Vegan...probably not. Vegetarian including dairy and eggs? Almost certainly. While eating too much meat is problematic for our health, not having enough protein is a serious problem for us. You can get enough from beans, legumes, and other non-animal sources, but having a boost from dairy and eggs should give anyone enough to live on.
My personal recommendation is pescatarian, for the bonus of healthy fats found in fish.
However, there has been a lot of anthropological and archeological studies that indicate our intelligence and survivability increased after we started eating animal flesh. So if you like being smart...eat meat?
http://www.livescience.com/23671-eat...-us-human.html