Life (cf. biota) is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate. Biology is the science concerned with the study of life.
Any contiguous living system is called an organism. Organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means. A diverse array of living organisms can be found in the biosphere of Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information.
Scientific evidence suggests that life began on Earth approximately 3.5 billion years ago. The mechanism by which life emerged is unknown and hypotheses are being formulated. Since then, life has evolved into a wide variety of forms, which biologists have classified into a hierarchy of taxa. Life can survive and thrive in a wide range of conditions. The meaning of life—its significance, origin, purpose, and ultimate fate—is a central concept and question in philosophy and religion. Both philosophy and religion have offered interpretations as to how life relates to existence and consciousness, and on related issues such as life stance, purpose, conception of a god or gods, a soul or an afterlife. Different cultures throughout history have had widely varying approaches to these issues.
Though the existence of life is only confirmed on Earth, many scientists believe extraterrestrial life is not only plausible, but probable. Other planets and moons in the Solar System have been examined for evidence of having once supported simple life, and projects such as SETI have attempted to detect transmissions from possible alien civilizations. According to the panspermia hypothesis, life on Earth may have originated from meteorites that spread organic molecules or simple life that first evolved elsewhere.
Read more about Life: Definitions, Origin, Conditions, Form and Function, Classification, Extraterrestrial Life, Death, Artificial Life
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... A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its ... In a medical context, the half-life may also describe the time that it takes for the concentration in blood plasma of a substance to reach one-half of its steady-state value (the "plasm ... For example, the biological half-life of water in a human being is about seven to 14 days, though this can be altered by his/her behavior ...
Famous quotes containing the word life:
“School divides life into two segments, which are increasingly of comparable length. As much as anything else, schooling implies custodial care for persons who are declared undesirable elsewhere by the simple fact that a school has been built to serve them.”
—Ivan Illich (b. 1926)
“O! the one Life within us and abroad,”
—Samuel Taylor Coleridge (1772–1834)
“Our life is a faint tracing on the surface of mystery, like the idle, curved tunnels of leaf miners on the face of a leaf. We must somehow take a wider view, look at the whole landscape, really see it, and describe what’s going on here. Then we can at least wail the right question into the swaddling band of darkness, or, if it comes to that, choir the proper praise.”
—Annie Dillard (b. 1945)