As animals, we owe a lot to the single-celled organisms that came before us. These are the organisms that laid the chemical groundwork for how we live, from the DNA and proteins within them to the molecules they released into the environment. There’s something humbling about looking at our hands or feet and imagining the mixture of cells within them, and realizing the lessons that keep those cells bound together physically and biologically are rooted in a very ancient study in cooperation.