That it teems with life makes Earth a precious oddity among the planets-although just how off, scientists cannot say. Certainly the conditions that made life possible were sensitive to the planet's surface temperature and therefore to its distance from the sun.
Abundant liquid water was critical to the planet's evolution. This water moderated temperatures, eroded rocks, dissolved minerals and supported complex chemical reactions, some of which yielded single-celled life close to four billion years ago. Macroscopic animals started proliferating only around 600 million years ago, eons after photosynthesis enriched the atmosphere with oxygen.
Earth's large moon probably formed from the debris after a collision between early Earth and another huge body. Because the moon and sun appear the same size from Earth, or planet is the only one to witness the beauty of the sun's corona during a total eclipse.