Is it alive?
crystals
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.
Image Credit: National Ignition Facility Programs
How to define "life" is a sweeping question that affects whole branches of biology, biochemistry, genetics, and ultimately the search for life elsewhere in the universe.
Comparing the semantic task to the ancient Hindu story of identifying an elephant by having each of six blind men touch only the tail, the trunk, or the leg, what answer a biologist might give can differ dramatically from the answer given by a theoretical physicist.
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.
Defining life takes on a more bewitching character when extended beyond the Earth's biosphere. The recent addition of extremophiles (archaea) to the tree of life underscores the notion that life is defined by what we know, what we have seen before, and often what we have succeeded in domesticating to a laboratory petri dish.
Astrobiology Magazine sought out expert opinion on this important question from Dr. Carol Cleland, who teaches philosophy at Colorado University in Boulder and is a member of NASA's Astrobiology Institute. While on sabbatical in Madrid, Spain, at the Centro de Astrobiologia (CSIC-INTA), she shared her thoughts on the power of definitions to shape science and philosophy.