Isotopes are different forms of an element, atoms with different molecular weights based on their varying numbers of neutrons. Depending on their diet and water supply, humans concentrate specific isotopes in varying ratios in the enamel of their teeth. For example, the ratio of strontium isotopes indicates whether the person grew up in a region underlain by very old bedrock, such as West Africa, or newer rock, such as Latin America. That is because older rock has a higher ratio of strontium 87 to strontium 86.

Ratios of carbon isotopes in the teeth reflect what foods a person ate. A diet heavy in corn, sorghum and other tropical plants yields more carbon 13, whereas grains such as barley and wheat produce more carbon 12. Europeans of Columbus’ time would have relatively little carbon 13 in their teeth; Mexicans would have much of the heavier isotope. Natives of Hispaniola and many Africans, who are believed to have eaten a mixed diet, would probably fall somewhere in between.

“Mexicans are about as heavy (on carbon 13) as you can get,” said James Burton, a geochemist at the University of Wisconsin involved in the La Isabela and Campeche projects. “Africans are in between, Europeans at the other end.”

Scientists are also analysing the oxygen composition of the teeth. Oxygen isotopes in water differ according to a region’s climate, with warmer climates yielding water with more “heavy oxygen” (oxygen 18) and cooler areas having “lighter oxygen” (oxygen 16).

Based on such analyses, scientists are certain that a number of people found in the Campeche cemetery were African. It looks highly possible they have found at least three Africans in La Isabela as well, although Burton warns against jumping to conclusions.

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