Considering there are many things we find humorous but never find threatening and many cases where we are relieved but don't find anything humorous, that theory

Considering there are many things we find humorous but never find threatening and many cases where we are relieved but don't find anything humorous, that theory seems dubious. A better explanation would be that we laugh at contradictions so as to signal to others that we recognize the contradictions. This both serves to demonstrate our intelligence and social skills (someone who gets jokes quickly comes off as smart and gregarious, the person who doesn't laugh seems either slow or weird), as well as to communicate information (if you laugh at someone who breaks a social rule, they very quickly learn the rule. You laugh at children doing dumb stuff, as well as friends and family, but random adults doing the same thing just appear sad). Especially in a pre-linguistic society, this ability to convey information is extremely important, and analogues can be seen in many species.