Some theoretical aspects of the problem of life origin.
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Amino Acid Sequence↗
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The organization of cell populations within lymph nodes: their origin, life history and functional relationships.
The normal lymph node comprises a superficial cortex, a deep cortex or paracortex and a medulla. In each of these regions there are three kinds of spaces: an intralymphatic space, an intravascular space and an extravascular space or interstitium. Both the vascular endothelium and the lymphatic endothelium are specialized in these different regions. The cell types in lymph nodes comprise lymphoid cells, accessory or non-lymphoid cells and stromal cells, and within these cell types a number of different sub-types can now be identified by means of enzyme- and immunocytochemistry. Based predominantly on experimental studies, the origin, migratory patterns, localization, inter-relationships and interactions between these various cells are reviewed.
Animals↗
Arteriolosclerosis of the human renal allograft: morphology, origin, life history and relationship to cyclosporine therapy.
In the decade 1979-1988, 658 biopsies were collected from 568 cadaveric renal allografts. In 118 grafts a non-proliferative insudative vasculopathy (IVA) was found in afferent vessels. Immunosuppression was based on azathioprine (AZA) or on cyclosporin A (CsA), from 1983. The prevalence and extent of IVA has increased significantly since 1984. Light microscopy showed fibrinoid and hyaline masses of varying extent; transmural insudative "knobs", intimal oedema with metachromasia, and microthrombosis were also seen with CsA. The ultrastructure of the insudates was unremarkable but CsA grafts displayed early oedema and hypergranulation of endothelial cells with a disarray of smooth muscle cell (SMC) microfibrils, and pronounced degenerative changes of SMC. Rebiopsy showed stationary IVA in AZA grafts and progression in one-half of CsA-treated patients. Nephrectomy specimens revealed, however, a marked predominance of late rejection endarteritis; in only 3 cases was IVA and/or microthrombosis the possible cause of nephrectomy. The mean donor age was higher in severe IVA in CsA grafts and the mean post-transplantation interval at the time of diagnosis of IVA was significantly shorter in CsA-treated patients. No important differences in cumulative graft survival were seen between grafts with absent, moderate or severe IVA. Unused cadaveric donors' kidneys of comparable age exhibited normal arterioles or a slight focal insudative or hyaline lesion.
Arteries↗
The genetic code and the origin of life.
The problem of the origin of life understandably counts as one of the most exciting questions in the natural sciences, but in spite of almost endless speculation on this subject, it is still far from its final solution. The complexity of the functional correlation between recent nucleic acids and proteins can e.g. give rise to the assumption that the genetic code (and life) could not originate on the Earth. It was Portelli (1975) who published the hypothesis that the genetic code could not originate during the history of the Earth. In his opinion the recent genetic code represents the informational message transmitted by living systems of the previous cycle of the Universe. Here however, we defend the existence of a certain strategy in the syntheses of the genetic code during the history of the Earth. The strategy of correlation between amino acid and nucleotide polymers made an increasing velocity of the chemical evolution possible, that is, it increased the velocity of formation of the genetic code. Thus, life with the recent genetic code could originate on the Earth within the present cycle of the Universe.
Amino Acids↗
Evolutionary oscillation in prebiology: igneous activity and the origins of life.
The processes of chemical evolution are responsible for the origin of life. Three such processes have special importance: oscillation, creation, and competition. An oscillation from one kind of environment to another provides a mechanism for instituting processes that can only take place under conditions far removed from equilibrium. Oscillating evolutionary processes are likely to have played an important part in the origin of life. It is a mistake to assume that life originated in any one environment. It did not arrive in a moment of time. It was the result of a long period of chemical evolution during which it passed through a variety of environments. Biopoesis took place in an environment in which a variety of different kinds of protolife were assembled and concentrated. One essential form of protolife involved in these processes is the protocell. The experiments of Fox suggest that the creation of protocells involves violent oscillations of temperature and hydration. Igneous activity is especially characterised by oscillating conditions. Volcanic eruptions consist of violent changes from one extreme condition to another. Temperatures, pressure, phase, concentration and hydration all oscillate violently, and are subject to shock pulses of many kinds. Protolife may well have passed through extremes of environment for wider that those that life itself can sustain. The most probable environment for the assembly of the various forms of protolife would be on mudbanks forming either at the mouth of streams draining regions of active vulcanicity, or round the edge of hot volclanic pools. In this situation one could fins concentrated not only the various stands of protolife necessary for the final act of biopoesis, but also perbiologically formed nutrients necessary as for the first eobionts. As soon as the first protocells start to grow, they start to compete with each other, and so initiate a new additional evolutionary process, that of natural selection. Only after such competition has been initiated is life itself likely to be established"20
Biological Evolution↗
Group selection of early replicators and the origin of life.
A major problem of the origin of life has been that of information integration. As Eigen (1971) has shown, a mutant distribution of RNAs replicating without the aid of a replicase cannot integrate sufficient information for the functioning of a higher-level unit utilizing several types of encoded enzymes. He proposed the hypercycle model to bridge this gap in prebiology. It can be shown by a nonlinear game model, incorporating mutation of a hypercycle, that the selection properties of hypercycles make them inefficient information integrators as they cannot compete favourably with all kinds of less efficient information carriers or mutationally coupled hypercycles. The stochastic corrector model is presented as an alternative resolution of Eigen's paradox. It assumes that replicative templates are competing within replicative compartments, whose selective values depend on the internal template composition via a catalytic acid in replication and "metabolism". The dynamics of template replication are analyzed by numerical simulation of master equations. Due to the stochasticity in replication and compartment fission the best compartment types recur. An Eigen equation at the compartment level is set up and calculated. Even selfish template mutants cannot destroy the system though they make it less efficient. The genetic information of templates is evaluated at both levels, and the higher (compartment) level successfully constrains the lower (template) one. Compartmentation together with stochastic effects is sufficient to integrate information dispersed in competitive replicators. Compartment selection is considered to be group selection of replicators. Implications for the origin of life are discussed.
Biological Evolution↗
How many genes to start with? A computer simulation about the origin of life.
A geneticist's view on the origin of life would focus on individual nucleic acid molecules rather than on their concentrations, on stochastics rather than on differential equations. The 'package model' envisages primordial compartments that contain ensembles of primordial genes. These are replicated independently from each other. During package fission they are distributed to two daughter packages. Packages with a complete ensemble of genes can continue to propagate. However, mutations as well as the stochastic nature of replication and package fission occasionally cause arising packages to miss genes from the ensemble, thus resulting in the death of those packages. A computer simulation, considering the complementarity of RNA as well as abortive termination of replication, yielded results that are similar to those of a preliminary simulation irrespective of these parameters: the results suggest that life could not have started with more than 3 genes, or else the primordial replicase would have to achieve at least a reduction of the replicational error rate by a factor of 13 and a reduction of undue chain termination by a factor of 10 to 25.
Computer Simulation↗
Fact and assumption in studies on the origins of life.
Proteins and nucleic acids organise metabolic systems very efficiently. Original forms of life may have depended on less efficient mechanisms which have been superseded in the course of evolutionary selection. Discussion and experiment on the origins of life should therefore not depend on the assumption that substances and mechanisms, now seemingly essential, were essential initially. Instead we should consider what might happen in a few million years on moist mineral surfaces, smeared with bituminous material, and exposed to UV light in a reducing atmosphere in the absence of predators. What little evidence can be gathered from differences in composition and metabolic behaviour between ancient and recently evolved species suggests that the trend in biochemical evolution is towards simplification.
Animals↗
An RNA Condensate Model for the Origin of Life.
The RNA World hypothesis predicts that self-replicating RNAs evolved before DNA genomes and coded proteins. Despite widespread support for the RNA World, self-replicating RNAs have yet to be identified in a natural context, leaving a key 'missing link' for this explanation of the origin of life. Inspired by recent work showing that condensates of charged polymers are capable of catalyzing chemical reactions, we consider a catalytic RNA condensate as a candidate for the self-replicating RNA. Specifically, we propose that short, low-complexity RNA polymers formed catalytic condensates capable of templated RNA polymerization. Because the condensate properties depend on the RNA sequences, RNAs that formed condensates with improved polymerization and demixing capacity would be amplified, leading to a 'condensate chain reaction' and evolution by natural selection. Many of the needed properties of this self-replicating RNA condensate have been realized experimentally in recent studies and our predictions could be tested with current experimental and theoretical tools. Our theory addresses central problems in the origins of life: (i) the origin of compartmentalization, (ii) the error threshold for the accuracy of templated replication, (iii) the free energy cost of maintaining an information-rich population of replicating RNA polymers. Furthermore, we note that the extant nucleolus appears to satisfy many of the requirements of an evolutionary relic for the model we propose. More generally, we suggest that future work on the origin of life would benefit from condensate-centric biophysical models of RNA evolution.
Origin of Life↗
Historical perspective: the problem of the origin of life in the context of developments in biology.
The structure of the history of scientific ideas on the origin of life, after Darwin's theory of evolution brought the problem into focus, is discussed. 19th-century theories in the mainstream of historical development already included some notion of chemical evolution. These theories were limited, however, by their reliance on a protoplasmic view of life, according to which the protoplasmic substance combines all vital properties. It was only when this holistic concept of protoplasm was abandoned that a clear distinction between different vital functions such as metabolism and replication was made. This led to two schools of thought in the origin of life field, one inspired by biochemistry and one by genetics. Oparin's theory, which was rooted in the metabolic traditions of biochemistry, provided a model which has had a lasting impact in methodological terms and which helped to transform the field from a largely theoretical one to an area of active research. Genetically based theories, on the other hand, had a delayed impact in this respect, because of long-lasting uncertainty regarding the structural basis of gene function.
Genes↗
Moral reflections on the new technologies: a Catholic analysis.
New technologies of conception bring into being beautiful babies. The wonder of these children, and of the technologies themselves, can tempt us to abbreviate ethical reflection on the moral appropriateness of initiating human life in this way. However, the moralists of the Catholic Church, along with many others, judge that human life should originate in acts of love between parents, not in productive acts of technologists. Scientific help for people desiring to generate a child out of their own being should be distinguished from scientific substitution for human acts of love in originating life. The child must be recognized as an equal, not as a product, subject to quality control. To radically alter our ways of generating human life without sufficient moral reflection is to generate human pain and moral dilemmas that we have not begun to fathom.
Catholicism↗
On the origin of life event.
On the assumption of a uniform sample space probability hypothesis it is estimated a maximum number of polypeptides (or other kind of polymers) that could be synthesized in the prebiotic Earth. Besides, on the basis of five premises that are postulated as indispensable requirements for the origin of a living system, under the constraints of a protein-nucleic acid chemistry, it is concluded categorically that the origin of life event could not be the result of unbiased polymerization phenomena. On the contrary, biased and specific patterns of polymerization had to be an essential component in this fundamental event. Finally, several theories on the origin of life and complementary concepts like hypercyclic organization and self-organization phenomena in dissipative structures are discussed in the light of the conclusions arrived at in this work.
Models, Genetic↗
On terminology in origin of life studies.
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Origin of Life↗
Terminology in origin of life studies.
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Origin of Life↗
Hypercycles and the origin of life.
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Mutation↗
An alternative view of the origin of life.
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DNA↗
Reversible suspension of metabolism and the origin of life.
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Cytoplasm↗
A proposal concerning the origin of life on the planet earth.
The widely accepted Oparin thesis for the origin and early evolution of life seems sufficiently far from the true state of affairs as to be considered incorrect. It is proposed that life on earth actually arose in the planet's atmosphere, however an atmosphere very different from the present one. Because of an extremely warm surface, the early earth may have possessed no liquid surface water, its water being partitioned between a motten crust and a fairly dense atmosphere. Early preliving systems are taken to arise in the droplet phase in such an atmosphere. The early earth, which resembled Venus then and to some extent now, underwent a transition to its present condition largely as a result of the evolution of methanogenic metabolism.
Animals↗