During the quark–lepton era the universe was dense enough for the average separation between unattached quarks to rival the separation between attached quarks.

Had Earth been much closer to the Sun, the oceans would have evaporated. Had Earth been much farther away, the oceans would have frozen. In either case, life as we know it would not have evolved.

These early, single-celled organisms unwittingly transformed Earth’s carbon dioxide-rich atmosphere into one with sufficient oxygen to allow aerobic organisms to emerge and dominate the oceans and land.

These same oxygen atoms, normally found in pairs (O2), also combined in threes to form ozone (O3) in the upper atmosphere, which serves as a shield that protects Earth’s surface from most of the Sun’s molecule-hostile ultraviolet photons.

This general flattening of objects that rotate is why Earth’s pole-to-pole diameter is smaller than its diameter at the equator. Not by much: three-tenths of one percent—about twenty-six miles.

Earth has also tidally locked the Moon, leaving it with identical periods of rotation on its axis and revolution around Earth. Wherever and whenever this happens, the locked moon shows only one face to its host planet.