| A | B |
|---|
| relative dating | estimate of the age of a fossil based on locations of fossils in strata |
| radiometric dating | technique that uses the natural decay rate of isotopes to calculate the age of material |
| isotope | form of an element that has the same number of protons but a different number of neutrons as another atom of the same element |
| half-life | amount of time it takes for half of the isotope in a sample to decay into its product isotope |
| index fossil | fossil of an organism that existed during only specific spans of geologic time across large geographic areas |
| geologic time scale | time scale representing the history of Earth |
| era | second largest unit of geologic time, lasting tens to hundreds of millions of years and consisting of two or more periods |
| period | unit of geologic time that lasts tens of millions of years and is associated with a particular type of rock system |
| epoch | smallest unit of geologic time, lasting several million years |
| nebula | rotating cloud of gas and dust |
| ribozyme | RNA molecule that can catalyze specific chemical reactions |
| cyanobacteria | bacteria that can carry out photosynthesis (singular: cyanobacterium) |
| endosymbiosis | ecological relationship in which one organism lives within the body of another |
| Paleiozoic | era of geologic time (from 544 to 248 million years ago) during which members of every major animal group alive today evolved |
| Cambrian explosion | earliest part of the Paleozoic era, when a huge diversity of animal species evolved |
| Mesozoic | era during which dinosaurs roamed Earth (from 248 to 65 million years ago) |
| Cenozoic | geologic time period that began 65 million years ago and continues today |
| primate | mammal with flexible hands and feet, forward-looking eyes, and enlarged brains relative to body size |
| prosimian | oldest primate group that includes mostly small, nocturnal primates such as lemurs |
| anthropoid | humanlike primate |
| hominid | primate that walks upright, has long lower limbs, thumbs that oppose the other four fingers, and a relatively large brain |
| bipedal | an animal using only two legs for walking |
