The theory of evolution proposes that life originated in an aquatic environment. Evidently, it is often observed that biochemical pathways like the urea cycle evolved to adapt to the changing environment with the gradual evolution of several terrestrial life forms. Most aquatic animals excrete ammonia and are called ammonotelic organisms; this process of excretion is known as ammonotelism. Ammonia is a highly toxic metabolic waste of protein metabolism which is produced in liver by the process of deamination and requires a large amount of water for its excretion. Terrestrial animals excrete urea as a nitrogenous waste product in order to conserve water. The excretion of urea is called ureotelism and such animals are ureotelic. Urea is produced in the liver from NH3 and CO2. Similarly, in the case of amphibians, the aquatic tadpole larvae of frogs are ammonotelic, while the terrestrial adult frog is ureotelic.