Smile! It makes you feel good, and it helps others to do the same. I found some interesting ideas about how did our ability to have facial expressions arose as I researched mammals lately. It all started long ago with the first synapsids, the lineage that led to mammals. Although Dimetrodon is a synapsid on a different branch of life than mammals, that animal has a piece of the puzzle on smiles.

“Dimetrodon” means “two measure teeth” or “two long teeth.” Compared to its reptilian sister lineage, the synapsids were experimenting with different shapes, sizes, and therefore, functions of teeth. The reptiles, even the dinosaurs, have more uniform teeth. This shows two different strategies for eating and digestion. Synapsids developed the ability to chew their food. Reptiles, with only a few exceptions, are bulk feeders, meaning that they eat large pieces of food (like a snake eating a rat) and digest them slowly or they use a muscular stomach to grind the food (like birds and other dinosaurs).

The study of mammal evolution focuses a great deal on the study of teeth. Not only do teeth fossilize well, they show the changes as mammals arose and lineages developed. A key characteristic of mammals is teeth that can do a better job of biting, chewing, and grinding.

What’s the big deal about chewing our food? It has to do with how fast the food can be broken down by digestive enzymes. All the action of enzymes is at the surface of the food. Lots of surface area equals fast digestion. And why would one want to digest food quickly? Quick digestion leads to quick metabolism and production of molecules that supply energy to cells. It is necessary if the animal is warm-blooded and has a large brain. Maintaining our body temperature and feeding our brains are very expensive in calories.

OK, we chew our food and digest it quickly, maintain our warm body temperature, and feed our expensive brains. What does that have to do with smiling? In order to chew food, an animal needs muscles that operate the jaw. That seems straight forward enough. Those muscles must attach to the head, and an array of muscles around the head brings another possibility – the ability to have facial expressions.

Have you ever seen a facial expression on a fish, amphibian, reptile, or any of the avian dinosaurs, aka birds? They don’t chew their food, and they have few, if any, facial expressions.

Most mammals, on the other hand, have many facial expressions. Evolution is not a straight line, climb-the-ladder sort of thing, so some mammals have lost the ability to have facial expressions. Or perhaps their expressions are so subtle that we can’t see them. I’m thinking of whales and dolphins. Their use of sound to communicate probably works better than small visual changes because of their aquatic environment.

The social mammals that live on land have many ways to communicate. Many mammals use the position of their ears to express themselves. A snarl shows teeth and speaks loudly across many mammal lineages. Facial expressions are important and versatile ways of communicating for the more visual lineages, like primates. Our smiles likely started millions of years ago, judging by the use of facial expressions in our fellow great apes.

I recommend a generous bestowing of smiles on your fellow humans. You can celebrate that your ancestors took the road to chewing food as you smile. It is a fine thing to celebrate the wonder of our long journey as a species as well.