At the end of the Cretaceous period, the last dinosaurs disappeared from the earth, setting off an evolutionary jubilee among the Milquetoast-like mammals that survived them, and preparing the ground for what was to become, 65 million years later, a permanent source of gainful occupation for scientists whose job it is to wonder why the dinosaurs died out. Scores of reasons have been given for this remarkable concatenation of extinctions. Global climate and sea level were changed by a city-sized asteroid striking the earth near what is now the Yucatan, or by a massive set of volcanic eruptions, or by the solar system passing through the core of a giant molecular cloud, perhaps colliding with a supercomet loosened from the Oort cluster, which orbits the Sun beyond Pluto. Theories of catastrophic extinction abound. Some of the most daring even conjure up the specter of an unseen companion star to our Sun, named Nemesis, whose eccentric orbit brings a wave of potentially deadly comet showers—and extinctions—every 26 million years. But there are also paleontologists who argue that the dinosaurs went away gradually, not suddenly, over a period of millions of years, and that toward the end they coexisted with the earliest hooved mammals, including ancestors of horses, cows, and sheep. If extinction was gradual, a different line of thought opens up: perhaps the dinosaurs died out because they couldn’t adapt and compete in a changing world. The big lummoxes were obsolete. I heard about the dinosaurs’ obsolescence back in my student days. It was as satisfying a notion then as it is today, especially if you didn’t think about it too hard. Here were these lumbering, pea-brained reptiles, barely able to walk and chew gum at the same time, while all around and underneath them, cleverly hiding behind clumps of primitive vegetation and cleverly burrowing in tunnels in the ground, were the nerdy but smart little mammals about to emerge from the shadows and begin their ascent to glory—somewhat, it occurs to me now, like Bill Gates in the waning days of heavy manufacturing.
On the Origin(s) and Evolution of Earth's Carbon