Ball 9. Going Fishing

Does an ancient fish fossil in the Caribbean have the potential to reshape the conventional wisdom about marine life in the Western hemisphere? Prosanta Chakrabarty thinks so.

Dr. Chakrabarty, an ichthyologist at the Louisiana State University in Baton Rouge, believes that the preserved remains of a 5-million-year-old member of the cichlid family on the island of Hispaniola—home to Haiti and the Dominican Republic—is evidence that a prevailing theory of how freshwater fish spread throughout Central and South America needs revisiting.

Biologists generally believe that the upwelling of the Isthmus of Panama out of the sea 3.5 million years ago led to a major migration of species in the region, a phenomenon with the automotive-sounding name of “the Great Biotic Interchange.” The new isthmus—the spit of land that bridges Central and South America—allowed freshwater fish and other creatures to colonize habitats previously forbidden them, diversifying the animal life in North and South America in the process.

Analyzing genetic material from more than a dozen fish families, Dr. Chakrabarty and a colleague, James Albert, of the University of Louisiana in Lafayette, found that many species appear to have spread across the region far earlier than can be explained by the isthmus model.

“Cuba and Hispaniola may have connected the continents long before the Panamanian isthmus did, allowing fishes to disperse across fresh waters from South America to North America 50 million years ago instead of just in the last 3 million years,” Dr. Chakrabarty says. “We find that the closure of the Isthmus of Panama was just the most recent event in the rich geological history of this region. The rise of the Isthmus of Panama actually allowed more fish to move south from Central America to South America—more species than in the other direction. This is a very different view than that proposed by the Great Biotic Interchange.”

In addition to fish, toads and some lizards also appear to have enjoyed the same mobility as fish, he says. “In 100 million years, when Central America breaks up and drifts away, no one will believe the current state of having these four disparate geologic masses aligned as a land bridge,” Dr. Chakrabarty says. “That fish can tell us how these land masses fit together and broke apart is pretty cool.”