Cutting edge

Was Anglo-Saxon Britain invaded en masse or did just a handful of warriors attack? The answer lies in tooth enamel.

Faced with two skeletons in an early Anglo-Saxon cemetery, how would archaeologists tell invader from invaded? And how would they know if the migrant came alone or brought the whole family? Unravelling ancient DNA can identify individuals who inherit the incoming gene pool but cannot pinpoint the pioneers who actually migrated.

Paul Budd and I are tackling this problem as part of a project funded by the Natural Environment Research Council. In a multi-disciplinary collaboration with biological anthropologist Charlotte Roberts of the University of Durham, and geochemists Jane Evans and Carolyn Chenery of NERC Isotope Geosciences Laboratory, we are using modern isotopic source-tracing methods to throw light on the travels of our ancestors.

Archaeologists employ a variety of methods to identify migrations. These include written records, skeletal attributes and sudden changes in grave goods, burial style and orientation. None is wholly objective: historical records are incomplete and often biased; skeletal traits are difficult to interpret and can vary more within than between populations; and how the burial was arranged may say more about the beliefs of the survivors than those of the deceased. Consequently, the scale of the Anglo-Saxon colonisation remains hotly disputed and theories have run the gamut from mass invasion to the arrival of a handful of high-status male warriors.

Fortunately, every immigrant brought along an ancient passport - the enamel on teeth. Once formed, enamel is a living fossil. It preserves not only the many chemical elements that were being ingested or inhaled while it was growing, but also their numerous isotopes. Certain elements contain enough isotopic information about their geological source (lead and strontium) or climatic regime (oxygen) to provide us with a skeleton fingerprint. This provides tantalising clues to a person's past.

Although isotopic provenancing is routine in a range of scientific applications, interpreting these archaeological visas is not clear-cut. Modern mass spectrometers measure isotopic ratios with a formidable degree of accuracy but the resulting numbers require interpretation using geological and archaeological knowledge of the period and places in question.

Considerable geographical variation in geology and climate exists but a move between two identical places would be invisible. When differences are found, we need to establish the normal variation within our archaeological population before we can pick out immigrant burials. Diet may fluctuate with status and lifestyle and also over time as trade, metal use and pollution increase.

Ruling out post-mortem contamination has been one of our biggest headaches. We are searching for individuals who are different from both the place of burial and other individuals in the cemetery because soil contamination cannot produce these false positives. Ambiguity reigns if soil and enamel are identical: are they indigenous or contaminated?

Recently, we faced the supreme test: a Roman "princess" from Spitalfields, London, sealed for 1,600 years in a lead-rich burial environment. Her coffin was made from British lead but was she indigenous? Contaminated or not, the lead in the coffin and her teeth were unlikely to be different if they were both British; lead products were ubiquitous throughout this period. However, not only was the tooth composition different, it was also very unusual. This passport remains a puzzle - but we are working on it!

Janet Montgomery, PhD student, department of archaeological sciences, University of Bradford.