JTW's Evolutionary Origins - Author: Lahav, Noam

Arguments For Hyperthermophilic Origins

Wachtershauser's Generalizations

• Since the temperature optimum of many enzymes is very close to their maximally tolerable temperature, the upward adaptation from mesophily to hyperthermophily would require a simultaneous increase of the optimum of many enzymes, which is unlikely. On the other hand, downward adaptation is much more likely, since enzymes retain their catalytic activity considerably below their optimal temperature.
  
  
• Condsiderations of optimized folding interactions suggest that hyperthermophiles' homologous sequences would be much more constrained in their folding than mesophiles' sequences. Therefore, the transition from hyperthermophily to mesophily is much more likely than the opposite transition.
  
  
• Enzymes of hyperthermophiles must consist of small discrete units, seperate or in domains... but they are often fused to large units in mesophiles... Reversal of these functions in upward evolution is unthinkable.
  
  
• Because hyperthermophiles metabolic pathways are kinetically guarded against degradation of unstable intermediates, the likelihood of simultaneous installation of such kinetic contrivances in a transition from mesophily to hyperthermophily is extremely low.

Consequences of Wachtershauser's Generalizations

• If organisms are never more thermophilic than their ancestors, then the common ancestor and the progenote were hyperthermophilic.
  
  
• It is generally accepted that our phylogenetic tree is monophyletic, and in some cases it was assumed that the mutational rate of certain homologous sequences throughout the evolution time was unchanged. The latter assumption, which was used in evolutionary time calculations (i.e., Eigen et al., 1989), is thus unwarranted. In one case (Doolittle et. al. 1996), calculations based on the relatively rapid evolutionary rate of mesophiles would result in a too-short evolutionary time.
  
  
• Inasmuch as the genetic code and its origin were related to protein folding, the deciphering of its origin should be based on hyperthermophilic proteins.
  
  
• The first proteins had a very small number of allowed conformations; this fits into Wachtershauser's scenario, which is based on 'surface metabolism' on pyrite.

• Lahav, Noam
  • Biogenesis: Theories of Life's Origins
  • Oxford University Press, Inc.
  • QH 325.L24 1999

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