Consider the claw. Frequently found on animals around the world, it's one of nature's most versatile tools. Bears use claws for digging as well as defense. An eagle's needle-like talons can pierce the skulls of their prey. Even the ancestors of primates used to wield these impressive appendages, until their claws evolved into nails. So what caus...

About 66 million years ago, a terrible extinction event wiped out the dinosaurs. But it wasn’t the only event of this kind -- extinctions of various severity have occurred throughout the Earth’s history — and are still happening all around us today. Borths, D'Emic, and Pritchard give a quick history of mass extinctions. [Directed by Juliette Mar...