Monitoring DNA contamination in handled vs. directly excavated ancient human skeletal remains.

Bones, teeth and hair are often the only physical evidence of human or animal presence at an archaeological site; they are also the most widely used sources of samples for ancient DNA (aDNA) analysis. Unfortunately, the DNA extracted from ancient samples, already scarce and highly degraded, is widely susceptible to exogenous contaminations that can affect the reliability of aDNA studies. We evaluated the molecular effects of sample handling on five human skeletons freshly excavated from a cemetery dated between the 11 to the 14(th) century. We collected specimens from several skeletal areas (teeth, ribs, femurs and ulnas) from each individual burial. We then divided the samples into two different sets: one labeled as "virgin samples" (i.e. samples that were taken by archaeologists under contamination-controlled conditions and then immediately sent to the laboratory for genetic analyses), and the second called "lab samples"(i.e. samples that were handled without any particular precautions and subject to normal washing, handling and measuring procedures in the osteological lab). Our results show that genetic profiles from "lab samples" are incomplete or ambiguous in the different skeletal areas while a different outcome is observed in the "virgin samples" set. Generally, all specimens from different skeletal areas in the exception of teeth present incongruent results between "lab" and "virgin" samples. Therefore teeth are less prone to contamination than the other skeletal areas we analyzed and may be considered a material of choice for classical aDNA studies. In addition, we showed that bones can also be a good candidate for human aDNA analysis if they come directly from the excavation site and are accompanied by a clear taphonomic history.

Genealogical discontinuities among Etruscan, Medieval, and contemporary Tuscans.

The available mitochondrial DNA (mtDNA) data do not point to clear genetic relationships between current Tuscans and the Bronze-Age inhabitants of Tuscany, the Etruscans. To understand how and when such a genetic discontinuity may have arisen, we extracted and typed the mtDNAs of 27 medieval Tuscans from an initial sample of 61, spanning a period between the 10th and 15th century AD. We then tested by serial coalescent simulation various models describing the genealogical relationships among past and current inhabitants of Tuscany, the latter including three samples (from Murlo, Volterra, and Casentino) that were recently claimed to be of Etruscan descent. Etruscans and medieval Tuscans share three mitochondrial haplotypes but fall in distinct branches of the mitochondrial genealogy in the only model that proved compatible with the data. Under that model, contemporary people of Tuscany show clear genetic relationships with Medieval people, but not with the Etruscans, along the female lines. No evidence of excess mutation was found in the Etruscan DNAs by a Bayesian test, and so there is no reason to suspect that these results are biased by systematic contamination of the ancient sequences or laboratory artefacts. Extensive demographic changes before AD 1000 are thus the simplest explanation for the differences between the contemporary and the Bronze-Age mtDNAs of Tuscany. Accordingly, genealogical continuity between ancient and modern populations of the same area does not seem a safe general assumption, but rather a hypothesis that, when possible, should be tested using ancient DNA analysis.