I Know What You Ate Last Summer

By Hanna Hayth

If you’re anything like me, and you have a diet that you’re not exactly the proudest of, I have bad news for you. Long after you’re gone, scientists are able to use your bones to determine what your diet was like.

But that’s not all!

Your bones give away so much information about you that you might not even mean to be giving away like that. When it’s just your bones left, you might as well be an open book. All your cards – or bones – are on the table.

If scientists want to know where you were born, what kind of foods you ate, or even when you started eating solid foods, they can find out for themselves.

How Is This Possible?

The ratio between different isotopes of the same element tell us a lot of information. They are specific depending on what you are looking at, for example, food sources or different climates. These ratios of isotopes act like a signature, the same way how we sign our name is unique to only us.

This is done by a method referred to as mass spectrometry. In fact, you only need 5 grams of a bone sample to determine information about several different isotopes, such as carbon, nitrogen, strontium, and oxygen!

What Can These Isotopes Tell Us?

Carbon isotopes give information specifically about what kind of diet a person had while they were living. There are three different types of pathways for photosynthesis that distinguish plants from each other based on their isotope ratio of carbon. These are C3, C4, and CAM plants. C3 plants cover a wide variety of plants, but some examples include rice, wheat, potatoes, spinach, apples, and peaches. C4 plants are a bit more specific as they include forage grasses such as maize. Finally, CAM plants include cactuses and pineapple.

One example of this in action was a study done by Beaumont and Montgomery in 2016 that showed that the Great Irish Famine showed a shift from potatoes, which is a C3 plant, to maize, which is a C4 plant. This was done by examining the teeth, which are formed earlier, and the ribs, which are formed later.

This information can also be combined with nitrogen isotopes to give a better picture. This gives you a better idea at where you are on the food chain as the higher up you go, the higher the levels of 𝛿¹⁵N. Another example of this is that terrestrial food sources have a low 𝛿¹⁵N while marine food sources have a high 𝛿¹⁵N. This is shown in the graph below.

The combination of nitrogen and oxygen isotopes can also tell you when a baby is weaned off of breastmilk and started to eat solid foods. This is because a baby who is breastfeeding is essentially “higher on the food chain” than the mother, so the nitrogen levels are higher, but. As the baby is weaned, the levels will decrease. Also, oxygen levels from water are high when a baby is breastfeeding, but lower when water comes from other sources that are “isotopically lighter”. Tracking these levels in the teeth gives an idea of when that person started eating solid foods early in life.

Strontium isotopes give a bit more of an interesting picture. When our teeth are developed, the strontium isotopes that are absorbed are from where we were living at the time, and that’s it. There’s no second chances to absorb different isotopes. Bone cells, on the other hand, are constantly changing, meaning that different strontium isotopes are being absorbed than the ones when they were first made. By comparing both of these pieces of information, you can figure out where someone grew up, and then where they lived later on in life.

What Does This All Mean?

Examining isotopes in bones give us a better picture of how we used to live in the past. We can learn more about how humans used to be, and what they were up to before history was more easily documented like it is today. We can learn about migration patterns, past ecosystems, and the diet of a population. It’s a great example of history and science working together.

Now that you know all about how your bones know all about you, I’ll leave you with a warning. If your burial site ever becomes an archaeological dig, just know that your eating habits might be published in a scientific article someday.