| A | B |
|---|
| geologic time scale | record of events in Earth History |
| era | largest subdivision of geologic time |
| periods | subdivisions of eras on the geologic time scale, based on life-forms and geologic events |
| epochs | subdivisions of periods on the geologic time scale |
| organic evolution | change in the hereditary features of a species over a long period of time |
| species | group of individuals that normally breed only among themselves |
| natural selection | process by which the best-suited individuals survive in their environment |
| endangered | a species in which only a relatively small number of members exist |
| habitat | a place where organisms live and grow |
| Precambrian time | geologic time with poorest fossil record |
| cyanobacteria | blue-green bacteria thought to be one of the earliest life-forms on Earth |
| Paleozoic Era | the second-oldest division of geologic time; began about 544 million years ago, when organisms developed hard parts |
| amphibian | vertebrate animals that reathe air and live on land but must return to water to reproduce |
| reptile | evolved from the same ancestors as amphibians |
| Mesozoic Era | the middle era of the Earth's geologic history; began 245 million years ago; reptiles and gymnosperms were the dominant land life-forms |
| Cenozoic Era | the geologic era in which we live |
| radioactive decay | the breakdown of radioactive elements, releasing particles of energy |
