The riddle of “life,” a biologist’s critical view

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    REVIEW

    The riddle oflife, a biologists critical view

    Heinz Penzlin

    Received: 5 March 2007 /Revised: 17 June 2008 /Accepted: 24 June 2008 / Published online: 2 September 2008# Springer-Verlag 2008

    Abstract To approach the question of what life is, we first

    have to state that life exists exclusively as the being-aliveof discrete spatio-temporal entities. The simplest unit thatcan legitimately be considered to be alive is an intact

    prokaryotic cell as a whole. In this review, I discusscritically various aspects of the nature and singularity ofliving beings from the biologists point of view. In spite ofthe enormous richness of forms and performances in the

    biotic realm, there is a considerable uniformity in thechemical machinery of life, which powers all organisms.Life represents a dynamic state; it is performance of asystem of singular kind: life-as-action approach. All life-as-things hypotheses are wrong from the beginning. Life is

    conditioned by certain substances but not defined by them.Living systems are endowed with a power to maintain theirinherent functional order (organization) permanentlyagainst disruptive influences. The term organization inher-ently involves the aspect of functionality, the teleonomic,

    purposeful cooperation of structural and functional ele-ments. Structures in turn require information for theirspecification, and information presupposes a source. Thissource is constituted in living systems by the nucleic acids.Organisms are unique in having a capacity to use, maintain,and replicate internal information, which yields the basisfor their specific organization in its perpetuation. The

    existence of a genome is a necessary condition for life and

    one of the absolute differences between living and non-livingmatter. Organization includes both what makes life possibleand what is determined by it. It is not something implantedinto the living beings but has its origin and capacity formaintenance within the system itself. It is the essence of life.The property of being alive we can consider as an emergent

    property of cells that corresponds to a certain level of self-maintained complex order or organization.

    Keywords Living state . Vital organization . Metabolism .

    Self-maintenance . Autonomy. Emergence

    Introduction

    The bondage of biology to the physical sciences haslasted more than half a century. It is now time for

    biology to take her right full place as an exactindependent science: to speak her own language andnot that of other sciences. Haldane (1922)

    Living matter exists in a wondrous array of emergentproducts, perhaps 10 to 20 million different species or evenmorenobody really knows the exact number. Life is so

    ubiquitous in our world, so evident that we fail to evenwonder at its existence. We take it for granted that a human

    being composed of an estimated 10 billion (1013!) tissuecells of about 350 different types and another 30 billion

    blood cells should develop from a relatively unstructuredseminated egg celland this, fortunately, with a remark-ably low rate of error. And nevertheless, embryogenesisdoubtlessly belongs to the most amazing and puzzling

    phenomena to be found in our natural world. What causesall the cells to differentiate in the way that they do so a

    Naturwissenschaften (2009) 96:123

    DOI 10.1007/s00114-008-0422-8

    H. PenzlinInstitut fr Allgemeine Zoologie und TierphysiologieFriedrich-Schiller-Universitt Jena,Erbertstrasse 1,07745 Jena, Germany

    H. Penzlin (*)Leo-Sachse-Strasse 10,07749 Jena, Germanye-mail: penzlin-jena@t-online.de

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    healthy child with a head, trunk, arms, and legs, with abrain, heart, and all the other organs comes into being andbegins to speak and to think? There is probably no one whobetter recognizes how remarkable life actually is than thedevelopmental biologist.

    Two hundred years ago, Treviranus, Lamarck, and othersintroduced the term biology to describe the science

    dealing with living beings. In the light of the multiple andheterogeneous questions it asks and approaches it takes, theterm life sciences is more usual nowadays. Strictlyspeaking, despite generations of trying, the subject matterof life sciences is as yet still undefined. My centralintention in this review therefore is not to try to definelife, which is probably a hopeless undertaking, but toattempt to work out what living entities are and todemonstrate that living beings are distinct from non-livingentities not only in their degree of complexity butfundamentally, in principle. Organisms have propertiesand facilities, which are unique to them and only to them.

    Life means being-alive of discrete entities:

    organisms

    Life exists exclusively as the being-alive of something

    To approach to the question of what life is, we first have tostate that life exists exclusively as the being-alive ofdiscrete entities, which we call living creatures ororganisms. No life exists outside and independently oforganisms; no independent agent makes inorganic matter

    alive. Life always means being alive. There is no entitylife, which we can make the object of our science.Therefore, biology is not the science of life but thescience of living entities in all their forms, aspects, andhierarchical levels. We can consider the living entities ashighly complex systems and life as the specific perfor-mance of these systems. This underlines the absolutenecessity of systems thinking in General Biology.

    The negation of a vital agent independent fromorganisms is not a trivial step by any means. It implicatesa fundamental point of view in the discussion of the

    problem of life. Many cultural groups, philosophies, and

    religions, for example, believed and believe in a vital agent,which also exists and acts outside of and independentlyfrom organisms. For Plato for instance, as already men-tioned, the soul is a distinct non-material entity, which

    bonds to certain objects and induces in them animatebehavior. However, daily experience teaches us that such aduality does not exist. Life existsas already mentionedonly as being-alive of highly complex, dynamic systemswith the fundamental property to autonomously maintainand replicate their internal organization. Each organism

    exists as a single, unique, spatiotemporally restricted entitywith a beginning and an end. Under present conditions onEarth, no organism comes into being spontaneously out ofnon-living matter. In every case, living entities derive fromliving entities: omne vivum e vivo (Lorenz Oken). Eachorganism has a unique, non-repeatable history. Becauseorganisms derive from other organisms (ancestors) and

    produce yet other organisms (descendents), a continuoussuccession of generations connects each present organismwith lifes origin on earth.

    Cells as elementary organisms

    Despite the tremendous diversity of living forms in ourworld, scientists in nineteenth century discovered that all

    present-day living organisms are made up of one, some,thousands, millions, or billions of cells. The entiremetabolism takes place in cells, and all cells are derivedfrom the division into two of previously existing cells:

    Omnis cellula e cellula (Rudolf Virchow). Life is cellactivityits uniqueness is the uniqueness of the cell.

    There are only two different types of cells (Fig. 1), the primitive prokaryotic cell (protocyte) of the archaea andbacteria and the essentially more complex eukaryotic cell(eucyte) of all the other organisms (protista, fungi, plants,and animals). The latter is about a thousand-fold morevoluminous and more complex than the protocyte. Nointermediate forms exist between these two types of cellsthat would guide a gradual evolutionary inference betweenthe prokaryotic and eukaryotic state. The protocyte has nomembrane-bounded organelles. Its genome consists in the

    minimum case of a single, double-stranded, closed loop ofDNA. In contrast, the eucyte contains organelles surrounded

    by double membranes, a nucleus with its contiguousendoplasmatic reticulum, a Golgi apparatus, and flagellawith a 9-+2-pattern of microtubule arrangement. During theinterphase, their hereditary material is concentrated in a setof complex chromosomes inside the cell nucleus.

    The whole cell is the most elementary unit that canmaintain life; it is the least complex thing that properlylives. When protozoan cells divide into two halves, onecontaining the nucleus and the other without, only the firstcan maintain life. A nucleus-less Amoeba is still able to eat

    and digest for some time. Later on, this capacity disappears,and the protozoan rejects the undigested food. Only cellfragments with an intact nucleus are able to regenerate thelost parts. The physiologist Ernst von Brcke characterizedthe cell as an elementary organism (Brcke 1851), andmany influential biologists (Walter Flemming 1882, E. B.Wilson 1907, Frederick Gowland Hopkins 1913, andothers) agree with him that life should be considered asthe activityor ensemble of activitiesof whole cells andnothing less. This conclusion expresses on the cellular level

    2 Naturwissenschaften (2009) 96:123

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    the holistic concept once formulated by Kant for theorganism as a whole. Below the cellular level, noindependent life is possible. Although some organelles,such as mitochondria and chloroplasts, undergo replication,this requires the functioning of the integrated cell. In tryingto define the essence of the living state (not of organisms!)fundamental for all living entities, we may confineourselves to considering life at its cellular level. When thefirst multicellular organisms appeared, life had alreadyexisted approximately 2.8 Ga. About three quarters ofevolution up to now has taken place on a cellular level or,in other words, was cell evolution1.

    Viruses are the most abundant biological entities on ourplanet (Breitbart and Rohwer 2005). There are 10311032

    virus particles in the biosphere; this is at least one order ofmagnitude more than the number of host cells! Virus

    particles (virions) indeed contain genetic information (DNAor RNA), but they are incapable of growth or division bythemselves. They lack the machinery to generate energy orto synthesize their own proteins. They are obligate parasitesand depend on the host cells ribosomes to synthesize their

    own proteins. Therefore, extant viruses are not truly alive.They were generally considered to be fragments of livingentities that have lost their capacity for autonomousexistence, but we must own up to the fact that we reallyhave no sound knowledge of how the entire domain ofviruses is organized, what the origins of viruses are, andhow they evolve (Bamford et al. 2005). Recently, Koonin etal. (2006) proposed an alternative hypothesis to thescenarios that describe viruses originating as genes thathave escaped from cellular organisms. In their concept ofan ancient virus world, the principal lineages of virusesand related selfish agents emerged from a primordial pool

    of primitive genetic elements, the ancestors of both cellularand viral genes. In the opinion of these authors, theemergence of substantial genetic diversity antedates theadvent of fully fledged cells. To explain the crucial step ofcompartmentalization in the primordial pool, a highlyspeculative model has been elaborated (Koonin and Martin2005). The assumption of a non-cellular last universalcommon ancestor of the three domains Bacteria, Archaea,and Eukaryotes, as favoured in this scenario, remains atopic of controversial discussion (e.g. Gogarten and Taiz1992).

    Last vital units below the cell level?life-as-thingshypotheses

    The assumption that single substances are the primaryvehicles of life has a long tradition beginning with the pre-Socratic philosophers. In the Middle Ages and earlyRenaissance, life was sometimes identified as a fluidsubstance known as liquor vitae. Felix Dujardins living

    jelly orsarcode was later succeeded by the more generalterm protoplasm which was considered by many as the

    1

    This cell-as-elementary-organism theorem is not inconsistent withthe well-known fact that the totipotency of the zygote gets lost in thedescendants of the zygote during the ontogenesis of a multicellularorganism. The cells become determined and differentiate into specifictypes of cells. This differentiation usually results from the differentialexpression of genes in the cell, i.e., from the differential regulation oftranscription, posttranscriptional events, or translation but not from aloss of DNA or irreversible changes in the genome. It is onlyirreversible in certain types of cells. In many cases, differentiation isreversible under the right environmental circumstances. Transdiffer-entiation of one differentiated cell type into another type has beenshown to occur for instance during regeneration and in cells in tissueculture.

    Eukaryotic cell Prokaryotic cellFig. 1 Diagrams of thelarger and more complicatedeukaryotic (animal) cell (left)and the simpler prokaryotic cell(right). Only the eukaryotic cellshave a separate compartment(nucleus) that contains theirDNA (left figure after Storchand Welsch 2005, right figure

    after Kaplan 1972)

    Naturwissenschaften (2009) 96:123 3

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    material basis of life until well into the twentieth century.In the second half of the nineteenth century under theinfluence of the blossoming discipline of organic chemis-try, the notion oflast units of life below the cellular levelwas widespread among theoretical biologists: physiologicalentities (Spencer), lebendiges Eiwei (Pflger 1875),Bioblasts (O. Hertwig 1906), Biophors (Weismann

    1892), Protomers (Heidenhain 1894), or Biogens(Verworn 1903). All these life-as-things hypotheses baseon the effort to explain the specific properties and

    performances of living systems in terms of mysterious units,molecules, or aggregates of molecules.

    Nowadays, it is clear that no single molecule, no singlecomponent of the cell per se is alive. Chemists have learnedto synthesize any protein or any nucleotide sequence but, indoing so, have never created life. The birth of life onearth did not coincide with the first appearance of a certain

    protein molecule as the physicist Pascual Jordan believed.Neither is it true that life began with the appearance of the

    first nucleic acid strand in the primeval soup, which was ableto replicate and to mutate and thus became the subject ofselection, as Kuhn and Waser (1982) once stated. If that werethe case, scientists would have succeeded in making artificiallife in a test tube many times over. We have to accept thatlife is the performance of complex internally organizedsystems and as such necessarily began as a minimal integralmultimolecular system. Translation and replication areconsequences of the total functioning of the whole.

    The role and importance of DNA is frequently exagger-ated. It is often uncritically elevated to...