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A newborn baby is an example of life

Life is easy to recognise, but hard to define.[1] The study of life is called biology and people who study life are biologists. A lifespan is the average length of life in a species.

All known life on Earth is powered by solar energy. Without energy from the sun no life could exist. Life is based on the chemistry of carbon compounds. In particular, all life uses long-chain molecules such as proteins and nucleic acid. With water, which is essential, the long molecules are wrapped inside membranes to form cells. This may or may not be true of all possible forms of life in the Universe: it is true of all life on Earth today.


Living organisms are open systems. They are always changing, because they exchange materials and information with their environment. They undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli and reproduce.

Through natural selection, they adapt to their environment in successive generations. More complex living organisms can communicate through various means.[1][2] Many life forms can be found on Earth. 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.

At present, the Earth is the only planet we have detailed information about. The question of whether life exists elsewhere in the Universe is open. There have been a number of claims of life elsewhere in the Universe. None of these have been confirmed so far. The best evidence of life outside of Earth is fossil evidence of possible bacterial life on Mars.

Definitions of life

One explanation of life is called the cell theory. The cell theory has three basic points: All living things are made up of cells. The cell is the smallest living thing that can do all the things needed for life. All cells must come from pre-existing cells.

Something is often said to be alive if it:

However, not all living things fit every point on this list.

  • mules cannot reproduce, and neither can worker ants
  • viruses and spores are not actively alive (metabolising) until the conditions are right.

They do, however, fit the biochemical definitions: they are made of the same kind of chemicals.

The thermodynamic definition of life is any system which can keep its entropy levels below maximum (usually through adaptation and mutations).

A modern approach

A modern definition was given by Humberto Maturana and Francisco Varela in 1980,[3] to which they gave the name autopoiesis:

  1. The production of their own components
  2. The correct assembly of these components
  3. Continuous repair and maintenance of their own existence.

Roth commented that "In short, organisms are self-reproducing and self-maintaining, or 'autopoietic', systems".[4] This approach makes use of molecular biology ideas and systems science ideas.

Gallery of images of life

Related pages


  1. 1.0 1.1 Koshland, Daniel E. Jr 2002. "The seven pillars of life". Science 295 (5563): 2215–2216. doi:10.1126/science.1068489 . PMID 11910092 . Retrieved 2009-05-25.
  2. "organism". Chambers 21st Century Dictionary (online). (1999). 
  3. Maturana H. & Varela F. 1980. Autopoiesis and cognition: the realization of the living. Boston: Reidel.
  4. Roth G. 2013. The long evolution of brains and minds. Heidelberg: Springer. p41, 48