Panama Land Bridge younger than expected

Inlet between North and South America closed only 2.7 million years ago

Basalt formation on the Pacific coast of Panama: It was created by volcanoes before the land bridge. © Aaron O'Dea
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Successful bridging: The land connection between North and South America is apparently still younger than previously thought. Instead of ten million years ago, as postulated last year, the divisional divide closed only about 2.7 million years ago, as revealed by a re-examination of all geological and biological evidence. The evidence for an older land bridge, however, are little evidence, according to the researchers in the journal Science Advances ".

The formation of the Panama Land Bridge is considered one of the formative natural events of the Earth's New Age: this new connection between two continents fundamentally changed life on land and in the surrounding oceans. Suddenly marine organisms in the Pacific and Atlantic were separated from each other, but land animals and plants were able to conquer completely new areas.

Three or ten million years ago?

"Knowing when and how the Panama land bridge formed is crucial to understanding this grand 'experiment of nature, '" said Aaron O'Dea of ​​the Smithsonian Tropical Research Institute in Panama and his colleagues. But that's exactly the contentious issue. For a long time it was assumed that a sea inlet still separated the continents until three million years ago. But in 2015, Camilo Montes of the University of the Andes and his colleagues published evidence that the land bridge existed ten million years ago.

But what's wrong? In order to clarify this, O'Dea and his colleagues have once again analyzed all geological, paleontological and molecular biological data on this topic and also re-examined the controversial evidence of Montes and his team.

The land bridge between North and South America - their disfiguring changed the living world on land and in the sea profoundly. © NASA / JPL / NIMA

Land bridge only 2.76 million years ago

Their result: A real, continuous land connection between the two continents could have originated only about 2.76 million years ago. Only then did the water exchange between the Pacific and the Atlantic end, and only then did a greater number of animal and plant species migrate from North to South America and vice versa. This is proven by biological and palo-temporal analyzes, according to the researchers. display

However, these changes have already begun: About 30 million years ago, volcanoes and the collision of earth plates created an island arc that connected the continents, as shown by rock analyzes. The individual islands were separated by wide, flat straits. "According to O'Dea and his colleagues, data suggest that large amounts of high-flow water were flowing through the straits into the Caribbean until at least 3.2 million years ago.",

Ice Age as Br ckenbauer

The arc of the island could explain why some South American sloths, small pikes and the terror bird appeared in North America five to nine million years ago: they used a combination of island H pests and natural streams of plant remains and driftwood to overcome the straits. "Some primeval primates and rodents have even crossed the Atlantic, " the researchers said.

The decisive turning point came only when an ice age began about three million years ago. The growing glaciers lowered sea levels worldwide and dried the straits in the Panama bow. This cap of the last connections between the Pacific and the Atlantic is reflected in marine sediments and fossils from that time and it is one of the phenomena that do not explain Montes' scenario of an early land bridge Can.

Zircon crystals store in their isotopes hints on their age and their origin - but the latter is not always clear. Angel Barbosa

Indicators of 2015 decongested

The new analyzes also reveal that the evidence cited in 2015 is less substantiated than expected. The researchers then investigated 13 to 15 million year old zircon occlusions in primeval fluvial deposits of northern Colombia and located their origin beyond the Great American Seaway in Panama. From there, the river water can only have drifted to the south, if the land bridges already existed at that time this is the conclusion of Montes and his team.

But now O'Dea and his colleagues contradict that. It is not correct that Panama could have been the only possible source for these zircons back then, they state. "In the Andean region of South America alone, more than 30 sites have been identified that contain zircons of exactly this age, " say the researchers. In addition, there are only sea sediments in the area that the prehistoric river was supposed to have flowed through at that time.

"Our review reveals that the evidence cited for an older land bridge is not conclusive, " the researchers say. "We therefore warn against an uncritical assumption of a land bridge before the Pliocene." (Science Advances, 2016; doi: 10.1126 / sciadv.1600883)

(AAAS, 18.08.2016 - NPO)