Defining Moment of Life

What Is Life?

If one is to name the longest running riddle, this one must rank high. All humans pass through life once, cherish it and are always fascinated by it. This fascination is precious common ground for humanity, cutting across all barriers of time, geography and language. For prebiotic evolution, solving the riddle is sheer necessity, for it is impossible to know when prebiotic ends and biotic begins without knowing what biotic means. Some definitions of life go back many decades:63

FG Hopkins in 1913—A minimum requirement for life is a "dynamic equilibrium in a polyphasic system."

JBS Haldane in 1952—a simple organism such as a bacterial virus contains about 100 bits of negative entropy or information and this is about the amount that would arise spontaneously in 109 years in the volume of the primitive ocean.

NW Pirie in 1957—"I argued twenty years ago that a rigid operational definition is not possible. This seems now generally to be accepted."

The 2002 collection of Palyi et al64 contains a wealth of definitions, e.g.,

GustafArrhenius—The basic ingredients are self-organization, self-replication, evolution through mutation, metabolism and concentrative encapsulation.

Andre Brack—Self-reproduction, mutation and evolution.

David Brin—Energy flows downhill and order, information and manipulative ability rise steeply inside local bundles of space-time.

David Deamer—Semi-permeable boundary structure, a system of polymeric catalysts and a system of polymeric instructions.

Christian de Duve—"Life is what is common to all living things."

Klaus Dose—Membrane, metabolism, control ofmetabolism, replication and mutability making possible evolution.

Ricardo Guerrero and Lynn Margulis—Matter that makes choices, binds time and breaks gradients.

Romeu Cardoso Guimaraes—Metabolism, growth and reproduction with stability.

Robert Hazen—Metabolism and reproduction with variation, concept of a sequence of discrete emergent steps.

Gerald Joyce—Gives the NASA Working Definition of"LIFE is a self-sustained chemical system capable of undergoing Darwinian evolution" in which self-sustenance is supported by genetically instructed metabolism.

Vladimir Kompanichenko—Organized form of intensified resistance to self-propagating processes of destruction.

Stanley Miller—The origin of life is the origin of evolution. Darwinian evolution requires replication, mutation, selection.

Eors Szathmary—Gives Tibor Ganti's definition based on metabolic network, template macromolecule and encapsulating membrane, all of which are autocatalytic.

Hubert Yockey—Having a genome and a genetic code.

There is deep-seated diversity in the definitions, but also general agreement that life is to be defined based on a combination of constituent attributes rather than any single attribute. The reason is, few of the individual attributes are really unique to life. For instance, self-reproduction/replication is observed with salts crystallizing from supersaturated solutions seeded with a few crystals, there are many geochemical cycles on Earth that are even more ancient than metabolic cycles, all soap bubbles have membranes and evolvability is displayed by ribozymes undergoing in vitro evolution. After all, if evolvability were an exclusive property of the living state, no prebiotic molecular assemblage could have evolved to generate the living state.

Besides having to employ a set of independent attributes for the definition of life based on the actual components of the first living cell, the need to pinpoint the source of the essence or new property that is endowed with purposefulness and unique to life is suggested by:

David Abel—"The more we can distil the essence of 'life', the greater the hope of elucidating the lost pathways of abiogenesis."

Hans Kuhn—"Physical objects come into being that behave as if they have apurpose...Let us call physical objects with this fundamentally new property (not present in any ancestralform in a prebiotic universe) living."

Furthermore, the challenge of addressing the defining moment when nonlife becomes life is posed by:

Janet Siefert—"One can begin to postulate on the defining moment or conditions for 'life' that arose here on Earth...I posit that life can be defined as the culmination and the eventual simultaneous occurrence" of the four events of replication, translation, control and cell wall.

This definition calls attention to the importance of protein catalysts produced from translation, but it might be too restrictive in precluding any possibility of the translation-less ribo-organisms of RNA World being living creatures. Translation cannot be an independent attribute. Instead, in order for natural selection to be effective, the phenotypes expressed through the catalysts must be determined unambiguously by the genotypes embodied in the replicators, which may choose to act as catalysts themselves as in the case of ribozymes, or to arrange the production of surrogate catalysts as in one-gene-one-enzyme coding. Candidate replicators

Figure 1.5. The defining moment of life, with thioesters (triangles) and ATP (circles) (Section 9.6) energizing the first cell division. The arrows depict coordination.

that know how to replicate but cannot contribute catalysis, e.g., replicating salt crystals, will remain irrelevant to life. Therefore the replicative function in life's emergence on Earth or elsewhere has to be fulfilled by replicators that are inherently not only mutable in order to make evolution possible, but also catalysis-competent by virtue of either their own catalytic ability (e.g., ribozymes), or that of catalysts specified by them through translation (e.g., enzymes). In view of this, translation in Siefert's simultaneous occurrence might be replaced by metabolism, in accord with the definitions of Arrhenius, Dose, Ganti and others.

Membranes, metabolism and replication, three constituent components that have come together from independent prebiotic origins, make the cell functional because, and only because, their partnership is a fully integrated one. The integration, painstakingly built with an evolved tool kit of catalysis, genetic encoding, encapsulation, REIM, REAS, energy coupling, transport gradients, signal transduction etc., opens up an organizational state ofmolecules with a higher level of complexity than pathways and autocatalytic cycles. The capability of the integrated whole so radically exceeds the individual capabilities of the three component parts that the towering excess amounts to a new property, an essence of life, undreamed of in the nonliving world. The extreme adaptability of this organizational state, acquired through heredity, mutability and evolution, equips it with the purpose-oriented behavior characteristic of the living. In time, this impressive adaptability would permeate from the cellular to the multicellular, neural and societal levels. On this basis, the defining moment of life is the moment when successful integration is signaled. Since self-reproduction is fundamental to life and prerequisite to evolution, it makes a suitable signal. Therefore the defining moment of life may be identified as follows:

Life is the integration of membranes, metabolism and mutable, catalysis-competent replication into a single entity.

The defining moment oflife is the moment ofsuccessful integration signaled by the self-reproduction of such an entity.

Accordingly, the birth of the living cell took place at its first division. This historic event is shown in Figure 1.5.

References

Further Readings

Highly readable books to provide an appreciation of the basic issues of prebiotic evolution and the diversity of scientific approaches to the subject:

1. De Duve C. Blueprint for a Cell. Neil Patterson Publishers, 1991.

2. Lahav N. Biogenesis. Theories of Life's Origin. Oxford University Press, 1999.

3. Luisi PL. Emergence of Life. From chemical origins to synthetic biology. Cambridge University Press, 2006.

4. Morowitz HJ. Beginnings of Cellular Life. Yale University Press, 1984.

5. Palyi G, Zucchi C, Caglioti L, eds. Fundamentals of Life. Elsevier, 2002.

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