confusion

To be confused is to be afraid, and fear weakens reasoning ability - which is why politicians and the media like to confuse you. It's hereditary. If you are confused whether the waving grass is the wind or a lion you may be dead.
Smiles arose from the same thing. Baring of teeth is normally a show of aggression/defence with animals; it is done for that reason when confused (because of fear). However, humans managed to harness many things which are confusing and potentially deadly; for example lethal weapons like bows and arrows, and as well of course, fire. So there is also an evolutionary benefit to the discovery of confusing new things for us. That novel moment of "aha" when putting together 2 things that make an absurd conclusion produces the same result for us; joy, but in another part of the brain, confusion and fear.
Laughter is basically you defending yourself with your teeth against the fake lion in the windy grass and then leaping to the joyous and momentous conclusion that, instead, YOU can eat the GRASS.