chapter  3
DNA → RNA → Proteins
Pages 20

Life on Earth began from some very simple molecules. The main characteristic of these molecules was that they normally contained a few different types of atoms that formed compounds that were somewhat stable, but were capable of carrying out simple reactions with other molecules. Primarily, they contained carbon and hydrogen; secondarily, nitrogen and oxygen; and, to a lesser extent, phosphorus and sulfur. On early Earth, the addition of water, time, heat, solar energy, pressure, and some compounds from extraterrestrial sources produced the rst biological reactions and eventually cells that were capable of absorbing nutrients for their metabolic processes and reproduction and excreting waste products. They used amino acids to build proteins, but also relied heavily on RNAs for structural elements, enzymatic functions, and genetic capabilities. Selection favored those that reproduced reliably over long periods of time. During this early evolution, a lipid bilayer cell membrane, use of carbohydrates, reliance on proteins and RNA, and translation of proteins by ribosomes became established. However, because of the high mutation rates for RNA, genomes would have been relatively small, and even then many of the progeny would have died because they would have inherited too many mutations to survive. But, one particular mutation, that which gave rise to the utilization of a 2′-deoxyribose for at least one gene, led to an advantage for the organisms that rapidly outcompeted the RNA-based cells.