Earth; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

That it teems with life makes Earth a precious oddity among planets-although just how odd, 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.