It doesn't take a very long time to irreversibly change the planet. In just a few dozen millennia — a geologic blink of an eye — three quarters of Earth's living things went extinct 66 million years ago. The cause was probably a massive asteroid impact, possibly accompanied by colossal volcanic eruptions. The consequences of what scientists call the Cretaceous-Tertiary (K-T) mass extinction were catastrophic: All the dinosaurs died (except for birds). Millions of microscopic organisms were killed off. Invertebrates vanished from land and sea. In North America, close to the site of the Chicxulub asteroid impact crater, more than half of all plant species were wiped away.