Origin of life
From Wikinfo
The scientific approach to the origin of life is also called "Chemical evolution" to distinguish it from religious or philosophical concepts. Chemical Evolution is the study of the processes which would have allowed the chemical elements which constitute the organisms (biogenic elements) to reach the degree of structural and functional organization characteristic of living matter. The fact that these processes require specific conditions, which can only occur in a few places of the universe, connects the study of the origin of life to that of astrobiology. Models proposed for the origin of life are attempts to recreate this evolution and it is important to emphasize that, in most steps of this process, there is no consensus among scientists whatsoever.
Contents |
Evolution of ideas
The heterotrophic (or classical) hypothesis.
The initial landmark in the modern scientific reasoning regarding the origin of life may be placed in Louis Pasteur's experiments, demonstrating that the formation of living organisms from inanimate matter (spontaneous generation), could not be, countering commonly held beliefs, a trivial phenomenon. From now on, scientists turned to the idea that research about the origin of life should focus the very special conditions required for this specific process, which would possibly only have prevailed in the beginnings of the history of planet Earth. One of the first approaches to the problem was the questioning about the probable nature of the first living entities. Survival in a primordial environment would either require that these primitive organisms be able to synthesize their own nutrients (i. e., they should be autotrophic), or that they had an external source of organic compounds (in which case they would be heterotrophic). The greater complexity of the autotrophic organisms, which must be able not only to consume but also to produce the materials from which they are made of, suggested to the scientists of the early 20s that the early Earth should have been an environment rich in organic compounds (Heterotrophic hypothesis). The discovery of the chemical composition of the atmospheres of the external planets, containing hydrogen, methane and ammonia, which are made up by the same elements as the organic compounds that constitute living beings, suggested to Oparin, one of the first researchers to address this issue, that our planet should have also had a similar atmosphere. It could therefore not contain molecular oxygen, for its presence would cause the decomposition of organic compounds by oxidation. A few years later, working independently, Haldane improved the model suggesting that, under the action of lightning or ultraviolet radiation, such compounds would be formed in the atmosphere and carried over by rainfall to the oceans, which would acquire the properties of a "hot thin soup". Another important theoretical contribution was Bernal's, who raised the question of the need for concentration of these components and suggested as important factors, the presence of evaporative surfaces such as coastal lagoons, and the absorption by mineral compounds in contact with the primordial oceans. He has likewise put forth the concept of biopoesis, a systematic approach deploying in progressive steps the development of the complexity of living beings. These formulations conquered a huge credence after Stanley Miller in 1953, working under the orientation of the Nobel prize winner Harold C Urey, succeeded to produce, in a historical experiment, a variety of amino acids. The real importance of this experiment is, nowadays, very disputed. Neither is the atmospheric composition employed held plausible under a geochemical standpoint, nor do the results obtained seem any close to throw light upon the succeeding steps in the history of life.
The role of clays.
Drawing on some of Bernal's ideas, Graham Cairns-Smith, developed the hypothesis that clay minerals would have been not only a support, but also the genetic system of primitive life, later to be supplanted by organic compounds (nucleic acids). His argument recalled the replication of crystalline surfaces, preserving failures and irregularities, and also the chemical complexity of the polymers involved in present-day reproductive processes.
Hydrothermal models.
The discovery of the Galápagos Trench (1979; Corliss, Baross, Hoffman), a rich ecosystem fed by compounds proceeding from hydrothermal activity, and therefore, independent from photosynthetic processes, was the basis for the hypothesis of an autotrophic chemosynthetic origin for life. Among the many variants of this proposal, Wächtershäuser provided the most elaborate, postulating a primitive stage of metabolism where biochemical processes would have been structured in a bidimensional pattern, over the surfaces of pyrite (FeS2), an abundant mineral in these environments.
Theoretical models.
Some conceptions about the origin of life tend to employ a generic or abstract approach. Instead of starting with the nature of the chemical constituents of a living system, they are oriented mainly by its functional properties. Among the most renowned of such models are the hypercycles, proposed by Manfred Eigen as prototypical primitive metabolic cycles. Some other proposals avoiding the specificity of postulating a primitive biochemistry are the "Garbage Bag World", put forward by physicist Freeman Dyson, endorsed by Robert Shapiro, and Stuart Kauffman's complexity model. According to Dyson vesicles containing random collections of chemical would compete for viability until one of them would bear every feature of a primitive living system. Accordingly he points to the possibility that metabolism and reproduction had a separate cellular genesis, and that modern organisms would stem from a symbiont of these two kinds of cells. Kauffmann claims, using purely mathematical models, that collections of chemical compounds exhibiting enough complexity could eventually "crystallize" metabolic cycles.
Metabolic models - The thioesther world.
The idea that present day metabolic processes can offer important clues to the comprehension of the biochemistry of the first organisms is the basis of a "metabolic" vision of the origin of life, championed by Harold Morowitz e Christian de Duve. Of special interest is De Duve's formulation of a more accurate model, postulating the congruence between primitive metabolism and today's biochemistry, where the central role of Acetyl-S-Coenzyme A in energetic metabolism would have been preceded by that of compounds derived from the estherification of carboxylic acids (RCOOH) with thiols (RSH), the thioesthers (RCOSR).
The RNA World.
The proposal of the RNA World put forward by Walter Gilbert in 1986, is based on the discovery that RNA molecules are able not only to store information (such as DNA in most present day living organisms), but also to promote metabolic reactions (like today's protein-based enzymes). Besides experimental evidence indicating a rich repertoire of catalytic activities and the replication and evolution capability of this material, there exist, within living organisms, a number of clues to this "RNA world". Among others, the chemical nature of enzymatic co-factors, structurally related to RNA monomers and the reproduction processes of many kinds of virus, held to be representative of primitive life forms. The most recent step of the origin of life, which is addressed by the RNA world hypothesis, is considered by most scientists to be the best known, and perhaps the only one where the domain of speculation has been clearly surpassed.
The Ecopoesis model.
The Ecopoesis model for the origin of life [1] (Félix de Sousa, R.A., 2000, 2006) proposes that the geochemical cycles of biogenic elements, driven by an early oxygen-rich atmosphere, were the basis of a planetary metabolism that preceded and conditioned the gradual evolution of organismal life. The idea that organisms, rather than shaping the environment, evolved their energy yielding processes along pre existing redox gradients, is consistent with growing evidence for an oxidizing atmosphere since an early stage of the Earth's history, and for the antiquity of aerobic metabolism, as compared to oxygenic photosynthesis.
References
- Biogenesis: Theories of Life's Origin by Noam Lahav
- Origin of life on Earth by Leslie E. Orgel
- Originally Adapted from the Wikipedia article, "Origin of life" December 4, 2003
