Life is the characteristic state of organisms and individual cells. Properties common to the known organisms found on Earth (plants, animals, fungi, protists, archaea and bacteria) are that they are carbon-and-water-based, are cellular with complex organization, undergo metabolism, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt in succeeding generations.
An entity with the above properties is considered to be an organism. However, not every definition of life considers all of these properties to be essential. For example, the capacity for descent with modification is often taken as the only essential property of life. This definition notably includes viruses, which do not qualify under narrower definitions as they are acellular and do not metabolise. Broader definitions of life may also include theoretical non-carbon-based life and other alternative biology.
Although it cannot be pinpointed exactly, evidence suggests that life on Earth has existed for about 3.7 billion years. Currently the entire Earth contains about 75 billion tons of biomass (life), which lives within various environments within the biosphere.
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