Maynard-Smith and Szathmary

In the last 20 years, a new problem, the evolutionary origin of levels of organization, or evolutionary transition, has been added to those celebrated since Darwin (Buss, 1987). In their recent book, The Major Transitions in Evolution, Maynard Smith and Szathmary (1995, p. 6) identified what they considered to be eight major originations of new levels of organization (Table 11.1). For most of these, they claimed a common feature . .. entities that were capable of independent replication before the transition... 

TABLE 11.1 The major transitions and levels in evolution (al ter Maynard Smith and Szathmary, 1995)... 

Notice that the common feature of many evolutionary transitions identified by Maynard Smith and Szathmary involves a change in the status of replicators. Once the replication of a class of entities becomes dependent on a larger whole, they do not become independent replicators again. So new replicators at the emergent level must exist and replicate independently in order for there to be a still higher level transition. Given the nature of replicators as analyzed by both Dawkins and Hull, there is a problem with this characterization of the common feature only one or possibly two evolutionary transitions are at all likely to have occurred in the history of life. 

Figure 7.12 A simple rendering of an hypercycle. Each of the units A, B, C and D is a replicator. The rate of replication of each unit is an increasing function of the concentration of the unit immediately proceeding it. Thus the rate of replication of B is an increasing function of the concentration of A, and so on round the cycle. (Adapted from Maynard-Smith and Szathmary, 1995.)...

Figure 8.10 Ganti s chemoton (redrawn from Maynard-Smith and Szathmary, 1995, based on the original by Ganti, 1984). The metabolic subsystem, with intermediates Ai -> A2 ->. . . A5, is an autocatalytic chemical cycle, consuming X as nutrient and producing Y as waste material pV is a polymer of n molecules of V which undergoes template replication R is a condensation byproduct of this replication, needed to turn into T, the membranogenic molecule the symbol Tm represents a bilayer membrane composed of m units made of T molecules.

Maynard-Smith and Szathmary (1995) discuss at length some of the implications of the chemoton, for example, the question of how the chemoton has heredity. They point out some of the difficulties in implementing the chemoton but conclude by saying (Maynard Smith and Szathmary, 1995) ... 

The replication paradigm requires that protein enzymes were not present at the beginning, and RNA replication was therefore performed by ribozymes. Some RNAs can in fact behave as polymerases and replicases, but they are far less efficient than the corresponding protein enzymes, and the accuracy of their replications was necessarily very low. The experimental measures, obtained from interacting coupled nucleotides, have shown that without protein enzymes the replication error e cannot be less than 0.01, which means, from formula 5.1, that primitive RNAs could not have, as an order of magnitude, more than 100 nucleotides (Maynard Smith and Szathmary, 1995). 

J. Maynard Smith and E. Szathmary. The Major Transitions of Evolution (New York W. H. Freeman, 1995). 

Ageing evolved at the same time as sex. By sex (I should come clean) I mean the production of sex cells such as sperm and egg and their fusion to form a new organism. The terms sex and reproduction are often used interchangeably, but technically they have completely different meanings. As John Maynard Smith and Eors Szathmary put it, "the sexual process is in fact the opposite of reproduction. In reproduction, one cell divides into two in sex, two cells fuse to form one." This poses a puzzle, which we shall see applies as much to ageing as to sex what is the benefit to the individual ... 

Maynard-Smith, J. and Szathmary, E. (2000). The Origins of Life — From the Birth of Life to the Origins of Language. Oxford University Press, Oxford. 

Maynard Smith, J. and Szathmary, E. (1998). The Origins of Life. Oxford University Press, Oxford. Morowitz, H. (2002). The Emergence of Everything. Oxford University Press Inc., New York. Mulkidjanian, A.Y., and Junge, W. (1999). Primordial UV-protectors as ancestors of the photosynthetic pigment protein. In G. A. Peshek, et al. (eds). The Phototropic Prokaryotes. Kluwer Academic Press Pub., New York, pp. 805-812... 

On the other hand, some accounts of evolutionary change emphasize a scientifically explicable thematic continuity and even a plausible, perhaps inevitable, directionality (Carroll, 2001 Conway Morris, 1998,2003 de Duve, 2002 Denton, 1998 Salthe, 1993 Szathmary and Maynard Smith, 1995 Williams and Frausto da Silva, 2003 but see Szathmary, 2002). Like a good melodrama, revisit the theater and one will see variations on a recurrent theme. 

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