"The big question now is what took the ancestors of modern mammals so long to diversify," says co-author Ross MacPhee, a curator at the American Museum of Natural History in New York City. "It's as though they came to the party after the dinosaurs left, but just hung around while all their distant relatives were having a good time."

The researchers speculate that a spike in atmospheric temperatures caused more floral diversity, giving mammals a new and plentiful food source.

Bininda-Emonds says the supertree is already being used by the London-based EDGE (Evolutionarily Distinct & Globally Endangered) program, which is an effort designed to establish a list of the 100 endangered animals most in need of protection. He says the supertree can be used to determine how "evolutionarily unique" a species is.

For example, the endangered red panda of northern India and southern China is 39 million years removed from its closest relative, according to the new research. "One might argue that it might be more worthwhile to save the red panda than an endangered species of mouse or rat, which may have diverged one million years ago," says Bininda-Emonds.

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