The Chemistry of Carbon

Carbon occupies a unique position in the periodic table. It has four electrons in its outer shell — exactly half of the eight needed for stability. This means carbon can form four bonds simultaneously, in almost any direction, with almost any element. No other element combines this bonding capacity with the stability needed to form large, complex molecules.

Silicon, the element most often proposed as an alternative basis for life, also has four outer electrons. But silicon-silicon bonds are weaker and less stable than carbon-carbon bonds, and silicon does not form the double bonds that give carbon chemistry its versatility. Silicon dioxide — "sand" — is a stable solid. Carbon dioxide is a gas that living things can process. The chemistry of life requires molecules that can be easily assembled and disassembled. Carbon provides this. Silicon does not.

The Carbon Cycle

Carbon cycles through Earth's systems — atmosphere, ocean, land, and living things — in a way that maintains stable conditions for life. Plants absorb carbon dioxide from the atmosphere and convert it to organic molecules through photosynthesis. Animals consume plants and return carbon to the atmosphere through respiration and decomposition. The ocean absorbs and releases carbon based on temperature and chemistry.

This cycle has maintained atmospheric carbon dioxide within a range compatible with life for hundreds of millions of years, despite volcanic eruptions, asteroid impacts, and changes in solar output. The buffering capacity of the carbon cycle — its ability to absorb perturbations and return to equilibrium — is a property of the chemistry of carbon combined with Earth's geological and biological systems.

Carbon in the Stars

Carbon is synthesized in the cores of stars through a process called the triple-alpha process: three helium nuclei fuse to form a carbon nucleus. This process depends on a resonance energy level in the carbon nucleus that was predicted by physicist Fred Hoyle in 1953 before it was experimentally confirmed.

Hoyle — an atheist — recognized that if this resonance level did not exist at precisely the right energy, carbon would not be produced in stellar interiors, and life based on carbon chemistry would be impossible. He wrote: "A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well as with chemistry and biology, and that there are no blind forces worth speaking about in nature."

The man who argued most forcefully against design in the universe was compelled by the evidence to use the language of design.