262 Alfred Sherwood Romer 



SO that by the end of that period a truly marine as well as a continental fish fauna 

 had been established. 



At the time that we were considering the fossil evidence, the same general question 

 was approached from an entirely different direction through the kidney studies of 

 Homer Smith ; these are to be found summarized in various papers and books by that 

 author (1932, 1936, 1953, etc.); his conclusions are briefly discussed in my text, "The 

 Vertebrate Body" (1955). His studies furnish extremely strong proof that the early 

 history of vertebrates was passed in fresh waters. It is well known that typical 

 vertebrates cannot exist unless their blood and body fluids contain a specific series of 

 salts in a given concentration which is considerably lower than that of sea water. The 

 most generalized type of vertebrate kidney is a structure that functions most especially 

 as a " pump " to rid the body of excess water rather than merely furnishing (as in the 

 case of many invertebrate nephridia) a means for disposal of metabolic wastes.* 

 Such kidneys are found in all fresh water fishes and amphibians. These animals live 

 in an environment where death would ensue, owing to osmotic dilution of the salt 

 content of their blood and body fluids, were not such a " pump " present to rid the 

 body of excess water, thus keeping up a proper salt concentration. Salt water fishes 

 live, on the other hand, in a water medium containing a higher concentration of salt 

 than their body fluids and hence equally dangerous to existence. A marine fish must 

 avoid the danger of becoming too salty in its body fluid content; it may do this either 

 by conserving water or by excreting excess salt, or both. Actually two very different 

 methods are found in the two major groups of marine fishes. Salt water bony fish 

 have solved the problem by (1) the development of salt excretion in the gills, and (2) in 

 many instances by kidney modification which reduces the water outflow. Sharks, in 

 contrast, have retained the " pump " type of kidney present in fresh water forms, but 

 nevertheless prevent excess salinity of the blood by a most unusual specialization — 

 the retention of considerable amounts of urea in the blood, so that blood and sea water 

 are in osmotic balance and hence loss of water through surface membranes (and 

 consequent dehydration) does not occur. 



These two contrasting types of marine adaptations are not derivable one from the 

 other; the two can have as a common origin only the kidney type found in fresh 

 water forms. This is, therefore, the ancestral type. Hence the ancestral fishes must 

 have lived in inland waters, and sharks and bony fishes must have independently 

 invaded the sea. 



1 had hoped, since the time of our earlier publication, to be able on some occasion 



* Amphioxus is remarkably similar to the vertebrates in almost every basic feature, and practically 

 every student of vertebrate history agrees that this little marine chordate is closely related, in some 

 fashion or other, to the primitive vertebrates. Amphioxus, however, does not have a kidney of 

 vertebrate type, but in contrast has nephridia comparable to those of certain marine organisms quite 

 unrelated to the vertebrates. For this reason Smith (1953) has felt forced to deny the relationship of 

 Amphioxus to the vertebrates — this despite the strong evidence to the contrary seen in almost every 

 other structural regard. This attitude seems to me quite unnecessary; in fact this contrast in kidney 

 structure can be fitted satisfactorily into the assumed history. Although I am not aware of any 

 precise functional study of the Amphioxus nephridia, these little organs appear to be (like comparable 

 structures in many other invertebrates) primarily for the elimination of nitrogenous wastes, and not 

 too efficient as eliminators of water. With the invasion of fresh water by the ancestral vertebrates, 

 water elimination in quantity was a basic necessity, and the vertebrate kidney was evolved de novo 

 as a water pump. But the kidney can also act as an eliminator of waste, and if nephridia were present 

 in the fish ancestors, they could be done away with in safety once the new kidney mechanism was 

 established. Thus the presence of nephridia in Amphioxus need not debar this animal from the 

 vertebrate " family circle ". 



