How do we scientifically define Life?

     The Origin of Life (Abiogenesis) is the natural process through which life has arisen from non-living matter such as simple organic compounds (Oparin 1953, Warmflash 2005, Yarus 2010). We see life around us in so many forms it is only natural that this subject startles our curiosity. In fact, life is a subject of study for thousands of years, and historical tracers show it as early as 3000BCE in Ancient Egypt and Mesopotamia (Lindberg D.C, 2007, Grant E., 2007). Still, we distinguish between living and non-living not really based on a definition, but rather on the connection we as human beings have with Nature. 

     When we try to construct a definition of life by moving the concept from our senzorial plane to our cognitive plane, complications appear. Currently, there are no less than 123 definitions of Life, some are even contradicting, some are obsolete. For decades efforts have been made to define life, discussions and debates and opinions have not yet led to a unique universal definition of life. Trifonov (2011) has compiled a definition with the common terms of all the previous definitions: 

“Life is metabolising material informational system with ability of self-reproduction with changes (evolution), which requires energy and suitable environment”. 

Why is so difficult to define “Life”? Well, philosophers of science argue that “a definition must give both necessary and sufficient conditions, and must do that as a matter of the meaning of the term”, hence a definition is “supposed to be an analytic claim—true solely in virtue of the meaning of the terms involved” (Banner 2010).  In this sense, a definition for Life is almost impossible to find. 

    Therfore, scientists took a different approach and proposed defining Life through the theory it encompasses (Benner 2010). One such theory is that life cannot exist without metabolism, where metabolism is a collection of processes that enable free energy to be transformed into matter. Another theory of life states that metabolism must be based on reduced carbon species which proved to be not true because metabolism at terrestrial temperatures is permitted by the parts of biomolecules that include non-carbon atoms.  One of the oldest theories of life is the “cell theory” which states that all living things are systems build of cells. One of the most famous definition is the NASA definition based on the theory that life is a chemical system capable of Darwinian evolution no matter what kind of chemistry supports that evolution in the environments where that life came to be. As you can see, a theory of life based on the life we see on Earth but valid for the life we might observe in the Universe is not trivial at all.

    That is why, meanwhile the search for the definition of life still continues, scientists are guided by lists of attributes of the life that we know, which is the life on Earth. Based on the document “The Limits of Organic life in Planetary Systems”, the Astrobiology department at NASA has defined the following list of life attributes:

• It is chemical in essence; Terran living systems contain molecular species that undergo chemical transformations (metabolism) under the direction of molecules (enzyme catalysts) whose structures are inherited, and heritable information is itself carried by molecules.
• To have directed chemical transformations, Terran living systems exploit a thermodynamic disequilibrium.
• The biomolecules that Terran life uses to support metabolism, build structures, manage energy, and transfer information take advantage of the covalent bonding properties of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur and the ability of heteroatoms, primarily oxygen and nitrogen, to modulate the reactivity of hydrocarbons.
• Terran biomolecules interact with water to be soluble (or not) or to react (or not) in a way that confers fitness on a host organism. The biomolecules found in Terran life appear to have molecular structures that create properties specifically suited to the demands imposed by water.
• Living systems that have emerged on Earth have done so by a process of random variation in the structure of inherited biomolecules, on which was superimposed natural selection to achieve fitness. These are the central elements of the Darwinian paradigm.

We might not have a rigorous scientific definition of life, but that is in no way an impediment to study life on Earth and in other Worlds in this Universe, to enjoy Life and to protect it as the miracle that embodies us all…