Cryptic contamination and phylogenetic nonsense.

Ancient human DNA has been treated cautiously ever since the problems related to this type of material were exposed in the early 1990s, but as sequential genetic data from ancient specimens have been key components in several evolutionary and ecological studies, interest in ancient human DNA is on the increase again. It is especially tempting to approach archaeological and anthropological questions through this type of material, but DNA from ancient human tissue is notoriously complicated to work with due to the risk of contamination with modern human DNA. Various ways of authenticating results based on ancient human DNA have been developed to circumvent the problems. One commonly used method is to predict what the contamination is expected to look like and then test whether the ancient human DNA fulfils this prediction. If it does, the results are rejected as contamination, while if it does not, they are often considered authentic. We show here that human contamination in ancient material may well deviate from local allele frequencies or the distributions to be found among the laboratory workers and archaeologists. We conclude that it is not reliable to authenticate ancient human DNA solely by showing that it is different from what would be expected from people who have handled the material.