Well-established facts form the foundation of scientific theory. But when new developments bring these facts into question, science as a whole progresses when research turns in another direction.
For years, geologists believed the Isthmus of Panama, the thin landmass connecting North and South America, formed 3.5 million years ago. But a recent comprehensive biological study revealed the isthmus formed nearly 30 million years earlier, changing the entire outlook of evolutionary history in the Western Hemisphere.
“Recently, [geologists] have been saying it is potentially a little bit older,” said Prosanta Chakrabarty, an LSU Associate Professor and Curator of Ichthyology at the LSU Museum of Natural Science. “We’re finding that the biology says that too. In fact, it could be three to five times older, which is a pretty big deal.”
The formation of the Isthmus of Panama drastically changed the Earth’s surface. By separating the waters of the Pacific and Atlantic Oceans, the isthmus disrupted ocean currents and altered sea levels, resulting in climate change.
Although previous studies primarily focused on the isthmus’ geological formation, Chakrabarty and his colleagues at the Smithsonian Tropical Research Institute, American Museum of Natural History and University of Gothenburg used a different approach, combining their biological and evolutional expertise.
By studying the migration of animals between the continents and their evolutionary history, the team was able to establish a detailed picture of the isthmus’ formation and its impact on the surrounding wildlife.
Specifically, Chakrabarty’s research focused on the migration of marine and freshwater organisms between the two continents and their genetic relationships to each other to determine when the isthmus developed.
“My role on that project was to look at all marine organisms, [saltwater] and freshwater, to look at when they connected,” Chakrabarty said.
Through his research, Chakrabarty found two opposite stories surrounding the formation of the isthmus — one from the freshwater perspective and the other from the marine perspective.
Chakrabarty found evidence that freshwater fish migrated to Central America much earlier than 3.5 million years ago, signaling that a landmass between the continents with freshwater must have existed before to allow movement through freshwater channels and lakes.
On the freshwater side, Chakrabarty and his fellow researchers discovered this earlier migration pattern by charting the evolutionary track of two major fish families in Central America — the cichlids and poeciliids.
After collecting samples from every country in Central America and sequencing their DNA, Chakrabarty and his team established their genetic relationship to one another and dated each species after checking their skeletal development to fossil records.
While the formation of the isthmus expanded the habitat for freshwater fish, it separated marine fish populations on either side.
“The marine system tells us the opposite because if there is no Isthmus of Panama then they can cross freely between the Caribbean-Atlantic to the Pacific,” Chakrabarty said. “When the Isthmus of Panama formed, it cut their populations in half.”
This clear separation is supported by the different evolutionary tracks populations took on either side of the Isthmus of Panama.
Although the populations were originally separated by the isthmus, Chakrabarty also found that the bridge was not always a concrete divider between the Pacific and Atlantic Oceans.
“When the formation happened, Panama is still a really slim, little landbridge,” Chakrabarty said. “It looks like even more recently than 3.5 million years ago there were times when there was flooding that may have connected the two oceans again. It’s not quite a hard barrier. It’s a fluid and dynamic area.”
After his years of researching fish in Central and South America and the Caribbean, Chakrabarty said he was not surprised with the finding of the study and will continue to examine how the evolution of freshwater fish can reflect geological changes.
You can reach Morgan Prewitt on Twitter @kmprewitt_TDR.