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A Fast Start for Mammals?

Mammals were already a diverse bunch during the age of dinosaurs, according to a molecular clock based on genes from hundreds of vertebrate species. The researchers argue in tomorrow's issue of Nature that the modern orders of mammals go back well into the Cretaceous period, but paleontologists are skeptical.

The long-standing view from the fossil record is that our furry ancestors first appeared 225 million years ago as small, shrewlike creatures living in the shadow of the dinosaurs. 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. Several molecular studies have hinted that many animal lineages are older than the fossil record shows, but most have relied on just a handful of genes and have not persuaded many doubters.

Evolutionary biologist S. Blair Hedges and molecular evolutionist Sudhir Kumar of Pennsylvania State University, University Park, improved on past studies by analyzing 658 genes and counting sequence differences between 207 vertebrate species. They assumed that the more differences between two species, the more time had passed since they diverged from a common ancestor. To calculate how fast the molecular clock ticks, the team started with a reliable date from the fossil record: 310 million years ago, when the mammal-like reptiles split from the birdlike reptiles.

For most species, the molecular dates matched those from fossils--but not for mammals. According to the genes, the modern orders of mammals arose much earlier than expected. Marsupialia (kangaroos and opossums) are pegged as originating 173 million years ago, rather than 94 million years ago as indicated by fossils, and Archonta (primates and tree shrews) at 86 million years ago, rather than 64 million years ago. "This doesn't mean that elephants and tigers were running around," says Hedges. "The animals themselves were probably small. But the lineages leading to different modern orders of mammals were already distinct."

But many paleontologists are deeply skeptical. "It suggests that the fossil record is horribly incomplete," says Michael Benton, a paleontologist at the University of Bristol in the United Kingdom. "They're saying side by side with lower Cretaceous dinosaurs, we should be finding ducks and hens and squirrels and rabbits." Instead, he thinks that the molecular clock can't keep time.