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Kidneys

Marcel F. Williams has devoted some study to the form and function of kidneys in Homo sapiens. In his paper: "Marine adaptations in human kidneys", he notes that humans are the only primate to possess multi-pyramidal lobulated medullas in their kidneys (fig.3), while most terrestrial animals have uni-pyramidal kidneys (fig.1). Conversely, he says, kidneys with multiple medullary pyramids are nearly universal in marine mammals. In salt water environments, renal medullary pyramids appear to function as a means to increases the rate that salt and nitrogenous waste is excreted by increasing the surface area between the cortex and medulla. They appear to have no functional value in freshwater environments, but there are some freshwater aquatic mammals with renal pyramidal kidneys. These can be phylogenetically traced to either marine ancestors or aquatic ancestors that frequented marine environments.

There are a number of terrestrial mammals with multi-pyramidal kidneys such as elephants, bears, and rhinoceroses but they also appear to have
had semi-aquatic ancestors that frequented marine environments. A notable exception is the Bactrian camel and Arabian camel (dromedary) whch do have multi-pyramidal kidneys (fig.2), but these animals live in very dry environments where they consume salt rich foods (halophytic plants) and drink water from brine pools with natural salinities higher than seawater.

The numerous vestiges of aquatic adaptations in the human body in addition to the abundant distribution of corporeal salt excreting eccrine sweat glands and the excretion of salt tears in humans, strongly suggest that the multiple medullary pyramids of the human kidneys probably evolved as an adaptation to a coastal marine ecology rather than to a xeric terrestrial environment [1].

Sheep kidney

Fig.1: Longitudinal section of a sheep kidney, displaying the typical uni-pyramidal morphology found in most terrestrial mammals.

Camel kidney

Fig.2: Longitudinal section of the kidney of a dromedary camel (Camelus dromedarius), displaying its renal pyramids.

Human kidney

Fig.3: The medullary of pyramids of a longitudinal sectioned human kidney.

Morphological evidence of marine adaptations in human kidneys.

Williams, MF

Abstract

Amongst primates, kidneys normally exhibiting lobulated, multipyramidal, medullas is a unique attribute of the human species. Although, kidneys naturally multipyramidal in their medullary morphology are rare in terrestrial mammals, kidneys with lobulated medullas do occur in: elephants, bears, rhinoceroses, bison, cattle, pigs, and the okapi. However, kidneys characterized with multipyramidal medullas are common in aquatic mammals and are nearly universal in marine mammals. To avoid the deleterious effects of saline water dehydration, marine mammals have adaptively thickened the medullas of their kidneys--which enhances their ability to concentrate excretory salts in the urine. However, the lobulation of the kidney's medullary region in marine mammals appears to be an adaptation to expand the surface area between the medulla and the enveloping outer cortex in order to increase the volume of marine dietary induced hypertonic plasma that can be immediately processed for the excretion of excess salts and nitrogenous waste. A phylogenetic review of freshwater aquatic mammals suggest that most, if not all, nonmarine aquatic mammals inherited the medullary pyramids of their kidneys from ancestors who originally inhabited, or frequented, marine environments. So this suggest that most, if not all, aquatic mammals exhibiting kidneys with lobulated medullas are either marine adapted--or are descended from marine antecedents. Additionally, a phylogenetic review of nonhuman terrestrial mammals possessing kidneys with multipyramidal medullas suggest that bears, elephants and possibly rhinoceroses, also, inherited their lobulated medullas from semi-aquatic marine ancestors. The fact that several terrestrial mammalian species of semiaquatic marine ancestry exhibit kidneys with multipyramidal medullas, may suggest that humans could have, also, inherited the lobulated medullas of their kidneys from coastal marine ancestors. And a specialized marine diet in ancient human ancestry could, also, explain the reactivation and enumeration of corporeal eccrine sweat glands and the copious secretion of salt tears. The substantial loss of genetic variation in humans relative to other hominoid primates, combined with the apparent isolation of early Pliocene human ancestors from particular retroviruses that infected all other African primate species, may suggest that such a semiaquatic marine phase, during the emergence of Homo, may have occurred on an island off the coast of Africa during the early Pliocene. [2]


References:

1. Williams, Marcel, F. Was Man More Aquatic In The Past? Fifty Years After Alister Hardy Waterside Hypothesis Of Human Evolution, Eds. Vaneechoutte M., Verhaegen M., Kuliukas A. eISBN: 978-1-60805-244-8, 2011. Ch 8: Marine Adaptations in Human Kidneys (pp 148 - 155)
2. http://www.ncbi.nlm.nih.gov/pubmed/16263222


 
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