Evolution 3 Essay Research Paper EVOLUTION People

Evolution 3 Essay, Research Paper

EVOLUTION. People have always wondered how life originated and how so many different kinds of plants and animals arose. Stories of a supernatural creation of life developed among many peoples. The Bible, for example, tells of God’s creation of humans and other higher animals over several days. Many people also believed that insects, worms, and other lower creatures spontaneously generated from mud and decay. Long after these stories became rooted in tradition, scientists began to question them.

Theories of special creation usually hold that life retains its original God-created form; it is immutable, or unchangeable. By contrast, theories of organic evolution hold that all organisms, including humans, are mutable; that is, they respond dynamically over time to changes in the environment.

Although the theory of organic evolution is accepted by the overwhelming majority of the scientific community, this theory has aroused controversy since the middle of the 19th century. Most objections have come from religious groups that support special creation, or the theory they term creationism, or creation science. Individuals and organizations that support this concept defend the belief that all beings were created by God. Fundamentalist Christians and others feel that the premise that species are continually changing conflicts with literal interpretations of the Bible. They have fought, sometimes successfully, to prevent the use of biology textbooks that teach evolution; or they have insisted that creation science be presented as an alternative theory. (For an extensive discussion of human evolution, see Human Origins.)

The Origin of Life

The first serious attack on the idea of spontaneous generation of life was made in 1668 by Francesco Redi, an Italian physician, who proved that maggots did not arise spontaneously in decaying matter, as commonly believed, but from eggs deposited there by flies. Proof that microorganisms are not generated spontaneously came in the 1860s, when Louis Pasteur, a French scientist, showed that they, too, develop from preexisting life (see Pasteur, Louis).

Many theories have been developed to explain how life first originated on Earth. Some people theorize that microorganisms reached Earth from another planet. Most scientists discount this idea, called panspermia, because the radiation in space would kill cells or spores before they reached Earth. Instead, they believe that terrestrial life evolved from nonliving matter on the primitive Earth. The way in which life first formed may always remain a mystery, but the following explanation is likely. (See also Extraterrestrial Life.)

In the 1980s scientists discovered fossil remains of microorganisms resembling blue-green algae in rocks that were about 3 billion years old. Since Earth is thought to be about 4.5 billion years old, the first living things probably evolved within a billion years of Earth’s formation. In the 1920s Aleksandr Oparin, a Soviet biochemist, pointed out that the atmosphere of the primitive Earth was probably very different from today’s. A so-called reducing atmosphere–one with much more hydrogen than oxygen–probably existed then. It also probably contained methane, other hydrocarbons, water vapor, and ammonia. Oparin suggested that the organic compounds in the first organisms could have been formed by the action of sunlight and the heat from volcanoes or lightning on the reducing atmosphere of the primitive Earth.

Oparin’s theory can be supported by the fact that all living things are alike in some respects. For example, they all consist of cells that must have protein enzymes to catalyze, or speed up, the biochemical reactions of life (see Cell; Enzymes). Enzymes consist of about 20 kinds of amino acids, nitrogen-containing organic molecules found in the proteins of all organisms. Also, all cells transmit hereditary traits through nucleic acids (see Genetics).

In 1953 Stanley Miller, then a young graduate student at the University of Chicago, passed an electric spark through an experimental atmosphere containing the chemicals suggested by Oparin. A mixture of organic chemicals resulted, including several of the vital amino acids. Since Miller’s pioneering work, other scientists have produced many naturally occurring amino acids and nucleic acid components in the laboratory under prebiological conditions. Radio astronomers have discovered related molecules in dust clouds far from our solar system.

Scientists think that simple prebiological organic chemicals concentrated in lakes and tidal pools, forming a rich prebiological “soup.” Influenced by ultraviolet light and mild heating, the simple molecules then condensed into more complex ones resembling proteins and nucleic acids. This might have taken place on the surface of minerals or in oily colloidal droplets called coacervates floating in the prebiological soup (see Colloid).

The next stage in the formation of life is the vaguest. For life to persist after becoming established, there must have been polymers (long-stranded molecules) capable of replicating, or making copies of, themselves. Presumably these first self-replicating polymers were like nucleic acids. Once polymers could replicate, those best able to cope with their surroundings survived. After certain other basic adaptations were achieved, the most pri