100 INTRODUCTION TO EVOLUTION 



1. Romer (1959) has concluded that geological and other evidence in- 

 dicates that the first vertebrates (jawless fishes called ostracoderms, Fig. 

 8.14, p. 159) lived in fresh water. If so the vertebrate kidney functioned 

 first as an organ for ridding the body of excess water, and only later took 

 on functions of excretion. 



2. Robertson (1957) has concluded that the geological and other evi- 

 dence indicates that the first vertebrates were marine, and that the kidney 

 was primarily an excretory organ. Its microscopic structure resembles that 

 of excretory tubules found in some invertebrates, in fact. According to this 

 point of view the ridding of the body of excess water came as a later 

 function of the organ. Robertson's opinion is that some marine verte- 

 brates, notably cyclostomes (lamprey, hagfish) and cartilaginous fishes 

 such as sharks did not have fresh-water ancestors; yet all have well de- 

 veloped kidneys. 



Both authors quoted agree, however, that evidence supports the view 

 that bony fishes and amphibians had fresh-water fishes in their ancestry. 

 Hence an organ that functioned in connection with osmotic regulation in 

 ancestors assumed (or resumed?) the function of excretion in descendants. 



The excretory function of the kidney, once inaugurated, or resumed, 

 became its main function. Reptiles, birds, and mammals develop horny 

 scales, feathers, hair, etc., with the result that water loss through the skin 

 is minimal. And water loss through the kidneys in birds and mammals is 

 reduced by development of a special section of the kidney tubules (loop 

 of Henle) which reabsorbs back into the blood much of the water in the 

 urine. Salts are also salvaged by regions of the tubules. Thus these truly 

 terrestrial animals excrete a concentrated (hypertonic) urine, ridding the 

 body of a maximum amount of waste dissolved in a relatively small 

 amount of water. As already mentioned, reptiles and birds which convert 

 nitrogenous wastes into relatively insoluble uric acid attain the peak of 

 water conservation in the excretory process (pp. 92-93). 



The solutions of problems of osmotic regulation afford striking examples 

 of adaptation to environmental requirements, as well as an example of 

 evolution of function in an organ of the body, the kidney. Structural 

 changes in the kidney accompany the functional changes, but the details 

 are beyond the scope of the present brief discussion. 



Biochemical Evidence of Vertebrate- invertebrate Relationships 



Phylum Chordata includes all vertebrates, possessing a vertebral column 

 ("backbone") as adults, and a few animals that lack a vertebral column 



