How is it possible that Earth's atmospheric temperature has remained perfectly suited to supporting life for billions of years? Why do oxygen levels in the atmosphere remain relatively stable when only a minuscule increase would cause everything on Earth to spontaneously combust? Why are the oceans salty, but not salty enough to make them uninhabitable? In the 1960s, an English scientist names James Lovelock formulated a groundbreaking and highly controversial explanation: Earth is alive.

Naming his theory after the ancient Greek earth goddess, Lovelock's "Gaia hypothesis" argued that...