We can contrast two distant relatives: the intestinal bacterium Escherichia coli and its host, ourselves. We span the spectrum of complexity in living organisms: the bacterium has minimal capability for perceiving and reacting to short term changes in its environment, whereas the major portion of our body is devoted to these tasks.
E coli commit less than 5% of their molecular machinery to motion and perception, allowing the simplest responses...Our bodies, in contrast, are built for specific, directed motion under the control of detailed, reasoned perception. The bulk of our body weight is dedicated to sense, reaction and motion. Cells in our retina are filled with arrays of opsin proteins for sensing light, light that is focused by layers of eye lens cells packed full of clear crystallin proteins. Cells in our skin spin enormously long strands of keratin protein into hairs, and other cells sense their slightest movement. These and other sensory data are transmitted and processed by nerve cells that carry electrical currents propagated by proteins and insulated by concentric layers of lipid. Fine control of movement is accomplished by an enormous skeleton of mineralized bone cells, moved by muscle cells filled with proteins that do nothing but contract, all glued together by connective tissue cells that build tough layers of sugar and protein. However, the common thread of life on Earth still shows through the diversity, tying the simplicity of the bacterium to the complexity of our bodies. All of these unique molecular machines are built of the same four molecular components -- proteins, nucleic acids, lipids, and polysaccharides.-- from The Machinery of Life by David S. Goodsell.
The title of this post is from an essay by Ken MacLeod. See also In the Waiting Room by Elizabeth Bishop.