Coming in the next issue: OUR ANCESTRAL PRIMATE

If we reset the dials in our Time Machine and travel to a point about 450 million years ago, we may be able to witness the start of another notable experiment by Nature. In the seas are the beginnings of the vertebrates, a long line of animals that will eventually evolve into the mammals and man. These are the animals with a more or less rigid backbone, containing the start of a well-organized and complex nervous and muscular system, and capable of reacting much more efficiently to their environment than the swarms of simpler, spineless invertebrates, which had apparently reached the end of their evolutionary rope. Nature was ready to embark on another revolutionary, and more complicated, experiment.

Because of the increased complexity of their nervous system and a fast-acting muscular system, these primitive vertebrate fishes were able to gather food better and avoid enemies and other perils, all of which had increased survival value. Before they could do this, however, they had to develop complex, specialized organ systems in which various biochemical processes were carried out. And their requirements for ascorbic acid were undoubtedly much higher because of their much increased activity. The simpler structures of the invertebrates no longer sufficed and required much modification to suit the needs of these more active and alert upstarts, the vertebrates.

The vertebrate fishes were such a successful evolutionary experiment that for the next 100 million years or so they dominated the waters. Nature was now ready to carry out another experiment -- that of taking the animals out of the crowded seas and putting them on dry land. It had experience in this sort of operation since the plants had long ago left the seas and were well established on land. The land was no longer a place of barren fields, but was covered with dense vegetation. Two lines of modification were tried: in one, the fish was structurally modified so that it could clumsily exist out of the water; in the other, a more complete renovation job was done. Modifications of the fins and the swim bladder ended in the evolutionary blind alley of the lung fishes, but the more ambitious program -- involving a complete change in the biochemistry and life cycle -- produced a more successful line -- the amphibians. These creatures are born in the water and spend their early life there and then they metamorphose into land-living forms. Frogs and salamanders are present-day denizens of this group. The next step in evolution was to produce wholly land-living animals -- the reptiles. These were scaly animals that slithered, crawled, walked, or ran; and some grew to prodigious size. Some preferred swimming and reverted to the water and others took to the air. It was these airborne species that eventually evolved into the warm-blooded birds. The birds are of particular interest to us because they solved an ascorbic acid problem in the same fashion as the primitive mammals, which were appearing on the scene at about this time.

We have gone into this cursory sketch of this period of evolutionary history to trace the possible history of ascorbic acid in these ancient animals. If we assume that the present-day representatives of the amphibians, the reptiles, the birds, and the mammals have the same biochemical systems as their remote ancestors, then we can do some more detective work on our elusive molecule. These complex vertebrates all have well-defined organ systems that are assigned certain definite functions. Usually an organ has a main biological function and also many other accessory, but no less important, biochemical responsibilities. The kidney, whose main function is that of selective filtration and excretion, is also the repository of enzyme systems for the production of vitally important chemicals needed by the body. The liver, the largest organ of the body, functions mainly to neutralize poisons, produce bile, and act as a storage depot for carbohydrate reserves; but it also has many other duties to perform.

In examining present-day creatures we find that in the fishes, amphibians, and reptiles, the place where ascorbic acid is produced in the body is localized in the kidney. When we investigate the higher vertebrates, the mammals, we find that the liver is the production site and the kidneys are inactive. Apparently, during the course of evolution the production of enzymes for the synthesis of ascorbic acid was shifted from the small, biochemically crowded kidneys to the more ample space of the liver. This shift was the evolutionary response to the needs of the more highly developed species for greater supplies of this vital substance.

The birds are of particular interest because they illustrate this transition. In the older orders of the birds, such as the chickens, pigeons, and owls, the enzymes for synthesizing ascorbic acid are in the kidneys. In the more recently evolved species, such as the mynas and song birds, both the kidneys and the liver are sites of synthesis; and in other species only the liver is active and the kidneys are no longer involved in the manufacture of ascorbic acid. Thus we have a panoramic picture of this evolutionary change in the birds, where the process has been "frozen" in their physiology for millions of years.

This evolutionary shift from the kidneys to the liver took place at a time when temperature regulatory mechanisms were evolving and warm-blooded animals were developing from the previous cold-blooded vertebrates. In the cold-blooded amphibians and reptiles, the amounts of ascorbic acid that were produced in their small kidneys sufficed for their needs. However, as soon as temperature regulatory means were evolved -- producing the highly active, warm-blooded mammals -- the biochemically crowded kidneys could no longer supply ascorbic acid in ample quantities. Both the birds and the mammals, the two concurrently evolving lines of vertebrates, independently arrived at the same solution to their physiological problem: the shift to the liver.