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What if I told you there’s a simple way for me to figure out every place you lived for the first, say, twenty years of your life? Because there is one. All you’d need to do is let me borrow a couple of teeth, so I could measure how much of the element strontium is in them.

If we actually tried this, the tooth-extraction part probably wouldn’t be too much fun for you. And there are also plenty of easier ways to get the same information... like by checking social media, or just asking you. However, the technique is super helpful for anthropologists who study ancient humans, because it’s not like skeletons can just tell us what their lives were like.

And knowing where a person lived can tell you a lot about them. For example, if someone grew up far from where he died, that might mean he was part of an invading army, or an immigrant seeking a better life. By the same token, if a person lived a long life and was buried in the same place where she was born, it might be logical to guess that she had a pretty cushy life, and didn’t need to move to find food or flee from enemies.

Your teeth hold a lot of information about your life story, and scientists are learning how to read it, thanks to the properties of strontium. For one thing, there’s more than one isotope -- or form -- of strontium. For anthropologists, the most useful are Strontium-87 and Strontium-86. They’re the same element, but 86 has one less neutron. But another thing that’s cool about it is that most strontium doesn’t start out as strontium. Instead, it’s one of the products that forms when another element, rubidium, decays. So, the place where you live has its own ratio of strontium-87 to strontium-86, based on what your local geology is like.

For example, older rocks have had more time to decay, so they’ll have more strontium-87 in them. Rocks that have lots of clay or granite in them also tend to be high in rubidium, so they’ll have more of the 87 isotope, too, and, whether you like it or not, all of this stuff in your local soil and rock ends up in your diet. It filters into the water, and the plants, and then travels up the food chain until it gets into your mouth. And then, the strontium you eat and drink gets incorporated into your teeth!

The enamel coating your teeth contains lots of calcium, and strontium is like calcium in a lot of ways. It reacts with a lot of the same compounds, and it’s a similar size, so, when your body’s making new teeth throughout your childhood, sometimes it’ll incorporate strontium instead of calcium -- and the strontium isotopes will show up in the same ratios that they had in the water that you drank. Anthropologists can measure those ratios in your teeth, and use that info to learn where on the planet you were living when those tissues formed.

For example, the strontium in the molars you got when you were around six will tell anthropologists where you lived for the first six or so years of your life, and the ratio of isotopes in your wisdom teeth will reflect where you lived from around the age of nine, when they start forming, until your body was done making the enamel -- which can be as late as your 20s. And once a tooth is formed, it’s done. It’s not getting any more new calcium, so the strontium ratio is set forever. So if anthropologists find a tooth from a skeleton, they can figure out where the person was born, and where they grew up, even if those were different places.

This is especially handy if the placement of the skeleton seems weird. Consider this case study: a mass grave in the ancient settlement of Cahokia, near modern-day St. Louis. The grave contains the remains of 39 people who were buried about a thousand years ago, many showing signs of some kind of violence, like stone points in their bones. For a long time, researchers thought these people were from outside of Cahokia, and were killed as war captives, or intruders, but recently, they tested the strontium in the victims’ teeth and it turned out that most of them were actually from Cahokia, born and raised in the same town where they were killed! This has made some anthropologists rethink their theories about the nature of life and death in America a thousand years ago -- just one of the discoveries made possible by studying a little element in human teeth. Strontium, man: It knows where you’ve been.

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