A Homeric view of kidney evolution: A reprint of H.W. Smith's classic essay with a new introduction

In From Fish to Philosopher (43), Homer W. Smith melds his groundbreaking physiological studies with geological and evolutionary musings to present an amazingly integrated view of how adaptation of a single organ may have contributed to the evolution of man. A condensed version of this thesis was presented in a lecture to the School of Medicine at the University of Kansas in 1943 and later published by the University of Kansas Press (Smith,41). This holistic view of how selection led to adaptation of the kidney in response to the move from salt- to freshwater, and in the example of the elasmobranchs and bony fishes back to saltwater again, is an extraordinary piece of evolutionary exposition. Knowledge has advanced since 1940 but as a work of integrative logic, and a beautiful piece of writing, it has few rivals. This essay begins with the cooling of the earth four billion years ago. The cooling of the crust produced periodic upheavals that had major effects on the earth's atmospheric conditions, which in turn led to changes in selective pressure. Smith describes how the migration from the oceans to freshwater challenged the physiology of invertebrates and protovertebrates and proposes how these pressures were dealt with through the evolution of the kidney. As upheavals drove the early freshwater-inhabiting vertebrates either back to the sea or onto the land, once again physiological barriers had to be overcome, largely by the kidney. Some of Smith's early physiological studies were conducted on the African lungfish and the changes that occur as these animals escape drought through estivation (summer sleep: the opposite of winter hibernation) in mud burrows. These animals can survive for years without food when encased in dry mud. In this state, the animal needs to adapt from a situation in which water influx and excretion are dominant processes to one in which there is absolutely no water intake and no production of urine. The lungfish therefore cyclically deals with similar physiological pressures to those faced by vertebrates in the course of evolution and musing on these changes probably contributed enormously to the thesis presented in the essay reproduced here. As a researcher investigating development of the kidney, I find this vision of evolution modifying organogenesis (and organogenesis modifying physiology) to be extremely useful. A wealth of comparative systems exist in nature that could allow us to link genetic change to developmental adaptation. One particularly useful example outlined by Smith is the aglomerular kidney's of certain marine fishes. Which of the genes postulated to be essential for glomerular development are no longer expressed during organogenesis in these animals? One such gene, WT1, is still present in the genome of the aglomerular (Marshall,38) pufferfish (Miles et al.,39), but has changes that may render it non- or partially functional. As the genomes of both glomerular (zebrafish) and aglomerular (Fugu) fish are now available, this problem is ripe for exploration. Amphibians initially live in an aqueous environment, but in many species spend large amounts of adult time terrestrially. These animals switch kidneys along with environments during metamorphosis with larval pronephroi dedicated to excreting water and adult mesonephroi dedicated to retaining it (Vize et al.,45). What developmental adaptations are associated with this transition? How do the permanently aquatic water excreting mesonephric kidneys of zebrafish differ from those of the water-retaining amphibians? Both Danio and Xenopus genomes will soon be completely sequenced and provide powerful tools for investigating such questions. Smith's essay provides a framework for thinking how genetic change may physiological and to a wealth of and this will be by the in From Fish to Philosopher by Smith a of his lungfish for physiological studies in an Homer and and the and with a by W. A from on the University University of Kansas years have since the that the in which live is but the the that and This environment, as is a from the and by a of physiological to the that by the and changes that the and that the since this there has in to which his of physiological be the of many that a can be The one of water the of the and there are the and the and and and This is to the of the as a The to the of the with to and and with this The for the of the in to largely the is no to that the of the is by the but by the kidneys are the of environment, to in the of or from the the of which are from these are to the major of in an are to do one of physiological kidneys are to an of can can can the can to without but when the kidneys to the of can again, as the of the the and a was the view of this that a and and of the of the of that have the of have have the of kidneys that that kidneys the major of physiological it be that the of the kidneys is to but in a view one can that the kidneys the of as a the kidney to be but on this is to of the which are of the is of one or These are and of a of a or which a These which through the the of the kidney, it the the In the in the the of the is a by the The of the glomerular in the kidneys this there are in time of or which is a of that of the this is still a in the for it in the to through it the the and similar large this of of are through the of the the the a of the an is for the to from to or from back the and is are as the glomerular the such as are and to the by processes of the time certain and are from the by the and to the urine. 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and that the in this since the of the or of the present or the These periodic as the have us have the and of the and and at of the have the and water in the barriers to and with the that the these have by and changes in the of the In of have by that in have to sea in in the had the to have the and and have a that was and to the a of evolution is the production of in of in the such of these as are to survive are by the to along as is to but be of to the a of selection the of of the by a in which are by from genetic through with the and probably in may that in the of the evolutionary as it and have and to either or selection certain to have one a few when many of have and of have can to find the that are to changes was the is in the development of of environment, of physiological in of the that can of evolution as The that evolution has a but an In it has a of in with the upheavals of the earth's was probably one of these with the that the vertebrates The in the of these with to those that have a the evolution of the kidney, are in of the evolution of the vertebrates in to saltwater and freshwater The which have this evolutionary The time is such that the and time are The problem of the of the or it was by the of the a A few years there was of on at one that the with the the and the a marine a similar to the of the The of this were that it a to that of and it one as it and through the the of this the of which no one has to the has on the that the as in the were and animals as in from as one can This is in for the of which has by that the may have from a or sea this will one of in a later As from the were can with the time of Some this evolution to the and to the The of the was by one of the of the earth has These have since but the to which were is to be in the of and through which the has the and from which the The has that the of the vertebrates of of a and with an for the of as to powerful of the the and of of of the to the of these and to an as the animal in the and of major in the of the These are such as to the with to it to move through the as an to live in a to one by and by the of the had from the sea the or freshwater of the when the the of the the of the and this the evolution of the by has it that the in freshwater the and in time by the of the marine that had to in the it is now that it was in the of the and in the that the and from the and early were in and I the comparative of the kidney and on the of the of the early vertebrates the freshwater thesis as by and were of on since that time the has by and and in the of this the freshwater can no longer be the of is and the evolution of the kidney is the of the evolution of the of the water of the in with the and therefore no problem of water it may be that in the or of the vertebrates the kidney was at with the excretion of but with the excretion of from the of comparative the marine of the had in of the of the a of which the or with the these were probably to the and of the the the had to a in and the which it with the and which were by an of the in the of the of the by from the or by this as it of the this of a and a of the to the of the In from the sea to and the to the these were probably a to for but were to have that as is by the which soon The vertebrates to in in the the and the and the the and the later advanced were encased from to in which the of bony or of the of from to that and in of the this has that the of the and early fishes were the that with the of these were the and and but were mud and with as or as do the may have The thesis that the of the early vertebrates to from is to I an these vertebrates had an which could but one which of the and and one from which there was no escape from to in the from the sea in freshwater, and the of marine were in for may on that at the of either or time the sea had or of present This be in but it could be without and The evolution of a environment, it had was in the freshwater the and of the water that by the to from to and in to to may that were the of water in freshwater be The the of water be to the as as by a that the present of the vertebrates of and time was a the of freshwater the and of the In the of evolution adaptation is to and and an to freshwater may have the of and and as later to such as had that it was from certain of these that the were this of that to be a was to the fishes with for and with and for and the with with which to on to the in changes in as The of the had to be and these had to be to the one which still the now the evolution of the may have by the of the with of the by a few had to be and in to in a powerful this on the with of the of the one or a few to the of the and to the development of and as in the The evolution of an the of which had to be in a the evolution of a or which in to the The development of the the of and the evolution of in the are in the of these and are from that it was in freshwater that the vertebrates to be in the the to be from the of at the early vertebrates it to to the and the of the of the the are as a of evolution in freshwater to the which is on many encased in a the the and the still by which of water could be The and early fishes had to for this influx by the excretion of this freshwater was by to and it that no could be to the of water of the to have the it and the to do this was to a by the with the to the as a of still in the of of the a was and the to the as in and of the in many fishes and the mesonephric still with the The essential is that the was of and the evolution of the it will be that in the development of the the is and with the of the time this has it is that this the development of the and the is an of the which the nature of a water to be of the but of the of the which the the these through the the which the from water through this with the of the it was to the that could these from the there was such an of water to be that the had to be it had to have a the as a of the evolution of the there a of large of and similar and of by the of the a that was to the this freshwater kidney was to The earth to the of the no were in but a the was in of which the of are that now were the and was by of of and The fishes of the early and to the saltwater and the freshwater which or mud Some of the powerful now to with the and marine by the the of the fishes to live in will be in a later The advanced of the in to survive in the of the to and and the for the evolution of the the of the the earth major in the periodic dry of the were by and and many of the fishes the example of the elasmobranchs and freshwater for in the sea the of marine certain of the freshwater the to for with which to from one to and the and which to to the water to and to a us have to the bony fishes that in the sea in the of these the marine since the of the but the physiology of is to the of from to the migration from to saltwater the and are the are the sea the of these is completely to and and The marine bony fishes a of freshwater but a of In could the of and water in the by excreting a but in do this for the fish kidney is to a which is the be as as that of the were it that have the of and the is the organ in the vertebrates of the such a organ could have a fish by the could have the from the water by excreting the of his in a the water for his or for the of urine. with the of the fish kidney still have had to his since for of be to a of by is that the marine spend in the in the of and no was to from the the bony fishes from freshwater to the the of the was no longer an but a the as as and with the years the and and to the in a of marine kidneys one of the the marine of glomerular and in certain of sea and in certain the kidney has is no by which the aglomerular is has that in the the to the and the adult has that in the and a in the evolution is the marine are kidneys back to the by the which the sea in or there is one of physiological that faced by the and the marine us to the had the marine migration in the These fishes the problem of in saltwater in an The four of the and from the and from in or the that is to the is the time at which can to four a four a and adaptation for in have the of by as it a from the glomerular as it the such as is present in this the of as the fishes to and this to the it of to The of this the of the that of the and water to move from the sea the through the and from at a to a for the excretion of and such as is present in the the bony fishes in a in the elasmobranchs this to the A is present in the kidney, to the which is to be the of the of from the glomerular of the in of in the is had water for there has no to is that the of in this is the of the the an in a The is the of Both the and those that have a or for which there exist in the and in the One that this of is with the of in the such time as kidneys and and are The of the but these were present in the and and and in the in the be to to us the time at which the as an adaptation to was now from the fishes to the evolutionary during the the were in and was a the was the was a by and and by that in water and on on the was as as the in which it was and in the to evolution or The of the the in the a of to was in the that now and The a and in the the of the was by and The and of the were the of the marine elasmobranchs were along with of the marine and freshwater and the was the of that the evolution of the which were in to permanently on These had and with which to from one water to the for the time in was encased in a and it the to the of the a of of which the of the environment, had to be to the from aquatic One of the of these adaptations in a change in the of of to as had the fishes and the and to is a it is in and it the it in the as a as the to the the of the water in the to be the the is as an dry This many is in the for the are but with and the in the sea many of completely as of water In view of the that in the and the marine the for water is one of to be aglomerular but no aglomerular or has The kidney in that for the once glomerular is to a in and a of is as these the largely but to the of the to the had of the of the and a of the in to the with a this are probably the of this is since of it has to the vertebrates and of it to the or to man. I this by that the but a of the and but one species The geological of is at a of the to years which are to the in the from this of the problem I that at this in the of the evolution of the kidney there is a in the the evolution of the The have the to the kidney since the to that is or the As in an the of this is in by the which is present in the of How this to a to be may on to the were from do the and the since the do live in freshwater, since in such as the can live dry such as the and can live in the sea without a of freshwater, have on the have the development in the us is early the the in the and the have the the of the these to on the and to for In the which the a of but one of and and to the of and to The advanced to on and the of the In the the to and the was but by the that the and in the evolution on the one in the the and probably the of that the earth has on the in the were still with and were still with at the of the when the and the were the is on this with a that is The the and and a of in the of the the as had the these had the and of the is still a The from the or early and these were advanced and no of a which could have to the have it be that were in in the early and probably in the the were still in a the the in such as the the and the of the and with the and it may be that the and the were of a be that this was it be that it had the of excreting it may have a that to as may was the in the from this it is to be that there were in and The was one of the of of the and the a the to be as a the to from one water to A could adapt to one of these of the us that the to in adaptation to to in adaptation to The evolution of a in the of the which in turn a in this in an of through the and this an for water by it from the of in the by the for is that the of the for water was a of the evolution of the state, which evolutionary may have the of excretion had in the the kidney had as an adaptation to it as an adaptation to as for the of water which it the to dry for dry with the dry the could the could and when the and the the and was the change in that the it may have the of the to this of and that led to the to both could This in the that in the kidney the are of a the in a the in such a the kidney is the and the the retaining the of the it can certain a The to the kidney in the is probably a of evolution by the of for the were from that were from the at the of the the the to the In the the to in the and the with both at the time and the of or In the a or to the which in the in turn the that and the an and such of the as were to the and to in the From a to there in the such as by and and by and by and by and to be man. a of the of The kidney is in and in but the kidney in from that organ in the and is that in the kidney has in the kidneys of the can with a back in the the of the kidney, be to find that it is from a In it is in many begins by of water the for this one of the by the of this of be This of is to the one the is to a large of water of the at the the are to the by of this water from and kidney are in on one of in to a state, but the in this on an of the kidney has to do work when it the of as the it and and the were to to the of kidney, in to the excretion of be no work at In of the of the and of a of and of to of and be from in the by from the is to a for is the to the by which The which the kidney is to is The such as is in of but of the the are of this back the again, that in of the the of the excretion is are certain which have the few which the kidney with it in of the that it has the kidney be to such as as it the in the and which it has excreting for and of The kidney is These from to such as the of the and of to and is a of the and As of on the the the are integrated of and an to that which or the of kidneys for a time to and is are those that the kidney was to the or to in an I I that the kidney is a but I that it was to or to the of the or to the physiological of in I that the kidney the of that it and it the in the which that this kidney has a certain and it that to or or but to the that the earth is an with a to the that for years have and and to the and and and and the of that have the vertebrates from sea freshwater, the dry land, from one to in of the and for one or to find when and and it and but by the in I in a presented the of the nature and physiological of in a A of of is one of the animal that have the by and of which the has at the time the to and the useful to as as the of This which with and which is and has in the of the a of and is an in the of which find the the which are of it in the and the which the of it to the the and the I for to the work of Homer W. and the University of Kansas School and Press and of University Press for in this for the

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