Definition of life

What are the differences between life and matter This question appears very simple, however it is not. A philosophical review on that question is given by Gayon, 

Hanstmeier, Water in the Universe, Astrophysics and Space Science Library 368, DOI 10.1007/978-90-481-9984-6 2, Springer Science+Business Media B.V. 2011 

2010 [138]. Aristotle defined life as animation, life can be also defined as mechanism or organization (Kant). Until very recently, viruses were not considered in discussions on the origin and definition of life. This situation is rapidly changing, and it has been recognized that viruses have played (and still play) a major innovative role in the evolution of cellular organisms (Forterre, 2010 [134]). Life scientists and chemists have not come to a conclusive definition of life. There are several characteristics for life as we know it from Earth  

Response to stimuli a major characteristic of all living things. Plant responses to stimuli are generally much slower than those of animals. 

Metabolism collective product of all the biochemical reactions taking place within an organism. New cytoplasm is produced, damage repaired and normal cells are maintained. Metabolism includes photosynthesis, respiration, digestion and assimilation. All these include complex chemical processes. 

Displaying no obvious hints as to the truly extraordinary behavioral complexity which dramatically emerges from it, the Game-of-Life is deceptively simple to state. Calling cells with value cr = 1, alive, and those with cr = 0, dead, the following defiuition makes obvious Life s relation to real population growth and decay. 

DEATH isolated living cells with no more than one live neighbor die  

SURVIVAL living cells with 2 or 3 neighbors survive 

More formally, Life is is an outer-totalistic (code OT224) k = 2 rule defined on 

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According to Luisi, a definition of life must satisfy the following criteria ... 

The definitions of life which have been formulated in the NASA Exobiology Program as general working definitions are as follows ... 

The search for life in the cosmos requires a generalised, universal definition of life. This must take into account the properties of systems ranging from viruses, prions, denucleated cells or endospores to life in a test tube, computer viruses or even to robots which are capable of self-replication. 

Results from philosophical considerations on language show that attempts to define life lead to a dilemma, similar to that which occurred when trying to define water before molecular theory existed. Since no analogous theory of the nature of living systems exists, an infinite controversy as to the definition of life is unavoidable (Cleland and Chyba, 2002). 

The definitions of life are extremely controversial . So begins a publication on the problem of the definition of life which appeared as late as 2004. This publication is written by three Spanish scientists from the Centre for Astrobiology (INTA/CSIC) in Madrid, the University of Valencia and the University of the Basque Country in San Sebastian (Ruiz-Mirazo et al., 2004). Their general definition of life introduces two new terms into the discussion autonomy and open-ended evolution capacities . 

Fig. 1.5 Schematic representation of the evolution of life from its precursors, on the basis of the definition of life given by the authors. If bioenergetic mechanisms have developed via autonomous systems, the thermodynamic basis for the beginning of the archiving of information, and thus for a one-polymer world such as the RNA world , has been set up. Several models for this transition have been discussed. This phase of development is possibly the starting point for the process of Darwinian evolution (with reproduction, variation and heredity), but still without any separation between genotype and phenotype. According to the authors definition, life begins in exactly that moment when the genetic code comes into play, i.e., in the transition from a one-polymer world to a two-polymer world . The last phase, open-ended evolution, then follows. After Ruiz-Mirazo et al. (2004)...

Popa R (2004) Between Necessity and Probability Searching for the Definition of Life, Springer, Berlin Heidelberg New York, p. 184 Shimizu M (1982) J Mol Evol 18 297 Spiegelman S (1967) American Scientist 55 221... 

The question is how primitive a cell can be in order for it to still fulfil the needs of the definitions of the term life . As already discussed (see Sect. 1.4), the definition of life is notoriously difficult (Luisi, 1998) and sometimes leads to misunderstandings in the community of biogeneticists. 

The formation of life and how to recognise it is the central problem in astrobiology. All searches for other life forms will be prejudiced horribly by the study of the only example we know to date - life on Earth. Despite attempts to classify the general roles of molecules in definitions of life as biomarkers, there have been many discussions of possible biomarkers, including ... 

The definition of life in Greek is pneuma means air. This air means to breathe. But 3.8 billion years ago there was no air to breath. In Prasna Upanishad prana is deva or God and it says, I alone, dividing myself fivefold, support this body and keep it . 

Another difficulty in attempting to give a definition of life is that in fact the term definition is too ambitious, too frightening. Probably the term description would be more acceptable. In the language of epistemology, there is the distinction between an intrinsic description, meaning a context-independent description based on first principles and an operational description. As Primas says in a different context (Primas, 1998) ... 

Actually, most of the definitions of life given in the literature comply to the above operational description. In the following pages, the term dehnition is used mostly as a way of habit, meant however in the above epistemological context. 

However, I still think - despite these intrinsic difficulties - that it is important to debate the question of the detiitition of life, both from an intellectual and a practical point of view. Everyone working in the held of the origin of life should be able to provide their own definition (or description) of life, simply because they work experimentally or theoretically on models of minimal life and they should state and define the subject of the inquiry and the final aim of the work. This corresponds already to a kind of definition of life. Considering the number of authors in the held, this may correspond to quite a large number of different research projects, but all should adhere to the same basic constraints. What are these ... 

Firstly, I believe that any of the above descriptions of minimal life should permit one to discriminate between the living and the non-living. All forms of life we empirically know about should be covered by such a dehnition - and conversely one should not be able to hnd forms of life that are contradictory to such a definition. Secondly, there is the intellectual challenge to capture in an explicit formulation the quality of life how can one express the common denominator of micro-organisms, plants, animals, mushrooms, and mammals which set them apart from the inanimate world of rocks and machines Clearly, even if we do not arrive at an unique definition of life, the two above conditions are capable of fostering useful discussion and progress in the field. 

After these preliminary considerations, we can look at a few definitions of life given in the literature. For a taste of them, the reader may refer to those mentioned in the monographs by Folsome (1979), Chyba and McDonald (1995), or in a book edited by the late Martino Rizzotti (Rizzotti, 1996 Popa, 2004). 

The oldest definitions of life, such as those given by Engels, Bernal, and Rolle, referred to what we have called life at the individual level. With the advent of molecular biology and the emergence of nucleic acids, genetics also became important when considering the definition of life. 

This section has indeed given a complex scenario of the question of the definition of life. The picture may appear confusing, and just for this reason one may ask whether or not it is not possible to go hack to base, and look for one definition of life that would he simple and at the same time general enough to include individual specimens and contain the Darwinian view as a particular case. 

In order to address these questions on the definition of life first at the individual level, let us use a well known metaphor, utilized for example by Oparin (1953) and by Monod (1971) in another context. It goes like this suppose than that an intelligent creature from a very distant solar system - a Green Man - comes to visit Earth in order to investigate what life is on our planet. He has has a long list of terrestrial things about which he is in doubt as to whether they are alive or not. He... 

Approaches to the definition of life Table 2.1. The game of the two lists... 

Figure 2.1 The Green Man s sketch of the living he schematizes a system composed by only one tissue S S decays into a P, but due to the internal activity of the system, the nutrient A is converted into the tissue S again. This pictorii representation corresponds to a definition of life that reads a physical system can be said to be living if it is able to transform external matter/energy into an internal process of self-maintenance and self-generation.

Do you believe in the utility attempting to give a definition of life If not, how would you answer the following questions ... 

Also, Varela asserts that to take reproduction into the definition of life would be ontologically wrong (Varela, 2(X)0), as... 

This brings us to the notion of the minimal cell, defined as that having the minimal and sufficient number of components to be called alive. What does alive mean Well, here we should go back to Chapter 2 and the various definitions of life however, a fairly general definition can be used here, which ought to keep everybody satisfied living at the cellular level means the concomitance of three properties self-maintenance (metabolism), self-reproduction, and evolvability (see Figure 11.2). 

About various definitions of life. Orig. Life Evol. Biosph., 28, 613-22. 

A short discussion of the minimal definition of life, on Earth and elsewhere. 

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