Among the seminal discoveries in biology in the twentieth century were the chemical nature and the three-dimensional structure of the genetic materia… - Albert L. Lehninger

The current understanding that all organisms share a common evolutionary origin is based in part on this observed universality of chemical intermediates and transformations, often termed "biochemical unity."

Perhaps the most remarkable property of living cells and organisms is their ability to reproduce themselves for countless generations with nearly perfect fidelity. This continuity of inherited traits implies constancy, over millions of years, in the structure of the molecules that contain the genetic information.

As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another (Fig. 2-lb), called a hydrogen bond.