Maybe when they work together in symbiosis, as all complex life form do now. Each part of an animal (or human) is really a complex system of cells working in symbiosis with other masses of cells--the skin, organs/gut/excretion, brain, nervous system, and sensory systems work in concert with each other to make up the entirety of of the complex organism, but at some point in our past, those masses of cells performing those functions were their own entity separate and apart from the others.

The great mystery is why they decided to work together at all.