7. THE DISTRIBUTION AND EVOLUTION OF VISUAL SYSTEMS
343
Widely different though the anatomy and derivation of these eyes,
all of them share very nearly the same chemistry. I see no alternative
to believing that they achieved this chemistry independently of one
another. I am willing to accept the implication that among the organic
molecules that animals can make and that are made available to them
by plants on this Earth, the carotenoids and vitamins A are particularly
fitted for the construction of photoreceptor systems. I suggested at the
beginning of this essay a number of aspects of the peculiar fitness of
this family of substances for this function. I believe that for this reason
animals, offered a wide choice of organic structures, have three times
independently selected these, and have not to our knowledge ever
selected any others.
Let me say this in a different way. Now that many of us believe
that life is part of the order of nature, we see no escape from the conclusion that it is distributed widely in the universe, on perhaps many
millions of planets like our own. I think that when biologists have had
the opportunity to examine the fauna of other well-lighted planets,
planets that have possessed life as has ours for perhaps several billion
years, they will find eyes. In the eyes will be light-sensitive pigments,
and these pigments will frequently, perhaps usually, have as prosthetic
groups such polyenes as the carotenoids and vitamins A.
References
1. E. Pettit, Astrophys. J. 75, 217 (1932).
2. E. S. Johnston, Smithsonian Inst. Misc. Publs. 92, No. 11 (1934).
3. G. Wald, Science 101, 653 (1945).
4. G. Wald, Vitamins and Hormones 1, 195 (1943).
5. (a) G. Wald, Harvey Lectures 41, 117 (1947); (b) G. Wald, in "Modern
Trends in Physiology and Biochemistry
,, (E. S. G. Barron, ed.), p. 337. Academic Press, New York, 1952.
6. K. V. Thimann and G. M. Curry, this volume, Chapter 6.
7. G. Wald and H. Zussman, /. Biol Chem. 122, 449 (1938).
8. L. Zechmeister, Chem. Revs. 34, 267 (1944).
9. R. Hubbard and G. Wald, /. Gen. Physiol. 36, 269 (1952-1953).
10. R. Hubbard and A. Kropf, Proc. Natl. Acad. Sei. U. S. 44, 130 (1958); A.
Kropf and R. Hubbard, Ann. N. Ύ. Acad. Sei. 74, 266 (1958).
11. G. Wald, Am. J. Ophthalmol. 40, 18 (1955); G. Wald, in "Enzymes: Units of
Biological Structure and Function" (O. H. Gaebler, ed.), p. 355. Academic
Press, New York, 1956; R. A. Morton and G. A. J. Pitt. Fortschr. Chem. org.
Naturstoffe 14, 244 (1957).
12. G. Wald, P. K. Brown, and P. H. Smith, Science 118, 505 (1953).
13. R. Granit, Ada Physiol. Scand. 1, 386 (1941); R. Granit, ibid. 2, 334 (1941).
14. G. Wald and R. Hubbard, /. Gen. Physiol. 32, 367 (1948-1949); G. Wald
Biochim. et Biophys. Ada 4, 215-228 (1950).
15. R. Hubbard and G. Wald, Proc. Natl Acad. Sei. U. S. 37, 69 (1951).
343
Widely different though the anatomy and derivation of these eyes,
all of them share very nearly the same chemistry. I see no alternative
to believing that they achieved this chemistry independently of one
another. I am willing to accept the implication that among the organic
molecules that animals can make and that are made available to them
by plants on this Earth, the carotenoids and vitamins A are particularly
fitted for the construction of photoreceptor systems. I suggested at the
beginning of this essay a number of aspects of the peculiar fitness of
this family of substances for this function. I believe that for this reason
animals, offered a wide choice of organic structures, have three times
independently selected these, and have not to our knowledge ever
selected any others.
Let me say this in a different way. Now that many of us believe
that life is part of the order of nature, we see no escape from the conclusion that it is distributed widely in the universe, on perhaps many
millions of planets like our own. I think that when biologists have had
the opportunity to examine the fauna of other well-lighted planets,
planets that have possessed life as has ours for perhaps several billion
years, they will find eyes. In the eyes will be light-sensitive pigments,
and these pigments will frequently, perhaps usually, have as prosthetic
groups such polyenes as the carotenoids and vitamins A.
References
1. E. Pettit, Astrophys. J. 75, 217 (1932).
2. E. S. Johnston, Smithsonian Inst. Misc. Publs. 92, No. 11 (1934).
3. G. Wald, Science 101, 653 (1945).
4. G. Wald, Vitamins and Hormones 1, 195 (1943).
5. (a) G. Wald, Harvey Lectures 41, 117 (1947); (b) G. Wald, in "Modern
Trends in Physiology and Biochemistry
,, (E. S. G. Barron, ed.), p. 337. Academic Press, New York, 1952.
6. K. V. Thimann and G. M. Curry, this volume, Chapter 6.
7. G. Wald and H. Zussman, /. Biol Chem. 122, 449 (1938).
8. L. Zechmeister, Chem. Revs. 34, 267 (1944).
9. R. Hubbard and G. Wald, /. Gen. Physiol. 36, 269 (1952-1953).
10. R. Hubbard and A. Kropf, Proc. Natl. Acad. Sei. U. S. 44, 130 (1958); A.
Kropf and R. Hubbard, Ann. N. Ύ. Acad. Sei. 74, 266 (1958).
11. G. Wald, Am. J. Ophthalmol. 40, 18 (1955); G. Wald, in "Enzymes: Units of
Biological Structure and Function" (O. H. Gaebler, ed.), p. 355. Academic
Press, New York, 1956; R. A. Morton and G. A. J. Pitt. Fortschr. Chem. org.
Naturstoffe 14, 244 (1957).
12. G. Wald, P. K. Brown, and P. H. Smith, Science 118, 505 (1953).
13. R. Granit, Ada Physiol. Scand. 1, 386 (1941); R. Granit, ibid. 2, 334 (1941).
14. G. Wald and R. Hubbard, /. Gen. Physiol. 32, 367 (1948-1949); G. Wald
Biochim. et Biophys. Ada 4, 215-228 (1950).
15. R. Hubbard and G. Wald, Proc. Natl Acad. Sei. U. S. 37, 69 (1951).