Earth

The presence of life on Earth is, as far as we know, unique. Men have walked on the Moon, and it seems certain there is no life on our barren, airless satellite. Unmanned spacecraft have landed on Venus and Mars and have flown close to every other planet in the solar system except Pluto. The most promising possibility, Mars, yielded nothing to the automated experiments performed by the Viking spacecraft that touched down there.

The origin of life on Earth is not understood, but a promising experiment was performed in 1952 that may hold the secret. Stanley Miller and Harold Urey simulated conditions in Earth's early oceans, reproducing the surface and atmospheric conditions thought to have existed more than three billion years ago. A critical element of this experiment was simulated lightning in the form of an electric arc. Miller and Urey found that under these conditions, amino acids, the essential building blocks of life, had formed in their primitive "sludge." Certainly this was a long way from humans—or even an amoeba—but the experiment proved that the early Earth may have been a place where organic compounds, the compounds found in living creatures, could form.

Life has existed on dry land only for the most recent 10% of Earth's history, since about 400 million years ago. Once life got a foothold beyond the oceans, however, it spread rapidly. Within 200 million years forests spread across the continents and the first amphibians evolved into dinosaurs. Mammals became dominant after the demise of the dinosaurs 65 million years ago, and only in the last two million years have humans come onto the scene.