The Appearance of Life
The oldest known evidence of life on Earth consists of the 3.5-billion-year-old fossils of primitive, plantlike organisms called cyanobacteria. Sometimes called blue-green algae, cyanobacteria are simple, one-celled organisms that are related to bacteria. The organic compounds that combined to produce Earth's first life forms, however, are certainly much older than these primitive organisms—perhaps as old as 4.2 billion years.
Scientists have developed two main theories explaining the origin of life—the theory of chemical evolution and the theory of panspermia. The more widely accepted theory of chemical evolution was developed independently during the 1920's by Soviet biochemist Alexander I. Oparin and by British biologist J. B. S. Haldane. Oparin and Haldane theorized that because hydrogen is the most abundant element in the universe, Earth's early atmosphere had large quantities of this gas. Under such conditions, the hydrogen-containing compounds ammonia, formaldehyde, hydrogen cyanide, methane, and water would also have been abundant. According to this theory, sunlight, lightning, and volcanoes powered reactions among these compounds that produced simple biological molecules, such as sugars and amino acids.
Two American chemists, Stanley L. Miller and Harold C. Urey, in 1953 provided the first experimental evidence in support of the theory of chemical evolution. They subjected a mixture of ammonia, hydrogen, methane, and water to the energy of high-voltage sparks for one week. After that time, amino acids and other simple biochemical compounds had formed. Scientists have repeated this experiment under various conditions. For example, some researchers have assumed that the early atmosphere contained little hydrogen but large quantities of carbon dioxide. They have replaced the hydrogen-rich “atmosphere” of the Miller-Urey experiment with various mixtures high in carbon dioxide and relatively low in hydrogen. These mixtures also have yielded biochemical compounds when exposed to sparks of energy. Other organic compounds on Early Earth may have come from carbon-rich meteorites known as carbonaceous chondrites. Scientists have found simple organic compound in several of this type of meteorite.
Most scientific research supports—or at least does not contradict—the idea that life arose through chemical evolution. For example, the surface of Earth experiences a continuous flow of energy as it gets light from the sun and radiates heat into outer space. Physics research has demonstrated that such an energy flow increases molecular organization. Thus, the evolution of complex biochemical molecules may be viewed as part of this natural process.
