All life on this planet is based on the four elements C, N, O, and H. Wherever we look, similar molecules are undergoing such similar reactions that we are forced to conclude that all terrestrial life has a common ancestry.

The web of life on this planet is universal and unbroken. Before we can ask the difficult question, "Could life have evolved elsewhere using a different chemistry or even different elements?" we must answer the simpler query, "How are C, N, O, and H especially suitable as the elements of life?"

Efficient computing machines cannot be built from ratchets, gears, and pulleys, as the computer pioneer Charles Babbage learned to his sorrow a century ago.

Intricate machines need intricate components. It is hard to imagine a biological machine complex enough to be classified as living, as made up from simple inorganic molecules and ions.

Of all the chemical elements, only carbon has the ability to form virtually endless straightand branched-chain molecules of infinite variety. We will develop this theme in the final chapters of this book. Nitrogen could never build the variety and complexity of molecules needed for a living organism, for long -N-N-N-N- chains are explosively unstable. Neither could boron; it does not have enough electrons to form continuous chains.

Metals could not become the basis of the molecules of life, because electrostatic forces are nondirectional, and the only structure found in salts and metals comes from the packing of ions.

Only carbon, with four electrons per atom available for making four electron-pair bonds, can construct the scaffolding needed for elaborate molecules.