Thermal Factors in Archaeometaboîism 651 



mediates in a thermal-reaction sequence. With each drop in temperature, a 

 selective advantage would be enjoyed by a unit which could catalyse the synthesis 

 of such intermediates from available precursors. As the stepwise evolution of 

 synthetic abihty followed the course suggested by Horowitz [y], chemical 

 catalysis would replace heat-driven reaction. Although the efficiency of chemical 

 catalysis may have been low at first, Calvin [8] has described how inorganic ions 

 may have evolved into chelates and metal-containing enzymes with high efficiency. 



If enzymes are substitutes for heat, then we should look for such evidence in 

 thermophiles. Indeed, Harvey [9] has noted that, like most anaerobes, hot-spring 

 algae lack catalase. At the high temperatures at which they Hve, the decomposi- 

 tion of hydrogen peroxide proceeds rapidly enough by itself. 



The thermal hmit of present-day blue-green algae and bacteria suggests that 

 early enzymes may have originated at temperatures above 85 °C. The persistence 

 of thermotolerant enzymes in present-day mesophiles as well as thermophiles 

 attests to the primitiveness of this characteristic. If efficiency at high tempera- 

 tures can be considered a primitive trait, then enzymes which have evolved 

 comparatively recently are probably less efficient at high temperatures. If 

 evolution proceeded from higher to lower environmental temperatures, can we 

 recapitulate biochemical evolution by reversing the thermal arrow? Can we 

 attempt to distinguish between archaic and recent metabolic pathways by using 

 elevated temperature as a tool for probing into enzyme evolution ? 



We have been studying the enhanced nutritional requirements of a variety 

 of protista (including algae, bacteria, protozoa as well as yeast), grown at incu- 

 bator temperatures above their usual optimum. Heightened temperature, by 

 fraying the biochemical fabric of the organism, brings to light otherwise poorly 

 accessible metabolic chains. The appearance of substrates as temperature factors 

 for photosynthetic forms such as Ochromonas and Euglena supports the idea of 

 reduced biosynthetic ability of a primitive thermotolerant heterotroph. In 

 addition increased requirements of metals and chelating agents in both photo- 

 synthetic and non-photosynthetic forms are reminiscent of a stage in Calvin's 

 chemical evolution. Finally, since these experiments are carried out in syn- 

 thetic media, the specific requirements of protista under thermal stress may 

 shed light on the contribution of abiogenic synthesis to the primitive environ- 

 ment. 



In summary, temperature factors appear to provide clues — could we but 

 interpret them — to archaeometaboîism, if we assume a thermal origin of life 

 and an early evolutionary tendency from heterotrophy to autotrophy. 



REFERENCES 



1. J. D. Bernal, The Physical Basis of Life. Routledge & Kegan Paul, London, 1951. 



2. A. I. Oparin, The Origin of Life on the Earth. Oliver & Boyd, London, 1957. 



3. J. J. COPELAND, Ann. N.Y. Acad. Sei., 36, i, 1936. 



4. J. B. S. Haldane, The Origin of Life. Rationalist Annual, 1929. 



5. S. W. Fox et al. Ann. N.Y. Acad. Sei., 69, 328, 1957. 



6. P. H. Abelson, Ann. N.Y. Acad. Sei., 69, 276, 1957. 



7. N. H. Horowitz, Proc. nat. Acad. Sei., Wash., 31, 153, 1945. 



8. M. Calvin, Amer. Sei., 44, 248, 1956. 



9. R. B. Harvey, Science, 60, 481, 1924. 



