Is there a rule that prohibits animals (like a gorilla) from playing professional sports?

Maybe there is something in the CBA's of each league that says HUMANS only...but that seems like something that could have easily been left out. I mean, if you found a loophole, and put a trained gorilla on an NFL team as a lineman...or as an NBA center, who exactly is going to knock them down or stop them from dunking? I'm just sayin...I'd really like to get some clarification on this. I think the GBA and the GFL are not far away from becoming a reality.