Move north or south from the lush, dense tropical forests and seas, and the numbers of species calling those places home drop off sharply. Scientists have known about this equatorial bulge of biodiversity for more than a century, yet its explanation remained elusive. Researchers focused on species formation and extinction to explain why the tropics had such diversity of life. In 1974 the highly regarded botanist G. Ledyard Stebbins declared that the tropics were either a cradle, where new life evolved more frequently than at other latitudes, or a museum, where old, stodgy life persisted there longer. But the evidence was not clear. Now there is a third option, researchers say, one that takes a little bit from the cradle and a little bit from the museum.

David Jablonski, professor of geophysical science at the University of Chicago, and his colleagues wanted to test the cradle or museum conundrum, so they headed to, of all places, museums. Specifically, they wanted to look at bivalve specimens. Today, bivalves, marine creatures with two hinged shells, are found at all the world's latitudes and have a substantial fossil record--giving the researchers a wealth of information. They rifled through museum drawers and sifted through old papers and monographs to trace back the lineages of hundreds of living bivalves to pinpoint where they first cropped up.

Once the researchers knew where the bivalves came from, they saw that these mollusks followed the "out of the tropics" theory, which states that more species originate and live longer in the tropics but that tropical species spread from their birthplaces to populate more temperate regions. "The dichotomy between cradle and museum is the wrong question," says Jablonski, who reported the analysis in the October 6 issue of Science. Instead, the bivalves showed the tropics to be both cradle and museum, where new life emerges and old life remains. Bivalves, among other living things, evolved in the tropics and spread to the cooler north and south. "The tropics really are the wellspring of biodiversity," says Jablonski. And he warns that damage to these vital birthing grounds and last preserves will have an outsize impact on biodiversity in the future.