Evolution of Enzymes 213 



in the primordial atmosphere but occurred in response to its function in other 

 oxidative dismutations not involving molecular oxygen, or that there was some 

 molecular oxygen in the Earth's atmosphere prior to the advent of the photo- 

 synthetic apparatus which now produces it. 



The development of such highly organized structures as are now found in the 

 organelles of the cells, such as the chloroplasts and mitochondria, seems to be 

 incompatible with the penetration of very much of the ultraviolet radiation 

 from the sun in the wavelength around 2500 Â which would have occurred had 

 there been no oxygen in the atmosphere. To-day, of course, the oxygen in the 

 upper atmosphere is photodissociated by this very ultraviolet light and is thus 

 converted into quantities of ozone which, in turn, strongly absorbs in the region 

 between 2200 Â and 3000 Â. While it is possible to conceive of the development 

 of heterotrophic organisms in the shelter of deep water and thus not exposed to 

 the destructive ultraviolet, it is difficult to conceive of the evolution of a photo- 

 synthetic apparatus, even a primitive one, also in the same environment, since 

 there would be httle visible Hght available either for the evolutionary selective 

 mechanism. 



Thus we are constrained to beheve not only was the evolution of higher 

 organisms delayed imtil the appearance of appreciable amounts of oxygen in the 

 atmosphere, but also the evolution of the porphyrin catalysis itself and some of 

 the more complex structural elements in Hving cells had to await this protection. 

 Presumably, it came about by virtue of the photodissociation of water into hy- 

 drogen and oxygen followed by the escape of the hydrogen from the gravitational 

 field of the Earth, leaving behind an appreciable quantity of oxygen, even though 

 it is now entirely photosynthetic in origin. We can thus envisage the develop- 

 ment of the highly organized structural units which we see to-day in the form 

 of mitochondria prior to the development of the chloroplasts and of the photo- 

 chemical apparatus. The similarity in the structure of these two organelles is 

 very striking, and it is not unreasonable to suppose that the latter had its origin 

 in the former (Fig. 4). 



Thus, the porphyrin molecule and the structural unit into which it is built 

 had independent evolutions. Only at a much later time did the coupling of the 

 optical properties of the porphyrin molecule to the energy demands of the carbon- 

 reduction cycle take place. This presumably occurred at a time when there may 

 have already been a rudimentary photosynthetic energy-yielding apparatus in 

 the near ultraviolet, perhaps using sulphur compounds as direct absorbers. The 

 more efficient energy-capturing molecule, which the porphyrin is, could not 

 be used in energy conversion until it was divested of the iron atom which was its 

 principal reason for being. The reason for this is that the requirement for an 

 efficient energy conversion carries with it the requirement for a long-lived 

 excited state after the capture of the quantum. The iron porphyrin, although 

 absorbing a good deal of visible hght, cannot have a very long life in the excited 

 state because of the presence of the magnetic iron atom in the molecule. The 

 inhomogeneous magnetic field surrounding the iron atom breaks down the triplet- 

 singlet selection prohibition and does not allow a long-Hved triplet state of 

 porphyrin. However, if the iron is replaced by magnesium, or some other dia- 



