Evolution reproduction and definition of life

Authors: Krzysztof Chodasewicz

Publish Date: 2013/05/15

Volume: 133, Issue: 1, Pages: 39-45

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Abstract

Synthetic theory of evolution is a superior integrative biological theory Therefore there is nothing surprising about the fact that multiple attempts of defining life are based on this theory One of them even has a status of NASA’s working definition According to this definition ‘life is a selfsustained chemical system capable of undergoing Darwinian evolution’ Luisi Orig Life Evol Bios 28613–622 1998 Cleland Chyba Orig Life Evol Bios 32387–393 2002 This definition is often considered as one of the more theoretically mature definitions of life This Darwinian definition has nonetheless provoked a lot of criticism One of the major arguments claims that this definition is wrong due to ‘mule’s problem’ Mules and other infertile hybrids despite being obviously living organisms in the light of this definition are considered inanimate objects It is strongly counterintuitive The aim of this article was to demonstrate that this reasoning is false In the later part of the text I also discuss some other arguments against the Darwinian approach to defining lifeDefining life is a nonstandard practice for biologists This is mainly due to the fact that biologists can make fruitful research without this definition Emmeche 1992 It is even true in case of objects having an unclear status such as viruses However this is unequivocally true only in case of ‘standard’ biology New biological fields of research such as astrobiology the origins of life artificial life and synthetic biology seem to be strongly dependent on the definition of life As suggested by Oliver and Perry some authors even believe that the lack of the definition of life is necessary for further progress in these disciplines 2006 Let us consider astrobiology if we want to project a space probe to search for life on other planets we have to assume some kind of life definition which allows us to differentiate living thingsWith regard to the origin of life an interesting analysis has been made lately by Addy Pross He argues that there are three major questions of modern biology what is life how can we make a simple living object and how did life emerge He also argues that these questions are strongly connected with each other We cannot answer one of these questions without making attempts to answer the remaining two questions Pross 2011 Pross’s analysis however could be extended if we consider that astrobiology is not only interested in the question how we can find extraterrestrial life but also in another fundamental problem where could life spontaneously emerge According to such an approach astrobiology is considered as strongly related to the research dealing with the origin of lifeAstrobiology however makes the problem complicated As we have no broadly accepted definition of life exploration of organic molecules and water is still the main way of searching for extraterrestrial life forms Chyba and Hand 2005 However astrobiologists often postulated that living beings could be built differently than their earthly counterparts SchulzeMakuch and Irwin 2006 This hypothesis has been recently extended to the socalled alternative forms of life which probably exist on our planet Carol Cleland and Shelley Copley claim that our methods of detecting and examining microbes are ‘blind’ when looking for different kinds of biochemistry So even on Earth we can expect organisms with different genetic codes using more amino acids than all the known forms of life or using amino acids with different chirality or maybe even more distinct from ‘standard’ life forms Cleland and Copley 2005 Observation of GFAJ1 bacteria from Mono Lake which are capable of using arsenic instead of phosphorus seems to be the first step to confirm this hypothesis WolfeSimon et al 2010 This case of course should be very carefully interpreted However if this assumption is true the question about artificial creation and the natural emergence of life can be more complicated and challenging The question whether it is possible to create a single theory of the origin of life is especially problematic How abstract can it be without the loss of the ability of explanation and prediction On the one hand we can create a definition that ‘captures’ all lifelike objects but it does not refer to the mechanisms of their emerging On the other hand we can construct a definition that implies a specific scenario of the origin of life but it is very restrictive Both solutions have their weaknessesAnother challenge of defining life is the artificial life further referred to as ALife The thesis that the form of living beings can be separated and implemented in a medium other than that made of organic compounds in other words the thesis about the multiple realization of life is broadly shared by theorists of strong ALife Emmeche 1992 Boden 2000 Contrary to astrobiology this claim is strongly supported by a lot of real artificial objects mainly computer simulations that manifest most of fundamental life’s properties such as reproduction evolution selfsustainability or purposefullike behavior The most popular ones are Christopher Langton’s Loops Thomas Ray’s Tierra and Craig Reynolds’ Boids Swan 2009 The thesis about the multiple realization of life makes all attempts to define living phenomena based on physicochemistry very complicated if not impossible If we accept it we have to deny the possibility of giving a single answer to the questions how can we “build” life and how can life spontaneously emerge On the other hand we can of course ignore ALife objects eg due to their different ontological nature But then we are faced with an uncomfortable dilemma as we must explain what the nature of differences and—more importantly—why we agree with the multiple realization hypothesis according to astrobiology denying at the same time multiple realization hypothesis according to ALife The concept of biological convergence also fully corresponds to the aforementioned thesis about the multiple realization of life indeed one of the multiple realization concept fathers—Jerry Fodor appealed exactly to this part of biological knowledge Block and Fodor 1972The above analysis demonstrates that there are a lot of problems connected with the definition of life and that one issue is especially important It is the conflict between the functional and the physicochemical approaches Some authors suppose that this state lack of consensus is normal and cannot be changed Edouard Machery claims that researchers from different branches of biology are interested in developing different definitions of life It results from the discrepancy between various disciplines Astrobiologists try to formulate a biochemical definition due to the practical facility of use but ALife aspires to a functional definition because it is an attempt for example to synthesize real life in a nonorganic medium 2011 Machery also believes that there is no instance of judgment which is able to choose between different definitions 2011 Similar skeptical arguments are proposed by Cleland and Chyba According to their view creation of a universal definition of life is impossible because we do not have any kind of mature biological theory The aforementioned authors compare our modern attempts of defining life to the attempts of defining water after molecular chemistry Cleland and Chyba 2002 So although we are able to generate a lot of definitions there is no reason to consider one of them as correctThe abovementioned reflections are surprising for many reasons First of all they seem to ignore the synthetic theory of evolution—the central fundamental and most integrative theory of modern biology The synthetic theory of evolution can be used at least in some degree to evaluate different propositions of life definitions Consequently there is no wonder that the synthetic theory of evolution has been often used to define life The most famous attempt is the socalled Darwinian or standard definition of life created by GF Joyce According to that definition ‘life is a selfsustaining chemical system capable to undergoing Darwinian evolution’ Luisi 1998 Cleland and Chyba 2002 RuizMirazo et al 2004 This definition is often said to be one of the most theoretically mature attempts to define life and has even reached the status of NASA’s working definition of life Luisi 1998 RuizMirazo et al 2004Before Joyce’s definition the evolutionary definition of life was put forward by many theorists including John Maynard Smith In the first chapter of his book The Problems of Biology he analyzed the conditions of evolution According to the author there are three fundamental prerequisites of evolution variability reproduction and inheritance A set of elements in other words a population of individuals will be changed in time evolve if its elements share these three properties and under the condition that at least a part of variability influences the chance of their survival and reproduction Maynard Smith 1986 In other words a living thing is an object which belongs to a set of elements characterized by variability reproduction and inheritance I propose to call this kind of definition a purely evolutionary definition There are many other examples of evolutionary definitions One of them is created by Kepa RuizMirazo Juli Peretó and Alvaro Moreno and it defines a living being as an ‘autonomous system with openended evolutionary capacities’ 2004 However according to this view evolution can be interpreted more widely than in accordance with the standard or the purely evolutionary definition With some objections also the cybernetic definition of life by Bernard Korzeniewski should be considered as one of the evolutionary definitions’ family The author of this conception reinterprets standard evolutionary requirements for life in cybernetic terms Korzeniewski 2001 2005 However some of his conclusions are contrary to the genetic interpretation of evolution selfish gene hypothesis Korzeniewski 2005 Notwithstanding this fact it seems to be a very original conception with great emphasis on formal and methodological aspects of life’s definition

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