49.4: Evolution of the Vertebrate Kidney

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Evolutionary pressure and the need to regulate body fluid homeostasis have led to pre-adaptation of the vertebrate kidneys to different environment conditions and to development of three kidney forms: the pronephros, mesonephros and metanephros. The kidneys of amniotes are unique compared to other internal organs, since three different kidneys are sequentially developed during embryogenesis, replacing each other and reflecting the evolution of the kidneys in vertebrates.^[28]

At the very beginning of vertebrates, when they evolved from marine chordates, their evolution probably took place in fresh or slightly saline water. There is a hypothesis according to which marine fish received their kidneys after a previous adaptation of the kidneys to fresh water. As a result, early vertebrates developed renal glomeruli capable of filtering blood and perhaps tubules that reabsorbed ions. Excretion of excess water from the body is the main characteristic of the pronephros in the case of species in which it develops into a functional excretory organ. In some species, the pronephros is functional during the embryonic stage of development, representing the first stage of kidney development, after which the mesonephros develops. The mesonephros probably appeared in the course of evolution in response to the increase in body mass of vertebrates, which also led to an increase in blood pressure.

The evolution of the kidneys, along with the evolution of the lungs, allowed vertebrates called amniotes to live and reproduce in terrestrial environment. Metanephros, the permanent kidney of amniotes, has the unique ability to efficiently retain water in the body. In addition to water conservation, terrestrial life also required maintenance of salt levels in the body along with the excretion of waste products. The first class of animals to become fully terrestrial without a larval stage were the reptiles, which were the first amniotes. The kidney takes a key role in maintenance of the constant internal environment. The relative ionic composition of the extracellular fluid is similar between marine fish and all subsequent species. Therefore, it can be said that the kidneys made it possible to preserve approximately the same composition of extracellular fluid in vertebrates as it was in the primordial ocean.

Text excerpted from Wikipedia https://en.wikipedia.org/wiki/Kidney_(vertebrates) Accessed 14 November 2023