Bones to pick

Roughly 540 million years ago, the diversity of animal life on earth increased at an extraordinary rate. This period, known as the Cambrian Explosion, produced a wide range of bones and fossils, and from them, scientists have pieced together an approximation of this biologically complex era. It's a "unique episode in earth history," says Charles Marshall, professor of biology and geology at Harvard, "when essentially all the animal phyla first appear in the fossil record." Marshall gives a lecture tonight, titled "Thinking Outside the Fossil Record: Explanations for the Cambrian Explosion of Animals." Free. 6 p.m. Harvard Museum of Natural History, 26 Oxford St., Cambridge. 617-495-2773. hmnh.harvard.edu [L.O.]