The long battle between humans and infectious microbes has left its marks all over us. It shows up most obviously in the way our bodies are constructed. The thousands of species of bacteria that swarm over us cannot penetrate our multilayered skin. Entry points, such as the eyes and the nose, are bathed in moisture to help flush out pathogens. The lining of our lungs releases bacteria-killing compounds. Viruses that manage to infect cells are greeted by proteins that attempt to shred them into genetic confetti. Any pathogen that sneaks past all these defenses then faces an army of immune cells, which can devour and destroy the invaders. Immune cells can also manufacture antibodies, which allow them to launch swift attacks if they encounter the same infection elsewhere in the body.
This elaborate defense system dates back billions of years. Our single-celled ancestors were infected with viruses; when they got bigger they were infected with bacteria; and after they evolved guts, those guts were infected with worms. Any mutation that offered even a little protection against those pathogens had a chance to be favored by natural selection. Over thousands of generations, mutation upon mutation built up our diversity of immune cells, signals, and weapons. There was never a point at which our defenses stopped evolving, because the pathogens were evolving as well. New generations of invaders slipped past our lines of defense, spurring the evolution of immune upgrades.
But it is not just our bodies that have been shaped by this tug-of-war. A number of scientists now argue that the battle against disease has left an indelible imprint on our minds as well.
