A chance of 1 out of 1,000,000,000,000,000 (quadrillion, 10 with 14 zeros) is considered a virtual impossibility. But when DNA co-discoverer Francis Crick calculated the possibility of a simple protein sequence of 200 amino-acids (much simpler than a DNA molecule) originating spontaneously, his figure was 10 with 26 zeroes after it.

Those who remember one fad of the past will appreciate British scientist Fred Hoyle's view of the odds against evolved life. "Anyone acquainted with the Rubik's cube," he wrote, "will concede the impossibility of a solution being obtained by a blind person moving the cube faces at random."

Mr. Hoyle's best-known analogy, however, has a tornado in a junkyard taking all the pieces of metal lying there and turning them into a Boeing 747. It might be possible for two pieces to be naturally welded together, and then two pieces more in a later whirlwind, but production of even a simple organic molecule would require all of the pieces to come together at one time.