This tendency has led to the suggestion that humans are paedomorphic primates. It's not necessarily a new hypothesis--a man named John Fiske made the argument as early as 1884--but it continues to be a reasonable one. There's more than just our playful nature that suggests eternal youth has played a role in our evolution. One of the defining characteristics of humans is our creativity, our willingness to try new things and new ways of interacting with our environment--all traits normally associated with youth.