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Geologic Time Geologic Time Scale • Earth’s history is divided into time units based on the life-forms that lived during certain periods. • When life-forms are not present then divisions of the geologic time scale are based on other criteria. Major Subdivisions of Geologic Time • Eons-The longest subdivisions of geologic time. Based upon the abundance of certain fossils. • Eras- The subdivision marked by major striking, and worldwide changes in the types of fossils present. • Periods-Units of geologic time characterized by the types of life existing worldwide at the time. • Epochs-Units of geologic time characterized by differences in life-forms, but some of these differences can vary from continent to continent. Epochs • Cenozoic epochs have special names • Epochs of other periods usually are referred to simply as early, middle and late. Organic Evolution • Fossils show that species of organisms have changed over time. This change is known as organic evolution. • Organisms that are not adapted to changes in the environment will die out. The organisms that have changed and survived will cause the species to change. Species • A group of organisms that reproduce with other organisms within their same group. • Occasionally organisms of different species will mate and reproduce, however the offspring are usually sterile. Mules are an example of mating between a horse and a donkey. Mules are sterile and cannot reproduce. • Organisms of different species sometimes look alike but do not mate and reproduce fertile offspring. Natural Selection • Charles Darwin is often referred to as the father of natural selection. • Natural selection is the process by which organisms with characteristics that are suited to a certain environment have a better chance of surviving and reproducing than organisms that do not have these characteristics. Natural Selection • During the industrial revolution two varieties of moth were present a light colored moth and a dark colored moth. Because of the coal burned in factories there was a dark coating over the exterior of buildings, trees etc. The light colored moths were eaten more than the dark because they were easier for birds to see. Therefore more of the dark moths lived and reproduced changing the color of the species. Artificial Selection • Humans crossing animals and plants in order to produce offspring with the most desired characteristics. Trilobite • An invertebrate that has an exoskeleton and a three lobed body, also a head (cephalon), a segmented middle section (thorax), and a tail (pygidium). • Different kinds of trilobites lived during different periods of time. • Their eyes show the result of natural selection. Eyes located toward the front of the organism indicates a swimmer, eyes on top of the organism indicate a bottom dweller. Trilobites cont. • The bodies and tails also changed over time to adapt to changing environments. • Olenellus is thought to be a very primitve trilobite because it has many body segments. • Organisms that could not adapt to the changing environment became extinct. Plate Tectonics and Earth History • One explanation for the extinction of the trilobite is the colliding and separating of continents many times. In the collision of continents mountains were formed and seas were closed. The formation of Pangea a giant landmass, the supercontinent, changed the environment in which trilobites lived. This is one theory that explains the extinction at the end of the Paleozoic Era.