Enlarge ImageMolecular fingerprints. Sedimentary rock formations in Oman, like this one, contain chemical evidence of the planet's first animals, which appeared at least 635 million years ago. Credit: David Fike, Nature Where did all the animals come from? The fossil record is virtually animal-free up until the Cambrian Explosion 540 million years ago, and then--boom--thousands of critters of all shapes and sizes show up. The mystery has plagued scientists for more than a century and a half, beginning with Charles Darwin. Now, with a brilliant bit of detective work, researchers have located our missing ancestors. The problem with the...