One prominent theory had proposed that frog species on the large islands of Cuba, Jamaican, Hispaniola, and Puerto Rico had walked there across land bridges that existed when those islands were connected in a geologic arc about 70-to-80-million years ago. A second major theory proposed that they arrived, instead, by rafting across the Caribbean Sea after the giant asteroid impact near Cuba 65-million years ago, which is widely believed to have exterminated the dinosaurs. "Both theories acknowledged that the frog faunas must have arrived by rafting over water to the smaller and younger islands, the Lesser Antilles, because they never were connected by land to South America, but neither theory proposed that all of the Caribbean-island frog species had a single common ancestor," Hedges said. __IMAGE_3
The anatomy of Caribbean frogs previously had led the advocates of both theories to conclude that species in Cuba and other western-Caribbean islands were related to different mainland species than were the species on Puerto Rico and other eastern-Caribbean islands, regardless of how they got there. "Discovering a single origin for all of these species from throughout the Caribbean islands was completely unexpected," Hedges said.
To make their discovery, the researchers sequenced the DNA of nearly 300 species of Caribbean, Central American, and South American frogs and used three mitochondrial genes and two nuclear genes in their study, building trees of relationships among the species and timing the divergences of the species with molecular-clock methods. "Molecular clocks work by counting the number of DNA-sequence mutations separating two spec