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adaptive radiation

Period of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill vacant ecological roles in their communities.

amino acid

An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of polypeptides.


A member of a group of shelled cephalopods that were important marine predators for hundreds of millions of years until their extinction at the end of the Cretaceous period (65.5 mya).

Cambrian explosion

A relatively brief time in geologic history when large, hard-bodied forms of animals with most of the major body plans known today appeared in the fossil record. This burst of evolutionary change occurred about 535-525 million years ago.


A process in which a unicellular organism (the "host") engulfs another cell, which lives within the host cell and ultimately becomes an organelle in the host cell; also refers to the hypothesis that mitochondria and plastids were formerly small prokaryotes that began living within larger cells.

geologic record

The division of Earth's history into time periods, grouped into three eons—Archaean, Proterozoic, and Phanerozoic—and further subdivided into eras, periods, and epochs.


The amount of time it takes for 50% of a sample of a radioactive isotope to decay.


Evolutionary change in the timing or rate of an organism's development.


Evolutionary change above the species level, including the origin of a new group of organisms or a shift in the broad pattern of evolutionary change over a long period of time. Examples of macroevolutionary change include the appearance of major new features of organisms and the impact of mass extinctions on the diversity of life and its subsequent recovery.


A mammal, such as a koala, kangaroo, or opossum, whose young complete their embryonic development inside a maternal pouch called the marsupium.

mass extinction

Period of time when global environmental changes lead to the elimination of a large number of species throughout Earth.


The retention in an adult organism of the juvenile features of its evolutionary ancestors.


The supercontinent that formed near the end of the Paleozoic era, when plate movements brought all the landmasses of Earth together.


The processes and functions of an organism and their study.


A collection of abiotically produced molecules surrounded by a membrane or membrane-like structure.

radioactive isotope

An isotope (an atomic form of a chemical element) that is unstable; the nucleus decays spontaneously, giving off detectable particles and energy.

radiometric dating

A method for determining the absolute ages of rocks and fossils, based on the half-life of radioactive isotopes.


An RNA molecule that functions as an enzyme, catalyzing reactions during RNA splicing.

serial endosymbiosis

A hypothesis for the origin of eukaryotes consisting of a sequence of endosymbiotic events in which mitochondria, chloroplasts, and perhaps other cellular structures were derived from small prokaryotes that had been engulfed by larger cells.


Layered rock that results from the activities of prokaryotes that bind thin films of sediment together.

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