Dinosaurs

A survey of prehistoric bones reveals that T. rex and some of its cousins had more than one way to reach enormous sizes. Evolution may have preserved that variation in modern animals too.

The paleontologist Pincelli Hull has nailed down the timing and speed of the extinction that killed off the dinosaurs — details that carry ominous warnings for today.

For decades, researchers have commonly assumed that higher oxygen levels led to the sudden diversification of animal life 540 million years ago. But one iconoclast argues the opposite: that new animal behaviors raised oxygen levels and remade the environment.

Modern birds emerged in a snap of evolutionary time through a series of simple genetic changes.