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.
Famous quotes containing the word life:
“The spring is here, young and beautiful as ever, and absolutely shocking in its display of reckless maternity; but the Judas tree will bloom for you on the Bosphorus if you get there in time. No one ever loved the dog-wood and Judas tree as I have done, and it is my one crown of life to be sure that I am going to take them with me to heaven to enjoy real happiness with the Virgin and them.”
—Henry Brooks Adams (18381918)
“Human life consists in mutual service. No grief, pain, misfortune, or broken heart, is excuse for cutting off ones life while any power of service remains. But when all usefulness is over, when one is assured of an unavoidable and imminent death, it is the simplest of human rights to choose a quick and easy death in place of a slow and horrible one.”
—Charlotte Perkins Gilman (18601935)
“Sometimes I think of life as a process where everybody is discouraging and taking everybody else down a peg or two.”
—Brenda Ueland (18911985)