Our closest cousins in the animal kingdom are also some of the most vital animals on the planet. They may be our closet link when it comes to understanding human history, but their true worth lies in the ecological role they play in the world. In tropical and sub-tropical climates, primates are what is known as a keystone species. They essentially act as the forests' gardeners, by dispersing seeds and pollen through their daily habits such as eating and - uh - defecating. Why is this important? Because tropical rainforests actually influence global rainfall patterns, so the trees that primates help grow actually work to sustain life on Earth.