Morphological evidence of marine adaptations in human kidneys
Marcel F Williams 2006 Med.Hypoth.66:247-257
doi 10.1016/j.mehy.2005.09.024

Amongst primates, kidneys normally exhibiting lobulated, multi-pyramidal, medullas is a unique attribute of the human species. Although kidneys naturally multi-pyramidal in their medullary morphology are rare in terrestrial mammals, kidneys with
lobulated medullas do occur in: elephants, bears, rhinoceroses, bison, cattle, pigs, 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 & the enveloping outer cortex, to increase the volume of marine dietary induced hypertonic plasma that can be immediately processed for the
excretion of excess salts & nitrogenous waste.

A phylogenetic review of freshwater aquatic mammals suggests:
most, if not all, non-marine 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 multi-pyramidal medullas suggest that bears, elephants & possibly rhinoceroses, also, inherited their lobulated medullas from semi-aquatic marine ancestors.

The fact that several terrestrial mammalian spp of semi-aquatic marine ancestry exhibit kidneys with multi-pyramidal medullas, may suggest:
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 & enumeration of corporeal eccrine sweat glands & the copious secretion of salt tears.

The substantial loss of genetic variation in humans vs other hominoids + the apparent isolation of early-Pliocene human ancestors from particular retro-viruses that infected all other African primate spp may suggest:
such a semi-aquatic marine phase, during the emergence of Homo, may have occurred on an island off the coast of Africa during the early-Pliocene.

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