Geologic Time Scale Geologic time is divided into units based on the types of life-forms living during certain periods. The major (or biggest) divisions in geologic time are eons. We don’t know a lot about the first 90% of Earth’s history because not many fossils formed and most rocks and fossils were destroyed by erosion, heat, and pressure.

Organic Evolution Birds evolved from dinosaurs. Reptiles were able to develop from amphibians because they laid an egg that had a thick skin so it could be laid in more places. Modern birds are related to Archaeopteryx because they have a common ancestor.

Trilobites Trilobites were excellent index fossils because they were widespread and lived for short periods of time. Trilobites can be used to study the passage of geologic time because their features have changed over time. For example, trilobites with no eyes were best adapted to life in deep water where light could not penetrate. Their skeletons changed because of competition, geographic isolation, and changing environments.

Cyanobacteria changed the atmosphere into one with oxygen. This helped create ozone, which protected complex organisms from ultraviolet radiation, so they could develop. Ediacarean organisms first developed during this time. Precambrian Time

Paleozoic Era At the end of the Paleozoic Era mass extinctions occurred as Pangaea formed and climates changed. During the Paleozoic era organisms with hard parts developed, unlike during Precambrian time when life was mainly soft-bodied. Reptiles first developed during the Paleozoic era.

Mesozoic Era Birds and mammals both appeared during this era. Dinosaurs also evolved during this time. Most life was marine (ocean) based, amphibians, and reptiles.

Cenozoic Era Dinosaurs went extinct and mammals and birds became a lot more common. Angiosperms became the dominant plant because their seeds let them reproduce in a wide variety of habitats. Humans first appeared during this era. Humans probably made some large mammal species go extinct by overhunting them (which changed the environment they lived in).

Cenozoic Era, cont. Two main Periods Quaternary – Most recent, has two main epochs. Pleistocene Holocene (Humans appear) Tertiary – When mammals and angiosperms became dominant. Several Epochs Paleocene Pliocene Miocene Oligocene Eocene

Plate Tectonics Plate tectonics cause environments to change, so animals and plants have to adapt or die out. In the Mesozoic era, plate tectonics caused Pangaea to separate. Reptiles adapted and became the dominant animal life (dinosaurs) of the era. The movement of plates also changes the earth’s surface and climate to change.

A final timeline First, cyanobacteria evolved. Then Ediacaran fauna (animals) disappeared. Dinosaurs became the dominant life form. Birds developed. Humans appeared.