Experts Clash on Age of Land Bridge Connecting Americas

This story was updated Aug. 18 at 5:53 p.m. EDT.

Despite reports that the land bridge connecting the Americas is "older than the hills," it is actually quite young, geologically speaking — only about 2.8 million years old, a new review of studies finds.

The finding contrasts with several recent studies purporting that the land bridge, known as the Isthmus of Panama, formed between 6 million and 23 million years ago, and reveals important details about the evolution and migration of animal species in different regions of the Americas.

These dates didn't jibe with data from other studies, the scientists of the new paper said. So they did an extensive review of studies on the geological, paleontological and molecular evidence from the isthmus and the animals that lived there, with the goal of deducing its true age, they said. [In Images: How North America Grew As a Continent]

"Our study had the simple aim of refining the timing of isthmus formation in a format intelligible for a wide audience of interested scientists, including geologists, ecologists, paleontologists, climatologists and evolutionary biologists, each of whom were begging for clarification of the question," study lead author Aaron O'Dea, a staff scientist at the Smithsonian Tropical Research Institute (STRI) in Panama, told Live Science in an email.

In the end, O'Dea and his colleagues found that while the isthmus developed slowly over a period of 30 million years, "all rigorous evidence" suggests that the land bridge formed 2.8 million years ago, thanks to a geologic uplift and a drop in sea level, he said.

Why age matters

Before the isthmus arose, waters from the Caribbean and Pacific mingled, allowing local marine life to call both oceans home. But the watery gap separated plants and animals that lived on land, meaning that those in South America stayed south and those in North America stayed north, for the most part.

Everything changed once the isthmus formed. The land bridge separated the oceans, meaning that marine animals in the Caribbean began to evolve independently from those in the Pacific. Moreover, the bridge connected the Americas, allowing land animals to travel freely between the two continents. For instance, South American terror birds and possums made their way north.

Finding the so-called isthmus birthday is crucial for evolutionary biologists, as the isthmus is "the only place on Earth" that allows them to examine the molecular rates at which ocean populations diverge, said review co-author Harilaos Lessios, a senior staff scientist of evolution of marine organisms at STRI.

O'Dea said that finding the age of the isthmus has even broader implications. "It's a fundamental question with major consequences for understanding ecology, evolution and the origin of life today in the seas and land of the Americas," he said.

Disputed findings

Since the 1970s, when researchers studied data from deep-sea drilling, some scientists have speculated that the isthmus was about 3 million years old. Some recent studies, however, upended that idea.

For instance, the isthmus must have existed earlier than scientists previously thought, according to a 2015 study published in the journal Science, because the researchers found evidence that a river carried 40-million-year-old zircon rock crystals, unique to Panama, across the isthmus to Colombia between 13 million and 15 million years ago.

However, the researchers who worked on the new review say those zircons could have come from places other than Panama, and that more evidence is needed to say that the isthmus existed before 3 million years ago. [Photos: The World's Weirdest Geological Formations]

The lead author of the 2015 study, Camilo Montes, a geologist at the University of the Andes in Bogotá, Colombia, contests this finding. The new review "can't point at a single magmatic rock that crystallized nearly 40 million years ago" that's not from Panama, Montes told Live Science in an email.

The new review also called into question work done by Christine Bacon, a postdoctoral fellow at the University of Gothenburg in Sweden. In a 2015 study, Bacon and her colleagues compared molecular data from land animals and plants, and said that the results indicated that phases of migration across the isthmus happened prior to 3 million years ago.

The authors of the new review said they found several problems with Bacon's study. For instance, they said she didn't analyze all of the available evidence. Furthermore, she assumed that mitochondrial DNA (found inside the powerhouse of a cell) diverges at a rate of 2 percent every 1 million years, when, in fact, different species evolve at different rates, Lessios said. But Bacon said this misrepresents her work.

"We assumed that rate for only 52 data points of the 424 total (ca. 12 percent of the entire dataset)," she told Live Science in an email. "It seems like misrepresentation … like I used a universal divergence rate over the entire dataset."

What's next

Though the new review doesn't make a definitive conclusion on the isthmus's age, it is an important step in figuring it out, said Carina Hoorn, a researcher at the Institute for Biodiversity and Ecosystem Dynamics at the University of Amsterdam in the Netherlands, who was not involved in the new research.

"Although there are no real new data in this study, the authors make it clear that the revolutionary new idea on 'old' Panama needs to be consolidated with further evidence," Hoorn told Live Science in an email. "Friend and foe agree that deformation of the region started over 30 million years ago… [but] when exactly the bridge was formed will need further evidence."

There needs to be more research into species evolutionand further examination of the fossil and marine record, Hoorn said. That's already happening, but it will require hard and cooperative work by many scientists, she said.

The review was published online today (Aug. 17) in the journal Science Advances.

Editor's Note: This article has been updated to correct the location of where the zircon crystals originated, according to Camilo Montes and his colleagues. It was Panama, not the northern Andes.

Original article on Live Science.

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