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Summary
The long-standing view from the fossil record is that mammals first appeared 225 million years ago as small, shrewlike creatures and that only after a mass extinction 65 million years ago at the end of the Cretaceous period killed off the dinosaurs were mammals able to evolve into everything from primates to rodents to carnivores. But in this week's issue of Nature, a pair of researchers compared genes from hundreds of vertebrate species and used the differences as a molecular clock to date when animal lineages originated. The molecules show, they say, that the modern orders of mammals go back well into the Cretaceous period, in some cases to more than 100 million years ago.
