PASS THE GRAVY: Of all the many traits shared by humans and rats, laughter is my favorite. Scientists have discovered that rats giggle when you — or other rats, for that matter — tickle their bellies. I have not tried this myself, but I will take their word for it. But this information flies in the face of everything scientists thought they knew about laughter.
We were told that laughter evolved when our brains got really big, and when we learned to stand upright and walk on two feet. Laughter was supposedly invented 10-16 million years ago as a means to create a joyous sense of unity among people otherwise struggling to shoehorn themselves into the increasingly complex social structures upon which their very survival depends.
In other words, laughter is a necessary social lubricant that keeps us from killing one another. Or, perhaps more immediately, from getting killed.