Geologists organize Earth's past by developing a time line called a geologic time scale.

While developing the geologic time scale, scientists chose the time boundaries based on what they observed in Earth's rock layers.

Geologists use sudden changes in the fossil record to mark divisions in geologic time.

A mass extinction is the extinction of many species on Earth within a short period of time.

Since all organisms depend on the environment for thir survival, if the environment changes quickly, and if they are unable to adapt, they will die.

Some examples of things that can affect the environment are gas gas and dust from volcanoes.

An example of organisms dying because an abrupt change in the environment, is the dinosaurs. Most died because of the impact the meteorite had.

Land bridges connect two continents that were previously separated. Over times organisms move across land bridges and evolve as they adapt to new environments.

Geographic Isolation is the separation of a population of organisms from the rest of its species due to some physical barrier such as mountain ranges or an ocean.

Scientists predict that multicellular life first appeared in the Cambrian period.

The rare fossils of multicellular life-forms in Precambrian rocks are from soft-bodied organisms different from organisms on Earth today.

Many of these species became extinct at the end of the Precambrian.

The Cambrian Explosion refers to the sudden appearance of new, complex life-forms.