Chemical evolution biomolecules

If the primeval atmosphere did not contain enough CO2 to maintain a greenhouse climate, the much lower solar irradiation at that time would have led to frozen oceans. But that would make almost all the assumed synthetic mechanisms for the formation of biomolecules impossible Bada et al. (1994) consider external help as a way out of this dilemma. They assume that the energy from meteor impacts (diameters up to around 100 km), converted into heat, would have sufficed to melt the oceanic ice. If such a process were to have occurred periodically, chemical evolution reactions (see Chap. 4) could have taken place in the ice-free periods and have led finally to biogenesis. 

Recent work suggests that there may have been a period in Europa s history when an extreme greenhouse effect led to temperatures which would have sufficed for reactions necessary for chemical evolution. According to this (unproven) hypothesis, building blocks for biomolecules or even primitive life forms could have existed. The authors assume that there is a high probability that bioelements could have been delivered by comets (Chyba and Phillips, 2002). 

The book consists of several parts. The first is a short introduction to the guiding principles and building blocks of chemical evolution. It contains a chapter on the important aspects of chirality or handedness of biomolecules. The second part is an attempt to describe up-to-date most plausible hypotheses and visions of the ancient world scenarios. References and specific websites will be helpful for those who wish to know more on specific topics. 

Chemical evolution is the increase in numbers and complexity of the newly formed compounds and polymers. The exogenous and indigenous compounds do have similar structures and more importantly, the built-in potential to react with each other to form complex biomolecules. They can be tested and reproduced in the lab, However, the nature of the formed key molecules depended on the environment, and since the environmental factors depended on the physical conditions of the early stages of the Earth (temperature, UV-irradiation, pressure), consequently, the nature of the compounds changed during the early phases of the Earth. One should not wonder why the composition of the Earth s ancient atmosphere is controversial. It changed quite often. 

Evolution (chamical and molecular) the processes by which biomolecules first arose, then gave rise to self-reproducing organisms. Chemical evolution, the process by which biomolecules arose from inorganic matter, is discussed under Abiogenesis (see). 

Evolution, the process by which systems under non-random selective pressure become more complex, has been a pervasive force over 4.5 billion years of Earth history. Evolutionary episodes, including the prebiotic synthesis of biomolecules, the selection and organization of those molecules into self-replicating systems, the subsequent co-evolution of the geo-and biospheres, and the modern-day acceleration of chemical evolution through human invention, serve to illustrate this recurrent theme. 

A large number of successful experimental studies which tried to work out plausible chemical scenarios for the origin of life have been conducted in the past (Mason, 1991). A sketch of a possible sequence of events in prebiotic evolution is shown in Figure 3. Most of the building blocks of present day biomolecules are available from different prebiotic sources, from extraterrestrial origins as well as from processes taking place in the primordial atmosphere or near hot vents in deep oceans. Condensation reactions and polymerization reactions formed non-in-structed polymers, for example random oligopeptides of the protenoid type (Fox... 

The differential solubilities exhibited by biomolecules thus should be appreciated as one of the most important aspects of the effects of water on living systems. Differential solubility is a critical principle in much of biochemical evolution, and it is a principle that is manifested in a number of contexts of adaptation to the environment. This is seen particularly clearly in the evolution of proteins in the face of different chemical and physical conditions. The amino acids selected to construct a particular protein reflect a finely tuned process that results in the generation of an appropriate three-dimensional structure and a correct balance between structural stability and flexibility—a balance termed marginal stability—that is essential for protein function. The marginal stability of the protein will be seen to be the consequence of complementary adaptations in the protein... 

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