Chapter 18 - The Origin of Life

Abstract

This chapter discusses the various theories related to the origin of life on earth. The belief in spontaneous generation of large plants and animals began to wane throughout the 17th and 18th centuries. According to some scientists, as no one has yet created life in the laboratory, this means that life cannot be created but must come from existing life. Thus if life can only originate from life, then life on Earth must have originated in outer space and come to Earth on meteorites in the form of cosmozoa, microbes, spores, or seeds. This theory is called panspermia. This leads to another assumption that life arose from lifeless matter in the earth. According to Harold Urey, who had been studying the atmosphere of Jupiter, the atmosphere of the early Earth, like that of Jupiter's, may have been reducing, and thus may have consisted largely of hydrogen, methane, ammonia, and water. Experiments conducted by Stanley Miller, in which a gaseous mixture of methane, ammonia, hydrogen, and water was connected to a flask of boiling water, resulted in the production of glycine and alanine. This result indicated that amino acids may have been present on the early Earth before the advent of life. Under prebiotic conditions, amino acids can polymerize into polypeptides without the aid of enzymes or a template. Even more complex structures like proteinoid microspheres can form under prebiotic conditions. Such proteinoid microspheres may have joined together with phospholipids, which can also be synthesized under prebiotic conditions to form the first plasma membranes in a process of self-assembly.