In many biological structures proteins are simply components of larger molecular machines.
This fact immediately suggested a singular event - that at some time in the distant past the universe began expanding from an extremely small size. To many people this inference was loaded with overtones of a supernatural event - the creation, the beginning of the universe.
Throughout history there have been many other examples, similar to that of Haeckel, Huxley and the cell, where a key piece of a particular scientific puzzle was beyond the understanding of the age.
In Darwin's time all of biology was a black box: not only the cell, or the eye, or digestion, or immunity, but every biological structure and function because, ultimately, no one could explain how biological processes occurred.
Skin is made in large measure of a protein called collagen.
Proteins are the machinery of living tissue that builds the structures and carries out the chemical reactions necessary for life.
