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
Other articles related to "life":
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Famous quotes containing the word life:
“You seem to have no real purpose in life and won’t realize at the age of twenty-two that for a man life means work, and hard work if you mean to succeed.”
—Jennie Jerome Churchill (1854–1921)
“Had Dr. Johnson written his own life, in conformity with the opinion which he has given, that every man’s life may be best written by himself; had he employed in the preservation of his own history, that clearness of narration and elegance of language in which he has embalmed so many eminent persons, the world would probably have had the most perfect example of biography that was ever exhibited.”
—James Boswell (1740–95)
“In soliciting donations from his flock, a preacher may promise eternal life in a celestial city whose streets are paved with gold, and that’s none of the law’s business. But if he promises an annual free stay in a luxury hotel on Earth, he’d better have the rooms available.”
—Unknown. Charlotte Observer (October 6, 1989)