Chapter 14 - Origin of Nucleic Acids and the First Cells

Abstract

Life, in the form of bacteria, first appeared on earth about 3.7 billion years ago. Oparin’s theory of the origin of life posits that simple organic compounds would have been produced spontaneously in the earth’s primitive reducing atmosphere. These molecules could have been concentrated into a “prebiotic organic soup,” which by chemical evolution gave rise to larger molecules and eventually protocells. The “RNA world” hypothesis assumes an early phase of life, wherein catalytic biopolymers consisted exclusively of ribozymes. Later, RNA was replaced by the chemically more stable DNA. Bacteria were the only cellular form of life for more than 2 billion years. During this time, they evolved most of the biochemistry present in all forms of life and diversified into two domains, termed Bacteria and Archaea. The Panspermia Theory considers that once the earth cooled it was infected with heat-resistant organisms from other celestial bodies.