Is it healthy for a vegan to eat only fruits and vegetables?

NO: The best vegan protein sources are nuts, seeds, beans & quinoa. It is very difficult to get all the protein you need solely from fruits & vegetables. But the clincher is essential fatty acids- i don't think there are any significant sources of omega 3 fats in fruits & veggies.If you aspire to a paleo diet it is not necessary to have beans or grains but you need nuts & seeds (hemp seeds esp. Good).