Waste...
First, man is one of the few creatures in the world that removes his fresh feces from the environment.
< Many carnivores bury their Faeces for obvious parasite-reduction purposes>
Think of what whales do. If we defecated into our atmosphere it would get pretty weird.
< What is a fart, except a direct expulsion of waste gases into our atmosphere?
As we know, some do it bigger and better than others (You should meet my mate "Tangles"msometime - He has a Ph.D. in Flatulence!) >