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
Famous quotes containing the word life:
“After I discovered the real life of mothers bore little resemblance to the plot outlined in most of the books and articles I’d read, I started relying on the expert advice of other mothers—especially those with sons a few years older than mine. This great body of knowledge is essentially an oral history, because anyone engaged in motherhood on a daily basis has no time to write an advice book about it.”
—Mary Kay Blakely (20th century)
“Shams and delusions are esteemed for soundest truths, while reality is fabulous. If men would steadily observe realities only, and not allow themselves to be deluded, life ... would be like a fairy tale and the Arabian Nights’ Entertainments.”
—Henry David Thoreau (1817–1862)
“Above all, we cannot afford not to live in the present. He is blessed over all mortals who loses no moment of the passing life in remembering the past.”
—Henry David Thoreau (1817–1862)