Term | Definition |
Relative dating | A method of determining whether an event or object, such as a fossil, is older or younger than other events or objects without referring to the object's age in years |
Radiometric dating | A method of determining the absolute age of an object, often by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope |
Isotope | An atom that has the same number of protons (or the same atomic number) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass) |
Half-life | The time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope |
Index fossil | A fossil that is used to establish the age of a rock layer because the fossil is distinct, abundant, and widespread and the species that formed that fossil existed for only a short span of geologic time |
Geological time scale | The standard method used to divide Earth's long natural history into manageable parts |
Era | A unit of geologic time that includes two or more periods |
Period | In chemistry, a horizontal row of elements in the periodic table |
Epoch | Smallest unit of geologic time, lasting several million years |
Nebula | A large cloud of gas and dust in interstellar space; a region in space where stars are born |
Ribozyme | A type of RNA that can act as an enzyme |
Cyanobacteria | Bacteria that can carry out photosynthesis |
Endosymbiosis | A mutually beneficial relationship in which one organism lives within another |
Paleozoic | 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 million years ago to 65 million years ago) |
Cenozoic | Geologic time period that began 65 million years ago and continues today |
Primate | A member of the order Primates, the group of mammals that includes humans, apes, monkeys, and prosimians; typically distinguished by a highly developed brain, forward-directed eyes and binocular vision, opposable thumbs, and varied locomotion |
Prosimian | Oldest primate group that includes mostly small, nocturnal primates such as lemurs |
Anthropoid | Humanlike primate |
Hominid | A member of the family Hominidae of the order Primates; characterized by bipedalism, relatively long lower limbs, and lack of a tail; examples include humans and their ancestors |
Bipedal | Animal that walks on two legs |