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Glossary: Life

All of the following terms appear in this unit. The terms are arranged here in alphabetical order.
adaptation — The capacity of living organisms to change from generation to generation, becoming better suited to their environments.
adaptive radiation — The rapid evolution of many new species that possess adaptations that allow them to fill previously empty ecological roles, or niches.
bacteria — Very simple unicellular, asexual, and prokaryotic organisms. The first forms of life were most likely similar to bacteria.
biodiversity — The variety of life forms in a habitat, whether that habitat is a local environment or an entire planet.
biology— The scientific study of living things.
biosphere — The entire network of life on Earth; the region of Earth in which living organisms can be found.
brain — A cluster of special nerve cells focused on coordinating the activities and vital functions of many different cells and cell groups (or organs).
Cambrian explosion — A time in the history of life during the Cambrian period (roughly 542 to 488 million years ago) in which many large fossils appeared and animal life developed an astonishing diversity of structural forms.
dinosaurs — Terrestrial vertebrates that first appeared roughly 230 million years ago and dominated the Earth from the beginning of the Jurassic period (about 200 million years ago) until the end of the Cretaceous (66 million years ago)—over 130 million years! Dinosaurs were an extremely diverse group of egg-laying reptiles, some as small as pigeons and others up to 28 meters in length. Fossil evidence suggests that dinosaurs gave rise to modern birds.
DNA (deoxyribonucleic acid) — The double-stranded molecule, present in all living cells, that contains the genetic information used to form and maintain the cell and passes that information to offspring cells.
eukaryotes — Cells more complex than prokaryotes, with distinct membrane- bound organelles (such as mitochondria) and a nucleus that protects the cell’s genetic material. Many single-celled organisms are eukaryotic, as are most multicellular ones.
evolution — Change over time. Applied most frequently to the development of living organisms according to the principles of natural selection, as identified by Charles Darwin in the nineteenth century.
extinction event — A specific time in the Earth’s history (or future) when very large numbers of species die off. These events may occur abruptly or over a longer period of time. In the last 600 million years, there have been five events in which over half of all animal species died.
fossil fuel — A carbon- based material such as coal, oil, or natural gas that can be used as an energy source. Fossil fuels were originally formed when the remains of living organisms were buried and broken down by intense heat and pressure over millions of years.
fossils — The preserved remains of organisms from the distant past. Fossils are usually mineralized or hardened remains of the organisms themselves, but can also include traces of an organism’s behavior (for example, footprints) that have been preserved.
gene — A segment of DNA which codes for the production of a specific protein. Genes dictate a particular sequence of amino acids, which when assembled, make up the protein.
homeostasis — The capacity of living organisms or cells to regulate internal conditions (for example, temperature) in order to maintain a stable state.
iridium — A dense chemical element with atomic number 77 that is more abundant in meteorites than on Earth; its presence at the K-T boundary offered an important clue to what caused the extinction of the dinosaurs.
K-T boundary — A layer of clay in the Earth’s geologic rock record between the Cretaceous period and  the Tertiary period. Unusually high levels of the element iridium at the K-T boundary provided scientists with an important clue to how the dinosaurs became extinct. (The K-T boundary is now often called the K-Pg boundary due to a change in how geologists name time periods.)
last universal common ancestor (LUCA) — The most recent organism from which all organisms now living on Earth descended; thought to date to about 3.8 billion years ago.
life — Four commonly accepted attributes of life are that it uses energy from the environment by eating or breathing or photosynthesizing (metabolism); it makes copies of itself (reproduction); over many generations it can change characteristics to adapt to its changing environment (adaptation); and it can regulate internal conditions in order to maintain a stable state (homeostasis).
mammals — Warm-blooded, hairy vertebrates that grow their young inside the bodies of females, and feed their young with milk from mammary glands.
marsupials — A group of mammals whose young are born in an undeveloped state and then develop and nurse in a maternal pouch.
metabolism — The capacity of living organisms to store, consume, and utilize energy through chemical reactions within cells. This can involve everything from creating nutrients (for example, through photosynthesis), to constructing components of cells, to breaking molecules apart to produce energy.
multicelled (multicellular) organism — An organism consisting of more than one cell. The possession of more than one cell allows for the specialization of certain cells, which enables these cells to perform specific vital functions.
natural selection — The process by which certain inherited traits become more common in a population because they improve an organism’s chances to survive and reproduce, passing along the traits to the next generation, while other traits become less common because they decrease an organism’s chances to survive and reproduce.
niche — An organism’s role within its ecosystem; a set of traits or behaviors employed by an organism within its environment to extract food, avoid predation, and reproduce. The description of an organism’s niche may include its habitat, its place in the food chain, and at what time of the day or night it is active. No two species can occupy the same niche in the same environment for a long period of time.
organism — An individual living thing.
photosynthesis — The conversion of light energy to chemical energy, which is stored in sugars or other organic compounds, and is performed by plants, algae, and a few other organisms. The first evidence of photosynthesis is from about 3.5 billion years ago. Photosynthesis supplies most of the energy necessary for life within the biosphere and is the source of most atmospheric oxygen, which is released as a byproduct of photosynthesis.
prokaryotes — Simple, single-celled organisms, including bacteria, which do not have distinct membrane- bound organelles, and in which genetic material is not bound by a nucleus. Life arose on Earth nearly 4 billion years ago; for roughly the first 2 billion of these years, all living things were prokaryotes.
proteins — Large biological molecules composed of long chains of amino acids. Proteins perform numerous functions necessary for life, such as speeding up chemical reactions in cells (enzymes), fighting foreign bodies within an organism (antibodies), providing structural support (structural proteins), and transporting matter within and between cells (motor proteins).
reproduction — The capacity of living organisms to create copies of themselves, some of which vary slightly, leading to natural selection and evolution.
RNA (ribonucleic acid) — Similar to DNA, but a single strand with slightly different chemistry, this molecule helps to carry out the instructions for protein synthesis specified by the DNA molecule.
species — The most specific category in biological classification; organisms are considered to be of the same species if they can interbreed in nature and produce viable, fertile offspring.

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