Eco is an action strategy game of sorts, which sets the player in charge initially of a small single-celled organism and challenges the player to guide its evolution.
The initial tasks are to roam around finding food and then find a potential mate. Depending on the organism's status, different predators will be after it - a radar is displayed to indicate where these are at any given time. Once this is done, the player is taken to a screen displaying the species' genes, and has the chance to modify one of them, thus resulting in evolution. Eventually the organism can evolve far enough to become plant, insect, fish, or even human.