ABSTRACT
In The Origin of Species (1859),1 Charles Darwin introduced the scientic theory of evolution, proposing that species continually adapt over time to their environments. Almost a century later, Homer Smith proposed that the concentrating capacity of the mammalian kidney was a major contributor to the evolution of various biologic species including humans.2 Early protovertebrates resided in a saltwater environment whose composition was similar to that of their own extracellular uid. erefore, these animals could ingest salt water without aecting the composition of their milieu intérieur.3 However, as early vertebrates migrated into freshwater streams, the development of a more water-impermeable integument was required to avoid fatal body uid dilution by the hypoosmotic freshwater environment. A vascular tu or primitive glomerulus thus developed, enabling the sh to lter excess uid from the blood. e subsequent development of proximal and distal tubules in vertebrates allowed for the preservation of sodium and the excretion of solute-free water, respectively. Importantly, there was now a means to uncouple sodium and water regulation.
