| A | B |
|---|
| cast | when a mold becomes filled in with hardened minerals |
| relative dating | a way to determine which of two fossils is older |
| absolute dating | a technique that allows scientists to determine the actual age of fossils |
| radioactive element | unstable elements that decay,or break down, into different elements |
| half-life | the time it takes fo half of the atoms in a sample to decay |
| fossil record | the millions of fossils that scientists have collected |
| extinct | when no members of species are still alive |
| gradualism | the theory that evolution occurs slowly over very long periods of time |
| punctuated equilibria | the theory that says species evolve during short periods of rapid change |
| homologous structure | similar structures that related species have inherited from a common ancestor |
| branching tree | a diagram that shows how scientists think different groups of organisms are related |
