In the Miocene, 10 million years ago, the Central American Seaway separated North and South America, leaving the southern continent in what scientists call \u201csplendid isolation.\u201d By 2.8 million years ago, the two continents were joined at the Isthmus of Panama. Mammals stampeded across the new land bridge in both directions.<\/p>\n
But the species trade wasn\u2019t equal. Today, more than half of the 217 mammal genera present in South America have origins in North America. Only four genera in South America can be traced to the north. The key to the North American mammals\u2019 success could be an extensive vacation. Over several million years, northern mammals moved into the low-lying \u201cholding pen\u201d of what is now the southern United States, Mexico and part of Central America, diversifying and adapting to the climate before the Isthmus of Panama arose, researchers report July 30 in Science<\/em>.<\/p>\n