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Life (jīvita) is the ability of the organisms that have it to respond to stimuli, grow, reproduce, absorb nutrition, excrete and persist over time. The two kinds of living things are plants and animals. In the case of most animals, a newly embodied life begins at conception or soon afterwards. The Buddha said that for conception to take place three things must coincide – the sexual union of the parents (sannipatita), the mother’s ovulation (utunī) and the presence of the gandhabba, i.e. the consciousness of the being who is to be reborn (M.I,266). This consciousness absorbs itself in the fertilised egg and begins to animate it so that it grows into a fully formed being. As with modern medicine, the Buddha recognized that the embryo passes through four stages: fertilization (kalaka), cleavage (abbuda), gastrulation (pesi) and organogenesis (ghana) before developing recognizably human features at around 17 weeks, after which it is known as the foetus (S.I,206). The Buddha said that because all beings cherish their life above everything else and struggle to avoid death at all costs (Dhp.130), to destroy the life of another being is the worst thing one can do to them. Thus respect for life (avihiṃsā, Sanskrit ahiṃsā),promoting life, improving the quality of life – is the highest Buddhist ethical ideal.
Life 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 on Earth is unknown although many hypotheses have been 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 confirmed on Earth only, many scientists think that extraterrestrial life is not only plausible, but probable or even inevitable. 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, microscopic life exists throughout the Universe, and is distributed by meteoroids, asteroids and planetoids.