When you meet someone you know, you extend your right hand for a handshake.

Why your right hand? Ninety percent of people world-wide are right-handed, and many of our customs reflect this. Imagine, however, that it's a hundred thousand years ago, and you met a Neanderthal of your acquaintance. You want to shake hands, but which hand should you use? Were Neanderthals right or left handed?

At first this question might seem impossible to answer. Neanderthals, one of our closest cousins on the evolutionary family tree, have been extinct for thousands of years. While Neanderthals had a complex culture, and an average brain size even larger than ours, they did not think to leave behind any etiquette books, or statistical studies of Neanderthal handedness. How can an anthropologist look at a Neanderthal's remains and decide which hand it preferred to use?

The answer is inside Neanderthal's mouth. Teeth are the most durable part of the body, retaining their integrity much better than the bones of the skeleton. Examining teeth from twenty different Neanderthals under an electron microscope, anthropologists found small scratches on the outward surfaces of the front teeth. These scratches, which are absent on the back teeth, may well have been caused by sharpened stone tools used to manipulate food in the mouth, the Neanderthal equivalent of silverware. Eighteen of the twenty Neanderthals studied had tooth scratches that angled in from the right. The remaining two had left angled scratches.

What does this mean? If these scratches were caused by right and left handed tool use, it would could be that, just like us, ninety percent of Neanderthals were right-handed!