Genesis - In The Beginning Precursors of Life, Chemical Models and E
This book deals with the Origin of Life on Earth and planets, which is currently a very “hot” topic dealt with at several international meetings, conferences and workshops by various societies and groups. The current volume is number 22 of the “Cellular O
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risantha Fernando
Vera Vasas
35 J. Seckbach (ed.), Genesis - In The Beginning: Precursors of Life, Chemical Models and Early Biological Evolution, Cellular Origin, Life in Extreme Habitats and Astrobiology 22, 35–53 DOI 10.1007/978-94-007-2941-4_2, © Springer Science+Business Media Dordrecht 2012
COOPTIVE EVOLUTION OF PREBIOTIC CHEMICAL NETWORKS
CHRISANTHA FERNANDO1 AND VERA VASAS2 1 Department of Informatics, University of Sussex, Falmer, Brighton BN1 9RH, UK 2 Departament de Genètica i de Microbiologia, Grup de Biologia Evolutiva (GBE), Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Spain 1. Introduction Natural selection can act on units that are capable of multiplication, variation, and heredity, given these units have differential fitness and can be stably selected (Maynard Smith, 1986). The fundamental problem faced by any such unit of evolution is how information can be stored and reliably transmitted between generations. One solution that is generally agreed to have once been found by evolution is the RNA world. Here, RNA strands capable of template replication also possessed catalytic activity (Gilbert, 1986) and were most likely enclosed in compartments (Fernando et al., 2005). However, the route to the RNA world is not known. RNA replication is template-based, see Fig. 1, meaning that the sequence of molecules on the parent strand is replicated (with mutation) to the child strand by a topographic mapping between the parent and child strand. The number of possible sequences that can be copied for a string of length L is nL where n is the number of nucleotide types and L is the length of the string. Even for n = 2 and L = 100, there are 1030 possible unique sequences. As this is more than could be realized in any realistically sized system such as the universe, heredity is effectively unlimited (Szathmary, 2000). In addition, template replication has an important feature that adds power to natural selection, although is not a necessary feature for a unit of evolution as defined above: selection can, in principle, move from any sequence to any other sequence by a process of small mutations that produce correlated variation between parent and offspring if the fitness landscape allows it. Hogeweg has referred to information carried by template replicators as storage-based because all possible sequences are equally stable and transmissible, using the same copying mechanism (Hogeweg, 1998). While unlimited heredity and correlated variation seems necessary to explain the wondrous diversity of life, there are several problems with the spontaneous emergence of RNA templates or other molecules capable of template replication in the first place. This chapter considers the mechanisms that could have led to the origin of nucleotides or, alternatively, of other monomers that made template-based 37
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CHRISANTHA FERNANDO AND VERA VASAS
Figure 1. Top: Template replication. The parent strand contains a sequence of molecules (circle, circle, square, etc.) and is connected by strong bonds. Complemen
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