By BRIAN SWITEK - SCIENCE AAAS
Added: Sun, 28 Aug 2011 14:44:23 UTC

Early cousin. The restored skeleton and body of the Jurassic mammal Juramaia.

Way back in the Late Jurassic, 160 million years ago, your closest relative looked like a shrew. That's not an insult but an evolutionary truth that stems from a new fossil discovery that pushes back the earliest appearance of the peculiar group of mammals to which we, as well as many other mammal species, belong. During the heyday of the Jurassic dinosaurs, our own ancestors were just getting their start in the dark corners of the Mesozoic world.

Living mammals are split into three subgroups: the egg-laying monotremes; the pouched marsupials; and, the most diverse of all, placental mammals, which includes everything from humans to bats to whales. Each group diverged at different times, and determining when marsupials and placentals split from each other has been problematic. Fossil discoveries point to the Cretaceous, about 125 million years ago, whereas estimates made on genetic differences among living mammals suggest that the split happened even earlier.

Now the discovery of a partial skeleton of a small, shrewlike mammal, described online today in Nature, pushes back the date of the divergence by 30 million years, to 160 million years ago. Found in the famous fossil beds of Liaoning, China, the newly discovered little mammal has been named Juramaia sinensis, or "Jurassic mother from China."

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TAGGED: EVOLUTION, PALEONTOLOGY