Scenarios for the origin of life Few problems in the history of science have raised such a tremendous interest and vivid controversial discussions as those related to the origin of life. Indeed, in this field modern science has gradually clarified the details of an extremely complex picture, still puzzling for both researchers and the great public, and refuelling centennial philosophical debates among the disciples of various theories. Astrophysics in connection to elementary particle physics present a novel image of the Universe, arising from a primordial break in symmetry some 15 billions of years ago, known as the Big Bang. This early hot ball of fire cooled gradually, condensing into nuclei and later into atoms. The long range interaction known as gravity led to the formation of galaxies of stars, where successive cycles of thermonuclear reactions led to accumulation of heavier elements indispensible for life. These elements were disposed in star dust by explosions of supernovae, triggered by the gravitational collapse when the primary nuclear fuel was insufficient to maintain a steady state, and recondensed in second order stars, like our own Sun, and in surrounding planetary systems. The timescale of events related to the appearance and evolution of life on Earth has been synthesized by Carl Sagan in a metaphoric view - the cosmic year. If all the events, starting with the Big Bang till now, would be compressed along the duration of a single year, some important landmarks would be placed as follows: - somewhere in April: appearance of our Galaxy, the Milky Way - September 9: formation of the Solar system - September 22: formation of Earth - beginning of October: the first assemblies of living matter - October 25: the first prokariotic cell (i.e. lacking a nucleus) - end of November: the first eukariotic cell - December 17: appearance of invertebrates - December 21: the first terrestrial animals - December 24: the dinosaurs ~ they survived only for 4 days - December 29: the first flower - December 31 at 10:30: the first human beings 23: the silex axe 23:59: the first signs of civilization in Europe - the last ten seconds: the written history of mankind second 53: the Trojan war second 56: the Roman Empire second 58: the Crusades second 59: the Renaissance In the Middle Age a valued theory of life was that of the vital force, a force that impregnates the living matter, in contrast to the non-living, resting matter; but the origin and properties of this subtle force remained misterious. Some speculative biologists (e.g. Spallanzani in Italy) built the theory of spontaneous generation: life appears spontaneously from non-living matter in appropriate conditions. The experiments proving such theories, like the appearance of microorganisms in resting water, were later correctly reinterpreted by Louis Pasteur as contaminations from the external environment. With the development of microscopy, a new theory, known as the cell theory, emerged. It was synthetized by Robert Hooke in the principle: Omni celula e celula. (Every cell derives from another cell). In the late 1800s, the cell theory was completed with the statement that the cell represents the basic structural and functional unit of living organisms, therefore defining the properties of a cell is equivalent to defining the properties of life. Also during the XIXth century, a fertile period for the development of natural sciences, the structure of chemical compounds within living organisms became gradually unveiled, and some of them were artificially synthesized in the laboratory. The first success in this field was the synthesis of ureea by Wöhler in 1827, marking the birth of organic chemistry. Relying on such type of knowledge, the famous biologist Ernest Haeckel pretended that the first precursors of life, the proteins, were formed by simple interactions of inorganic molecules in the primordial sea, in appropriate conditions. Today this phenomenon no longer occurs because the temperature and chemical environment have changed. A bright demonstration of this theory was produced one century later. Within an outstanding experiment, published in Science in 1953, the American chemist Stanley Miller proved that within a reaction bottle filled with methane and ammonia, exposed to high temperatures (150 °C), ultraviolet radiation and electric discharges, the elementary bricks of biologic macromolecules (aminoacids and nucleotides) form spontaneously. But the opponents of spontaneous generation of life were still not convinced, claiming that generation of the building bricks is one fact, and their intelligent combination in macromolecular self-replicating assemblies is a completely different issue, which could by no means occur by chance alone, reflecting the essence of divine creation. The microscopic study of fossils led during the first half of the XXth century to the theory of coacervates, sustained by Pflüger, Haldane, and Oparin. Coacervates are primitive living systems consisting in assemblies of macromolecules that perform redox reactions, using the chemical energy of various molecular substrates. In the years 1960, Fox showed that by dry heating of a mixture of aminoacids at ~ 150 °C, primitive proteinoid compounds with typical and atypical peptide bonds form spontaneously. These proteinoids in water form microspheres with catalytic activities (glucose decomposition, esterases, peroxidases), including synthetic activities: peptide synthesis from aminoacids and ATP, and polynucleotide syntesis. When deposited on lecithin membranes, these microspheres generated a transmembrane potential (-20 -70 mV) and transient phenomena resembling action potentials, which were until then attributed only to evolved specialized cells. More recent theories rely on the self-catalytic properties of nucleic acids. Thus it is conceivable that a primitive self-replicating RNA (called ribozyme) could accomplish both roles: genetic information carrier and enzymatic effector. However, spontaneous errors in replication impose severe restrictions on the maximal length of such a primordial macromolecule – below 100 nucleotides. Thus, before the coupling between nucleic acids and specialized proteins, that would detect and correct replication errors, transmission of a significant amount of genetic information would be prohibited. Modern theorists tried to depict, using computer simulations, the development of nucleic acid – protein coupling leading to the appearance of the genetic code, a universal feature of living beings on Earth. A well-known contribution is the hypercycle theory of Manfred Eigen: selfreproductive autocatalytic cycles that became gradually interconnected by dual autocatalytic links, with possibilities of specific cooperative interconnections. Proteins functioned at the beginning just as enzyme cofactors, but, with the development of the genetic code, ended by acting as the main catalytic effectors. Another famous theory is Stuart Kauffman’s autocatalytic set theory. It is based on mathematical graph theory and states that a sufficiently complex system of polymers, where the synthesis of chemical bonds is thermodynamically favorable and where at least a few of them have some more or less specific catalytic activity, will result in an increase in the maximum length of polymers. Thus many new polymers will arise, and even more catalytic links. From graph theory it follows that as the ratio between the elements of a system and their connections grows in favor of the latter, the system’s inner links become more and more interconnected via cycles. An Israeli biochemist of Romanian origin, Doron Lancet, shifts the point of emphasis from RNA or protein chemistry to phospholipid assemblies. In the socalled lipid scenario of life appearance, he claims that small phospholipid assemblies show the essential features of living beings: self-replication, compositional diversity and segregation via selection of a limited repertoire of monomers, achieving a kind of primitive “genetic” information. Fig. 1 shows two different outcomes of phospholipid micelles, according to the ratio between N, the average number of monomers within an assembly, and NG, the total number of monomer types (i.e. the repertoire). In large random-formed assemblies (N >> NG) each type of phospholipid is present in multiple copies, therefore all assemblies are virtually identical and selection is impossible. On the other hand, in small assemblies (N >> NG) there is a wide diversity, each assembly featuring a distinct composition (e.g., for an assembly of N = 100 molecules and a repertoire NG = 1000, there are 10100 possible assemblies). In these conditions, fragmentation (fission) leads to a loss of replication capability, the two fragments being very different. In order to reach an intermediate regime, where both diversity and fidelity of replication are ensured, small assemblies should undergo a process of molecular repertoire restriction. As shown in the insert of Fig. 1, this is possible by occurrence of organized mutual catalysis networks, according to Kauffman’s theory. Repertoire restriction is not possible at equilibrium (insert B), but far from equilibrium (insert C). Thus, such assemblies will reach a critical point, called the Morowitz barrier, where self-replication via fission occurs, leading to natural selection. Other authors stress the importance of weak electromagnetic fields in the development of living systems, recognizing their role in molecular recognition and orientation of biochemical reactions. Such stable microscopic electromagnetic fields, contributed by polar macromolecular compounds, biomembranes, etc., may have helped to the generation of a genetic code, offering a kind of primitive heredity, and may still be important for morphogenetic processes like development of the cytoskeleton or evolution of the embryo. Supplementary lectures Eigen M, Schuster P: The Hypercycle, Springer Verlag, Berlin, 1979. Fox SW, Nakashima T: The assembly and properties of protobiological structures, Biosystems 12:155-166, 1980. Jerman I: Electromagnetic origin of life, Electro- and magnetobiology 17:401-413, 1998. Kauffman SA: The Origins of Order, Oxford University Press, New York, 1993. Miller SL: Science 117:528-529, 1953. Miller SL, Orgel LE: The Origins of Life on the Earth, Prentice Hall, Englewood Cliffs, NJ, 1974. Morowitz HJ: Beginnings of Cellular Life, Yale University Press, London, UK, 1992. Nicolis G, Prigogine I: Self-organization in Nonequilibrium Systems – From Dissipative Structures to Order through Fluctuations, John Wiley & Sons, Toronto, Canada, 1977. Oparin AI, Gladilin KL: Evolution of self-assemblies of proteins, Biosystems 12:133-145, 1980. Segré D, Lancet D: Composing life, EMBO Reports 1:217-222, 2000.