There is another reason why H, C, N, and O were used in the development of life: Like Mt. Everest, they were there.

The primary atmosphere on the Earth that was present when it was formed by accretion of material from the primal dust cloud around the young sun, was mainly hydrogen, helium, and some other inert gases. In time, these light atoms were lost from the planet's weak gravitational field, and the Earth became an airless ball of rock.

The secondary atmosphere, in which life evolved, arose later by outgassing of the interior of the planet. This atmosphere was mainly a mixture of hydrogen, and of methane, ammonia, and water, the hydrogen compounds of the second-row nonmetals.

Only such small, covalently bonded molecules as these were gases and hence were exuded into the secondary atmosphere. The heavier elements, and the elements that favored ionic compounds, remained locked in the minerals of the crust. It is natural that if life develops on a planet, it makes use of materials that are readily available.

To complete the story in capsule form, the best evidence we have suggests that lightning discharges, ultraviolet radiation, heat from vulcanism, and energy from radioactive decay caused the molecules of the primitive atmosphere to react and to condense (polymerize) into formaldehyde, amino acids, and other simple organic compounds, which were washed into the seas by rain. This thin "organic soup" then was the medium in which self-perpetuating chemical systems sufficiently complex to be called "living" gradually developed.

Our planet did not decide, 4.5 billion years ago, to bring forth life. Instead, the Earth was fortunate enough to have in abundance the kinds of chemical elements that could form the types of compounds that led to the reactive systems of living organisms. It is possible that, given the right conditions on a young planet, life develops as inevitably as crystals develop in evaporating salt water.