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.
Scientific evidence suggests that life began on Earth at least 3.5 billion years ago. The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. The mechanism by which life emerged on Earth is unknown, although many hypotheses have been formulated.
The planet Earth
Though the existence of life is confirmed only on the planet Earth, 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 radio transmissions from possible alien civilizations.
According to the panspermia hypothesis, microscopic life exists throughout the Universe, and is distributed by meteoroids, asteroids and planetoids. 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.
Theories of life
Some of the earliest theories of life were materialist, holding that all that exists is matter, and that life is merely a complex form or arrangement of matter. Empedocles (430 BC) argued that every thing in the universe is made up of a combination of four eternal “elements” or “roots of all”: earth, water, air, and fire. All change is explained by the arrangement and rearrangement of these four elements.
The various forms of life are caused by an appropriate mixture of elements. Democritus (460 BC) thought that the essential characteristic of life is having a soul (psyche). Like other ancient writers, he was attempting to explain what makes something a living thing. His explanation was that fiery atoms make a soul in exactly the same way atoms and void account for any other thing.