Apatite is a mineral that gives structure to bones and teeth, and can be used to determine where you have traveled based on what you have eaten—apatite records your appetite! Apatite is the most abundant mineral in your body and is composed primarily of calcium (Ca) and phosphate (PO4) [as Ca5(Po4)3(F,OH)] that are bound together in a rigid crystalline framework (Fig. 1). Joined together with collagen (your body's most abundant protein), tiny apatite crystals provide the stiffness in bones that support your body and the hardness in teeth that allow you to eat tough foods. One reason that we find fossil skeletons of dinosaurs today is because they contain apatite, which is readily preserved for millions of years.
But apatite is more than just a strong mineral. The ability for elements to substitute in trace quantities for calcium (Ca) and hydroxyl (OH) in apatite (Fig. 1) can provide paleontologists and archeologists with a life-long record of body chemistry. Trace amounts of the element strontium (Sr) provide a special tool for tracking ancient animal movements through analysis of the ratio of two different strontium isotopes (also see the “Nitty Gritty Details” box)—87Sr and 86Sr. So, how does this work? Geochemically, “you are what you eat,” meaning that your body's chemistry, including the apatite in your skeleton, reflects the composition of the food you eat and water you drink. The food and water that you consume contain trace amounts of Sr. And the two flavors of Sr, 87Sr vs. 86Sr, occur in different amounts, depending upon the local geology and soils in which plants grow. So plants absorb Sr (and other elements), animals eat plants, and then humans eat plants and animals, which all happen to contain lots of Sr. The ratio of 87Sr to 86Sr (symbolized by 87Sr/86Sr1) depends on rock type: old igneous and metamorphic rocks have high 87Sr/86Sr (meaning there is more of the 87Sr isotope relative to 86Sr), whereas limestones and young volcanic rocks have low 87Sr/86Sr. So, if an animal moves around during its lifetime, say between areas underlain by limestone vs. old granite, where food and water 87Sr/86Sr values are different, the animal's 87Sr/86Sr ratio will change correspondingly and be recorded in its apatite. These differences, captured in tiny samples of apatite, can be easily measured by a mass spectrometer.
Archeologists use these types of Sr isotope changes in bioapatite to reveal ancient human movements. The key here is that different tissues, such as the bioapatite in bones and teeth, grow and match sequential changes in chemistry that occur at different times, much in the same manner that tree rings, for example, grow at different times. So by analyzing different tissues, and knowing when they equilibrate with the body, an isotopic history of location relative to soil with different 87Sr/86Sr can be developed. A famous application involves “Ötzi,” a mummified ~46 year-old man who lived about 5000 years ago in the central European Alps (Müller et al. 2003). Analysis of his teeth, bones, and intestinal contents reveal that he generally lived within ~60 km of the discovery site, along Alpine valleys to the south that are underlain by old metamorphic rocks, known as gneisses and phyllites. But he also moved around within that area (Fig. 2). Such analyses provide clues about the prehistoric lifestyle of the only human we have found from that time.
Isotopes: Isotopes refer to the different masses of the atoms of an element. The nucleus of a specific element always contains the same number of protons, equal to its atomic number, but it can contain a different number of neutrons. For example, all Sr atoms contain 38 protons, but the four natural varieties can contain 46, 48, 49, or 50 neutrons, making the four isotopes, 84Sr, 86Sr, 87Sr, and 88Sr. The superscripts represent the number of protons (38) plus the number of neutrons (46, 48, etc.). The ratio 87Sr/86Sr (“Strontium eighty-seven–eighty-six”) is commonly used as a tracer of rock age or type.
Why we use 87Sr/86Sr: Although four isotopes of Sr are stable, so do not radioactively decay, the slow decay of radioactive 87Rb makes extra 87Sr. Therefore, rocks can develop a high 87Sr/86Sr if they are old (lots of time for 87Rb to decay), and/or have high Rb contents (shales and granites or their metamorphic equivalents—phyllites, schists, and gneisses have high Rb). Rocks can have low 87Sr/86Sr if they have low Rb, such as limestones and/or basalts, or if they have high Rb are very young. Analyzing 87Sr/86Sr allows us to discriminate whether an animal got its food and water from an area whose bedrock was old metamorphic and igneous rocks (high 87Sr/86Sr) vs. young sedimentary rocks (low 87Sr/86Sr).
Mass spectrometer: A mass spectrometer is a modern analytical instrument that separates atoms with different masses and allows us to measure the amount of 87Sr and 86Sr in a material.
Mastodon or Mammoth?: Both are members of the order Proboscidea, which included many different species in the past, but now is populated solely by elephants. Mastodons had lumpy teeth and ate a lot of leaves and twigs. Mammoths had banded teeth and preferred eating grass. Both became extinct at the end of the last Ice Age.
Another study involved Sr isotope zoning within a fossilized mastodon tooth from Florida, which revealed annual migration patterns of these elephant cousins (Fig. 2; Hoppe et al. 1999). These patterns would have been impossible to figure out any other way. A key observation is that teeth form from top to bottom (Fig. 2b), and in large herbivores teeth require more than one year to reach their maximum size. So by measuring such growth zoning in teeth, we can identify where an animal lived seasonally, sometimes over multiple years. Zoning in the Mastadon tooth (Fig. 3) shows that this individual mostly lived in areas with moderate 87Sr/86Sr, but occasionally migrated to areas with lower and higher 87Sr/86Sr. Local geologic variations in 87Sr/86Sr show that these animals must have migrated at least 100 km each year, and perhaps more than 500 km.
Apatite's ability to record the geochemistry of past diets provides an important way to study the life history of humans and other animals long after death. This information helps us evaluate hypotheses about how human cultures evolved, and how ecosystems functioned in the past.
This work was supported by National Science Foundation grant EAR-1349749. Thanks to Steve Shirey for initiating this project, teachers Tanya Gordon and Julie Ekhoff for comments on an early version, and Dave Mogk, Holly Godsey, and Alex Speer for reviews.