the timing is essential. This has proven surprisingly difficult to resolve, but a paper in Science Advances makes the case for a date 3 million years ago.

Between 25 and 23 million years ago, the already volcanic province that became Panama started to interact with South America in a way that changed the volcanic activity. In combination with some biological evidence, this inspired claims for a similar age for the landbridge. Sediments thought to have eroded from the isthmus led others to propose a date closer to 6 million years ago. 

However, a team led by Dr Aaron O’Dea of the Smithsonian Tropical Research Institute argue that marine sediments demonstrate that a deep water connection between the tropical Atlantic and Pacific survived until 4.6 million years ago. The sediments on each side of the isthmus share a common isotopic fingerprint, suggestive of deep ocean currents flowing between.

More recently, the salty deep water at the bottom of the Atlantic Ocean became trapped. On the Pacific side, upwelling of deep water increased from 4.2 to 3.8 million years ago, consistent with models that the powerful incoming current had been cut off.

Even after the deep connection was cut off, marine species on each side remained similar, suggesting surface waters could still flow between the two great oceans. According to the authors, this ceased between 3.5 and 3.0 million years ago, when species on each side started to evolve independently.

^{Marine snails had similar shapes in the Caribbean and eastern Pacific until 3 million years ago, when they started to diverge. Jon Todd}

These findings are consistent with volcanic processes pushing up an undersea ridge, probably dotted with volcanic islands. Only 3 million years ago did the ridge rise to the point where the islands joined.

Although South American sloths, among other species, appeared in the north as far back as 9 million years ago, the paper dismisses claims that these arrivals are proof of a complete land bridge. Monkeys and rodents are thought to have crossed the Atlantic on fallen branches. Giant sloths may have found their way across the much smaller gap between islands with a strong current behind them.

Ironically, the 3 million year estimate is consistent with the earliest research on the topic, dating from the 1970s, which only recently came under challenge.