To amuse people and make them laugh
Right after humans learnt to laugh.
God, when he created man. Funny people created jokes.
That depends on what joke you're talking about. Try to be more specific, and someone might be able to answer you.
as cavemen grew and evolved the found things funny and as words evolved different words and phrases set of thoughts and nerves that made them laugh and now we have Alan carr (boo!) good to help!