Earth Science, 10^th edition

Chapter 11: Earth’s History

 

 

    I. Precambrian era

       A. 4.5 billion to 540 million years ago

       B. 88% of Earth's history

       C. Earth's atmosphere

             1. Primitive atmosphere formed from volcanic gases

                  a. A process called outgassing

                  b. Water vapor, carbon dioxide, nitrogen, and several trace gases

                  c. Very little free oxygen

             2. Water vapor condenses and forms primitive oceans as Earth cools

             3. Bacteria evolve

             4. Plants evolve and photosynthesis produces oxygen

             5. Oxygen content in the atmosphere increases

             6. By about 4 billion years after Earth formed, abundant ocean-dwelling organisms

                  that require oxygen existed

       D. Precambrian fossils

             1. Most common are stromatolites

             2. Microfossils of bacteria and algae have been found in chert

             3. Plant fossils date from the middle Precambrian

             4. Animal fossils date from the late Precambrian

             5. Diverse and multicelled organisms exist by the close of the Precambrian

  II. Paleozoic era

       A. 540 million years ago to about 248 million years ago

       B. First life forms with hard parts

       C. Abundant Paleozoic fossils

       D. Early Paleozoic life

             1. Restricted to seas (invertebrates)

             2. Vertebrates had not yet evolved

        F. Late Paleozoic history

             1. Supercontinent of Pangaea forms

             2. World's climate becomes very seasonal, causing the dramatic extinction of many species

       G. Late Paleozoic life

             1. Organisms diversified dramatically

             2. Land plants

             3. Fishes evolve into two groups of bony fish

             4. Insects invade the land

             5. Amphibians diversify rapidly

             6. Extensive coal swamps develop

III. Mesozoic era

       A. 248 million years ago to about 65 million years ago

       B. Often called the "age of dinosaurs"

       D. Mesozoic life

             1. Reptiles (first true terrestrial animals) readily adapt to the dry Mesozoic climate

             2. Dinosaurs dominate

             3. One group of reptiles led to the birds

             4. Many reptile groups, along with many other animal groups, become extinct

                  at the close of the Mesozoic

IV. Cenozoic era

       A. 65 million years ago to the present

       B. Often called the "age of mammals"

       C. Smaller fraction of geologic time than either the Paleozoic or the Mesozoic

       D. Cenozoic life

             1. Mammals replace reptiles as the dominant land animals

             2. Angiosperms (flowering plants with covered seeds) dominate the plant world

                  a. Strongly influenced the evolution of both birds and mammals