| Term | Definition |
|---|
| trilobite | small hard-shelled organisms crawled on the seafloor and swam through water. Considered index fossil |
| Geologic Time Scale | division of Earth's history |
| eon | longest subdivisions of geologic time basd upon the abundance of fossils. |
| era | second longest subdivisions that are marked by major, striking, and worldwide change in types of fossils present. |
| period | units of geologic time characterized by types of life existing worldwide at the time |
| epoch | smaller units of time characterized by differences in life forms. |
| organic evolution | change through time |
| species | group of organisms that normally reproduces only with other members of their group |
| natural selection | proces by which organisms with characteristics that are suited to a certain environment have a better chance of surviving and reporducing than organisms that don't have the characteristics. |
| Pangaea | the continents came together and formed one large landmass |
| Precambrian time | longest part of the Earth's history and includes the Hadean, Archaean, and Proterozoic Eons. |
| cyanobacteria | blue-green algae thtought to be one of the earliest life forms on Earth. |
| Paleozoic era | an era of ancient life that began about 544 million years and ended about 248 million years ago. |
| Mesozoic era | the middle life when many changes on Earth happened. |
| Cenzoic era | an era of recent life that began about 65 million years ago and continues today |
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