Where does life come from? The theory of evolution describes how one species slowly develops out of another, how humans evolved from apes, but does not explain the ultimate origin of life. Charles Darwin, who first published his theory of evolution in 1859, was always baffled by this mystery. At one point, even though he was an atheist, he even desperately suggested that God must have “breathed” life into the earliest organisms.

For most scientists, this mystery was solved by the famous experiments of the chemists Stanley Miller and Harold Urey, performed in 1953 at the University of Chicago. In a large, glass beaker in their laboratory, Miller and Urey approximately mimicked the conditions on Earth long before there was any life. They added some simple inorganic chemicals, heated them and shocked them with bursts of artificial lightning. After a week, they opened the beaker to see what products had formed. To their surprise, they found large quantities of amino acids, the simple organic molecules that are the building blocks of our bodies. Although these were not alive, this result proved for many that mere random mixing on the primitive Earth would produce organic molecules and eventually lead to simple life-forms. That is, given enough time and a chemical beaker the size of Earth's surface, evolution would begin spontaneously.

Physicists tell us that Earth formed about four-and-a-half billion year ago, and recently fossils of very simple one-cell organisms have been found in Australia that date from less than a billion years later.