alien life

Four and a half billion years... half way through the lifetime of a planet... for some of that time molten and dead from bombardment in the early formation of the solar system... for most of the remainder inhabited only by single-celled life forms. And for a 'mere' half billion years, a flourishing of plants and animals. Then at last, for the merest flicker of geological time, there are human beings. How very long it took until a tool-making, syntactic language-using, self-reflecting species arose—for the first time and (for all we know) the only time in the entire galaxy.

But life on Earth is dangerous. Mass extinctions can happen slowly through geological changes that cause vulcanism and planetary cooling, or quickly through a collision with a meteorite. A nearby star could become a supernova. At least twice in its long lifetime, Earth has been frozen solid or nearly solid all the way to the equator—the sort of ice age which, if it happened now, would exterminate all multicellular life. Bacteria would once again be the only life forms. By the greatest of good fortune, our planet has survived until it is within reach of safety from cosmic disasters: its latest creation, ourselves, is slowly maturing in its capacity to develop the means to safeguard the planet for all life.