Many scientists think that the seaway separating the two continents closed about 3 million years ago, sparking mass animal migrations and an ice age in the Northern Hemisphere. While the surrounding rock was laid down 10 million to 13 million years ago, the zircons were as much as 60 million years old. But even if a river did run from Panama to South America 13 million years ago, that doesn’t mean the Americas were completely connected, says geologist Anthony Coates of the Smithsonian Tropical Research Institute in Panama City.