Americas’ hookup not so ancient after all

A debate over when the gap between North and South America closed has opened a rift in the scientific community.

Analyzing existing data from ancient rocks, fossils and genetic studies, a group of researchers has assembled a defense of the conventional view that the Isthmus of Panama formed around 3 million years ago. That work rebuts papers published last year that concluded that the continental connection started millions of years earlier (SN: 5/2/15, p. 10). The authors of the new paper, published August 17 in Science Advances, caution against the “uncritical acceptance” of the older formation date.
“Those of us who are advocating the traditional view are in danger of being seen as old fuddy-duddy conservatives,” says study coauthor Harilaos Lessios, a molecular evolutionist at the Smithsonian Tropical Research Institute in Panama City. “But sometimes the traditional view is the correct one.”
The American continents drifted apart following the breakup of the Pangaea supercontinent around 200 million years ago. Eventually, the landmasses slid back together. As they reconnected, a volcanic mound on the Caribbean tectonic plate collided with South America and rose above the ocean. This new land closed a seaway between the Pacific and Atlantic oceans, rerouted ocean currents and sparked animal migrations, leaving clues that scientists on both sides of the debate are using to determine the age of the Isthmus of Panama.

Aaron O’Dea, a paleontologist at the Smithsonian Tropical Research Institute, Lessios and colleagues revisited several of those lines of evidence to date the seaway closure. For instance, fossil records reveal that land animals began migrating more frequently between the Americas around 2.7 million years ago, possible evidence of a newly available land route, O’Dea’s team concludes. Critics, though, counter that those migrations were instead driven by climate and ecosystem changes that allowed animals to migrate.
In the oceans, the closed seaway divided populations of marine organisms such as sand dollars. Over time, these populations’ genetic makeups diverged. Based on the degree of genetic change between the groups as well as fossil evidence, O’Dea’s team estimates that the seaway closed roughly 3 million years ago.

Christine Bacon, an evolutionary biologist at the University of Gothenburg in Sweden, and colleagues analyzed similar evidence last year but came to a different conclusion. The seaway closed between 23 million and 7 million years ago, Bacon and colleagues estimated in the Proceedings of the National Academy of Sciences. That study assumed a different rate of genetic divergence and looked at more species than the work by O’Dea and colleagues, Bacon says.

Rocks also trace the isthmus’s rise from the sea. Chemical traces from ancient ocean sediments record when seawater stopped mixing between the Atlantic and Pacific. Analyzing those traces, O’Dea and colleagues estimate that the seaway became relatively shallow around 12 million to 9.2 million years ago and completely shut around 2.7 million years ago.

Other rocky evidence tells a different story, proponents of the older age claim. Volcanically-forged crystals, known as zircons, found in South America date back to around 13 million to 15 million years ago. The only possible source of those crystals was in Panama, suggesting that a river washed the crystals down a land connection between Panama and South Americaaround that time, geologist Camilo Montes of the Universidad de los Andes in Bogotá, Colombia, and colleagues concluded last year in Science.
Those South American crystals may have formed closer to home, O’Dea and colleagues argue in the new paper. Similar crystals have been found elsewhere in South America, so the crystals reported by Montes and colleagues may have originated from a source in South America, not Panama, O’Dea says.

Some of the disagreement between the two sides stems from the fact that the seaway closure wasn’t a single event, says Carlos Jaramillo, a paleontologist at the Smithsonian Tropical Research Institute who coauthored the studies by Montes and Bacon. The seaway would have closed in stages, with various segments shortened and closed off over millions of years, Jaramillo says. “You can’t just use one date for everything, it depends on what you’re looking at,”he says.

Bacon is holding her ground. “They basically rehashed a mishmash of old papers,” she says of the new work. “We need to gather new data and collaborate rather than hold on to old ideas bitterly.”