Life: A definition

[Zeno]

Decent article here on definition of life: Life


The salient points are:

"However, some initial agreement is possible. Living things tend to be complex and highly organized. They have the ability to take in energy from the environment and transform it for growth and reproduction. Organisms tend toward homeostasis: an equilibrium of parameters that define their internal environment. Living creatures respond, and their stimulation fosters a reaction-like motion, recoil, and in advanced forms, learning. Life is reproductive, as some kind of copying is needed for evolution to take hold through a population's mutation and natural selection. To grow and develop, living creatures need foremost to be consumers, since growth includes changing biomass, creating new individuals, and the shedding of waste.

To qualify as a living thing, a creature must meet some variation for all these criteria. For example, a crystal can grow, reach equilibrium, and even move in response to stimuli, but lacks what commonly would be thought of as a biological nervous system.

While a "bright line" definition is needed, the borderline cases give life's definition a distinctly gray and fuzzy quality. In hopes of restricting the working definition at least terrestrially, all known organisms seem to share a carbon-based chemistry, depend on water, and leave behind fossils with carbon or sulfur isotopes that point to present or past metabolism.

If these tendencies make for a rich set of characteristics, they have been criticized as ignoring the history of life itself. Terrestrially, life is classified among four biological families: archaea, bacteria, eukaryotes, and viruses. Archaea are the recently defined branch that often survives in extreme environments as single cells, and they share traits with both bacteria and eukaryotes. Bacteria, often referred to as prokaryotes, generally lack chlorophyll (except for cyanobacteria) and a cell nucleus, and they ferment and respire to produce energy. The eukaryotes include all organisms whose cells have a nucleus - so humans and all other animals are eukaryotes, as are plants, protists, and fungi. The final grouping includes the viruses, which don't have cells at all, but fragments of DNA and RNA that parasitically reproduce when they infect a compatible host cell. These classifications clarify the grand puzzle of existing life, but do little to provide a final definition. "

Another link with a reasonable definition:

Ask a Scientist

Of course asking a scientist is anathema to some people.



-Zeno, Scientist and a sentient terrestrial life form.