The evolution of multicellularity made it possible to enclose and therefore create an internal fluid environment that bathed all cells of the organism. Since that internal environment provided for all of the needs of the individual cells, individual cell lines could become specialized as to what they contributed to the internal environment. Just as with people, specialization in one function usually goes hand in hand with losses in other functions. But those functional losses in each cell line were compensated for by the constancy of the internal environment. The evolution of organs composed of multiple tissues and the evolution of informational systems to control organ activities improved the abilities of animals to maintain a constant internal environment. In essence, the evolution of physiological systems to maintain various aspects of the internal environment made it possible for multicellular animals to become larger, and more complex, and allowed them to occupy many different environments.

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