Lifebiological idea that has no simple definition. Something is often said to be alive if at least once during its existence it:
- Takes in food, uses the food for energy, and passes waste products. See Metabolism.
- Moves, either moving itself, or has movement inside of itself
- Reproduces, by creating more-or-less exact copies of itself
- Reacts to conditions in its surroundings
- fire is alive
- mules are not (cannot reproduce)
- viruses are not (cannot grow)
The study of life is called biology and people who study life are biologists.
Currently (2003), the Earth is the only planet in the Universe known by humans to support life. The question of whether life exists elsewhere in the Universe remains open. There have been a number of false alarms of life elsewhere in the Universe, but none of these apparent discoveries have so far been confirmed.
Currently the closest that scientists have gotten to finding life outside of Earth is fossil evidence of possible bacterial life on Mars.
All life on Earth is based on the chemistry of carbon compounds. Some say that this must be so for all possible forms of life throughout the Universe; others describe this position as "carbon chauvinism".
Most successful animal of the earth in terms of total weight: Antarctic krill, Euphausia superba, with a biomass probably over 500 million tonnes.
Lifespan is the length of life in each species.
Death is the end of life in a living system, or in a part of it.
Life insurances, including pensions and life annuities, provide payments depending on life or death of a particular person. Because of this, documents that may be required for payment are:
- a life certificate stating that a person was alive at the date of issue;
- a death certificate stating that a person died on a particular date.
See also: Meaning of life, Vitalism, Materialism, Artificial life, Value of life, Afterlife