But, while evidence of previous life forms does play a central part in our understanding of evolution, it isn't necessary to go on a dig to see it in action -- you might just look inside your mouth.

We all learn that human beings have thirty-two teeth. They aren't all the same, though; the pointy incisors in the front are easily distinguished from the flatter, double-edged bicuspids that run along the side. Our teeth serve different functions: those incisors are great for biting and holding on, while the bicuspids do more of a saw-motion for chewing.

Now here's something you may not know: strictly speaking, it isn't true that everyone has thirty-two teeth. Even excluding people with dental problems, some folks have only thirty, or even twenty-eight. The ones that are missing are often the farthest back in the mouth. Where did they go?

One likely answer is found in evolution. A long time ago --say ten thousand years -- human beings ate much more raw meat. With a rough diet, you need lots of grinding teeth to help prepare your food for digestion. Later, with the gradual advent of cooked foods, not all those teeth are needed anymore. If someone is born without them, she or he doesn't suffer any ill consequence; a scientist would say there's no evolutionary pressure to keep those teeth.

The result? Many thousand years later, some of us don't have as many grinding teeth as others. The dimensions of the human mouth, the jaw, and the skull are evolving -- right underneath your nose.  

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A Moment of Science®