| A | B |
|---|
| Geologic time scale | Division of Earth's history into time units based largely on the types of life forms that only lived during certain periods |
| Eon | Longest subdivision of geologic time scale, divided into eras periods and epochs |
| Era | Second longest division of geologic time, based on abundance of certain types of fossils |
| Period | Third longest division of geologic time, divided into epochs |
| Epoch | Smalle division of time after period, differences in life forms |
| Organic evolution | Change of organisms over geologic time |
| Species | Group of organisms that reproduces only with other members of their group |
| Natural selection | Organism better suited to a particular environment and are better able to survive and reproduce then other organisms |
| Trilobite | Organism with a three lobed exoskeleton that was abundant in Paleozoic oceans, is an index fossil |
| Pangaea | Large ancient land mass that was composed of all the continents joined together |
| Precambrian Time | Longest part of Earth's history, lasting from 4 billion to about 544 million years ago |
| Cyanobacteria | Chlorophyll containing photosynthetic bacteria thought to be one of Earth's earliest life forms |
| Paleozoic Era | Era of ancient life which began about 544 million years ago, when organisms developed hard parts, and ended with mass extinctions about 245 million years ago |
| Mesozoic Era | Middle era of earth's history, during which Pangaea broke apart, dinosaurs appeared, and reptiles and gymnosperms were the dominant land life-forms |
| Cenozoic Era | Era of recent life that began about 66 million years ago and continues today; includes appearance of homo sapient (humans) |
