Summary
Many types of evidence support the fact of evolution. A rich record of fossils demonstrates that life began simply and increased in complexity over time. The older a rock, the more its fossils are likely to differ from modern forms, because the vast majority of life forms have become extinct. Biochemical evidence also supports the fact of evolution. Not only do all forms of life employ the same biochemical mechanism for translating DNA into proteins, but many of those proteins are similar in very different species. Comparison of structural details reveals that closely related species, like humans and chimpanzees, have nearly identical proteins, while those of more distantly related animals show more protein differences. Vestigial organs, such as the human appendix, provide yet another piece of evidence in the evolution story.
Life on Earth evolved in two stages. The first period of chemical evolution was characterized by the gradual buildup of organic chemicals by the gradual buildup of organic chemicals in the primitive oceans. The Miller-Urey experiment showed that simple compounds, including water, methane, ammonia, and hydrogen, subjected to electrical sparks or some other energy source, combine to make the building blocks of life—amino acids, lipids and other molecules. Through a sequence of events not yet well understood, a primitive but complex self-replicating chemical system developed. All subsequent life evolved from that first cell.
The first cell, free from competitors, quickly multiplied in the nutrient–filled oceans. As oceans became crowed and competition for resources increased, a phase of evolution—natural selection—began. The theory of natural selection, introduced by Charles Darwin in his 1859 monograph Origin of Species, this is based on two facts¦every species exhibits variations in traits, and some traits enhance an individual's ability to survive and produce offspring. Just as breeders develop new varieties of animals by selects traits through the struggle for survival. These are called adaptations. In the way, over immense spans of time, new species arise. Geological time is divided into four eras¦Proterozoic, Paleozoic, Mesozoic, and Cenozoic. Theses eras depend on the kind of fossils found from the period when the rocks formed. While extinction is a continuos process, there was a number of catastrophic, episodes of mass extinction, when most species disappeared in a brief interval. Asteroid impacts many account for some of these events.
Human evolution is traced back approximately 4.5 million years ago to Australopithicus, a hominid that walked erect but had a brain about the size of a chimpanzee's. Our knowledge of Australopithicus, is to a great degree on the partial skeleton of a female, known as Lucy. Homo habilis, the first member of our genus, that appeared about 2 million years ago, was distinguished by a larger brain and the first appearance of stone tools. Homo erectus, who learned to use fire, evolved at about the same time but disappeared about 1.5 million years ago. Modern Humans of the species Homo sapiens are in fossils as old as 200,000 years. The status of the so–called Neanderthal man is still under debate¦some say Neanderthal is a separate species, now extinct, while others argue that it is merely a subspecies of Homo sapiens.
