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The profound physiological differences between land and marine mammals can be divided into five basic categories:
1. Weight-bearing: Sea-dwelling creatures have no problem in bearing their own weight in the sea. However, most land-dwelling creatures consume 40% of their energy just in carrying their bodies around. Creatures making the transition from water to land would at the same time have had to develop new muscular and skeletal systems (!) to meet this energy need, and this could not have come about by chance mutations.
2. Heat Retention: On land, the temperature can change quickly, and fluctuates over a wide range. Land-dwelling creatures possess a physical mechanism that can withstand such great temperature changes. However, in the sea, the temperature changes slowly and within a narrower range. A living organism with a body system regulated according to the constant temperature of the sea would need to acquire a protective system to ensure minimum harm from the temperature changes on land. It is preposterous to claim that fish acquired such a system by random mutations as soon as they stepped onto land.
3. Water: Essential to metabolism, water needs to be used economically due to its relative scarcity on land. For instance, the skin has to be able to permit a certain amount of water loss, while also preventing excessive evaporation. That is why land-dwelling creatures experience thirst, something sea-dwelling creatures do not do. For this reason, the skin of sea-dwelling animals is not suitable for a non-aquatic habitat.
4. Kidneys: Sea-dwelling organisms discharge waste materials, especially ammonia, by means of their aquatic environment. On land, water has to be used economically. This is why these living beings have a kidney system.
Thanks to the kidneys, ammonia is stored by being converted into urea and the minimum amount of water is used during its excretion. In addition, new systems are needed to provide for the kidney's functioning. In short, in order for the passage from water to land to have occurred, living things without a kidney would have had to develop a kidney system all at once.
5. Respiratory system: Fish "breathe" by taking in oxygen dissolved in water, which they pass through their gills. They cannot live more than a few minutes out of water. In order to survive on land, they would have to acquire a perfect lung system all of a sudden.
It is most certainly impossible that all of these dramatic physiological changes could have happened in the same organism at the same time, and all by chance.
