For a strip of land 400 miles wide—less than the distance from Boston to Washington, DC—the Isthmus of Panama has been a hotbed of geological research. Until now, conventional scientific wisdom had it that the terrestrial link between North and South America formed three million years ago. The timing is convenient for connecting the land bridge's formation to major global changes that occurred around the same time, such as the onset of glaciation in the Arctic and the migration of land animals between the Americas.
But a study published in Science on Friday throws a wrench in these explanations. Its authors, led by a team of scientists from the University de los Andes in Colombia, the National University of Colombia, and the Smithsonian Tropical Research Institute in Panama, found new evidence that the land bridge may have actually started to form 13 to 15 million years ago, over 10 million years earlier than the timing traditionally assigned to the event.
Analyzing rocks in Colombia, the scientists found grains of the mineral zircon dating back 13 to 15 million years ago, which could be traced to volcanic eruptions in Panama. They also found coal and soil with preserved tree roots in the river deposits. The scientists believe the zircon was deposited in Colombia by an ancient river system that flowed across land.
"We know that particular zircon age population was present in Panama but not in northwestern South America," said Camilo Montes, a geologist at the University de los Andes who led the study. The date of the zircon suggests that a land bridge connected Panama to South America by 13 to 15 million years ago, explained Montes.
The new geologic evidence builds on researchers' earlier discoveries of fossilized trees and freshwater crocodilians in Panama that date back as far as 20 million years ago. Put together, the zircon grains and fossils paint a compelling argument that Panama started to form much earlier than three million years ago.
Hanging between Costa Rica and Colombia like a chain lock, Panama separates the Atlantic and Pacific Oceans. Before tectonic activity drove the isthmus out of the sea, a corridor of water called the Central American Seaway conjoined the Atlantic and Pacific. By shutting down the exchange of water between the two oceans, Panama gave rise to the Gulf Stream, which originates in Florida and carries warm water to northwestern Europe. Scientists believe the warmth delivered by the current makes Europe more habitable in the winter.
Researchers, most notably a geologist named Gerald Haug at the Swiss University ETH Zurich, have proposed that the Gulf Stream kicked off a glaciation event in the Arctic three million years ago. Haug believes that evaporative cooling of the current's warm waters provided the moisture needed for snow and ice formation.
But the new evidence from Montes and his collaborators calls Haug's hypothesis into question. "His hypothesis was very elegant," said Montes, "but it loses some support if we say the isthmus formed 13 or more million years ago."
It's possible that the revised date could instead explain a warming event that happened roughly 14 to 20 million years ago, during the middle Miocene, Montes told me. "But it's important not to confuse causality with synchronicity," he cautioned.
The formation of the transcontinental highway has also been connected to a major biodiversity event around three million years ago, called the Great American Biotic Interchange. During that time period, a mass migration of flora and fauna, including the ancestors of bears, cats and horses, took place between North and South America.
With new evidence that the land bridge may have started to form much earlier than three million years ago, investigators will have to re-examine their understanding of these major climate and biodiversity events.
Some prior research already offers explanations: Scientists have suggested that the uplifting of Indonesia and closure of the Indonesian seaway may have actually triggered ice sheet formation in the Arctic. Other scientists have proposed that the Great American Biotic Interchange occurred three million years ago because climate events around that time caused Panama's dense jungle environment to turn into arid savanna. The transition might have made it easier for many animals, such as big cats, to travel from North America to South America.
Not all scientists are convinced that the new findings mean a land bridge was fully formed by 13 to 15 million years ago. "There seem to be clear clues that a bridge started forming much earlier than once thought," said Marcelo Sánchez, professor of paleobiology at the University of Zurich in Switzerland. "However, this finding does not exclude the possibility of intermittent or remaining sea connections between the Pacific Ocean and the Atlantic."
Montes is aware that some scientists are skeptical of the new dates because of evidence that marine organisms existed in the region before three million years ago. But he points out that these arguments do not conflict with his team's hypothesis, which acknowledges that there could have been remaining shallow water connections between the Atlantic and Pacific.
"There is clear evidence of marine invertebrate faunas. We are not denying that," he said. "We allow for shallow and transient openings that connected the Caribbean and Pacific up until three million years ago."
Moving forward Montes hopes to continue piecing together the history of the Panama land bridge. In the coming months he will bring his students to Azuero Peninsula, west of Panama City, to do a variety of field measurements. He is open-minded about what they might find. "The puzzle [of how the isthmus formed] is still incomplete," he said. "We're interested in anything that may be related."