Vitals

Our bodies are made of cells. Mitochondria are the parts of our cells that
generate the energy-rich molecules we use every instant of our lives.
Billions of years ago, mitochondria were bacterial invaders, parasites of
early cells. They joined forces with their hosts; now they are essential.
"My mitochondria compose a very large proportion of me. I cannot do
the calculation, but I suppose there is almost as much of them in sheer dry
bulk as there is the rest of me. Looked at in this way, I could be taken for a
large, motile colony of bacteria, operating a complex system of nuclei,
micro tubules, and neurons, for the pleasure and sustenance of their
families, and running, at the moment, a typewriter."
--Lewis Thomas, "Organelles as Organism," 1974