SPECULATIONS ON THE ORIGINS AND EVOLUTION OF PHOTOSYNTHESIS

There'is a constant urge in man to seek beginnings. In biology, the beginning means the origin of life. Nearly a century ago Pasteur performed decisive experiments that led to the development of the important theory that all life develops from living cells. However, Pasteur himself realized that this did not hold the answer to the origin of life.' A number of young sciences such as biochemistry, geochemistry, nuclear chemistry, and solid-state physics already have added important generalizations to our knowledge. Instead of discussing the question of the origin of life solely from a general philosophical point of view, we now have the knowledge to seek experimental approaches to this question, not only in terms of life on this planet, but on other planets and planetary systems as well. I have been asked to discuss the possible origins of photosynthesis in relation to hypotheses on the origin of life. By way of introduction I shall first summarize recent proposals on the origin of life and photosynthesis, and outline briefly the speculations that I wish to present. I shall then proceed to discuss these speculations in detail. More recent proposals concerning the origin of life assume the following sequence of First, a variety of complex organic molecules accumulated in the sea due to the action of ultraviolet light and electrical discharges on a mixture of ammonia, water, hydrogen, and methane or the oxides of carbon. Sufficient time then elapsed to allow the formation of a heterotrophic organization of units of protoplasm that used the organic compounds around them for growth. Clay minerals could have served as inorganic catalysts. Gradually, as these organic compounds were depleted, photosynthesis occurred, with chlorophyll or related compounds as the photosynthetic pigment. Then came the development of biosynthetic chains that originated from the functional end products as we know them today and, working backward, the production of genes and enzymes that would provide the precursor steps for the end product. The formation of a living entity was a one-way process. Once the complex organic materials of the environment were exhausted, new life could be started only from existing ce!ls. Today I wish to present speculations that differ in a number of ways from the above hypotheses. I shall propose that the first organization of preprotoplasm would be a primitive energy-conversion unit that could perform the elementary processes of photosynthesis and respiration; that this unit originated from some common minerals; that the minerals that contain metal ions served both as coordinating templates and catalysts for various reactions, and that around this unit were formed organic molecules that gradually became organized into units of ever-increasing complexity. Gradually, biosynthetic chains de-