Why is it so important for animals to keep the concentration of their body fluids constant

Organisms in aquatic and terrestrial environments must maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion (getting rid of metabolic nitrogen wastes and other substances such as hormones that would be toxic if allowed to accumulate in the blood) through organs.

Explanation:

the fossil record provides evidence of a common ancestor to many species.