Water

My discussion yesterday may seem entirely reasonable and logical, but there is one major loophole in the assumption - and the actual fact as well.

The very point is: terrestrial vertebrates did not come from marine animals, but freshwater ones.

Of course freshwater fishes originated from the sea. Nonetheless, by settling in an environment free of sodium chloride, the animal is adapted to an entirely opposite logic to handle minerals - to conserve ions, and get away with the water.

(Honoured to be a nephrologist, the latter adaptation results in the development of glomerulus - the very structure that filters a huge volume of blood and excrete it as some dilute solution. Seawater fishes, on the other hand, have no glomerulus; metabolic waste is actively secreted by kidney tubules - an ability that most terrestrial animals still preserve.)

The real problem, therefore, becomes: why should freshwater fishes - rather than marine ones, which obviously out-numbered the former - get to the shore and start walking ?