A child’s teeth carry a chemical record of the ground they grew up on. Strontium from local rock enters the body through water and plant food, and it settles in tooth enamel as the teeth form. Enamel then stays fixed for life, so an adult’s molars still show where they lived as a small child.
Bone keeps changing. National Geographic notes that bone cells turn over constantly and pick up the strontium of wherever a person later lived. Compare the two, and you can tell whether someone moved between childhood and death.
Why the rocks leave a signature
Strontium comes in more than one isotope, and the ratio of strontium-87 to strontium-86 depends on the local geology. Older rocks generally hold more strontium-87 relative to strontium-86. A person who eats and drinks from one region carries that region’s ratio in their body.
The timing differs by tissue. Archaeology magazine reported that enamel forms in the first few years of life, while the strontium in bone shifts over seven to ten years. Bone therefore reflects roughly the last decade before death.
The pueblo that tested the idea
In the late 13th century drought drove people across the American Southwest from their homes. Some settled in a well-watered Arizona valley at the site now called Grasshopper Pueblo. At its height it had 500 rooms.
The pueblo’s mix of building styles, pottery and burial customs made archaeologists suspect several cultures had crowded in together. Nobody could test it.
In the late 1980s T. Douglas Price and geochemist Jim Burton, of the University of Wisconsin’s Laboratory of Archaeological Chemistry, took up the new strontium method there. North-central Arizona’s geology is a patchwork, which suited the test. Of the first six people they sampled, three matched the local rock, including two from the earliest burials. The other three had grown up far away, perhaps to the north.
A king who was not from far away
The method has since moved from villages to dynasties. Inscriptions say Yax K’uk Mo arrived at the Maya city of Copán, in western Honduras, in AD 426. Some archaeologists thought he came from Teotihuacán in the Mexican highlands. Price’s analysis of his remains pointed instead to a birthplace much nearer Copán, possibly in the Peten of northern Guatemala.
Price told the magazine he had once thought ancient people stayed put. “Almost every place we look we see a lot of mobility,” he said.
What else the atoms record
Strontium is one tool among several. Carbon isotope ratios in bone, teeth and hair reflect the plants an animal or person ate, and nitrogen can show when a child started on solid food. National Geographic lists the limits too. The work is expensive, fails on burned material and needs care to avoid contamination. Bone collagen also breaks down after about 50,000 years.
