Ancient DNA Comes of Age

Feature T he study of genetic material from ancient specimens was, in its early years, dominated by a race to sequence DNA from extinct species like the dodo and the woolly mammoth. Now that the supply of these crowd-pleasing curiosities has run dry, scientists are starting to ask new questions of ancient DNA (aDNA) that are revealing how the genetic make-up of prehistoric populations changed through time. These fi ndings look set to trounce assumptions about how evolution really unfolded. However, there is still concern that many studies are not paying enough attention to the exacting protocols needed to overcome the technical challenges of the discipline and to defend it from the ridicule that has plagued it in the past. In 1994, while Jurassic Park was still taking in millions of dollars at the box offi ce, scientists claimed to have extracted and sequenced DNA from an 80-million-year-old dinosaur [1]. When sceptical researchers took a look at the sequence, it turned out to be of human rather than dinosaur origin. " To make that mistake, you'd have to try really, really hard, " says Alan Cooper, head of the Henry Wellcome Ancient Biomolecules Centre at Oxford University in the United Kingdom. If you think you've sequenced some dinosaur DNA, the fi rst thing you'd do is run a phylogenetic analysis on it, he says. " Had they done that properly, with any mammal at all involved in the tree,…they would have found that their sequence was grouping with the mammals and not with the reptiles or the birds, " says Cooper. Perhaps they'd watched Michael Crichton's inventive fi ction one too many times, he suggests. It was this kind of bungling study that highlighted the need for an exacting protocol that would steer researchers around the signifi cant pitfalls posed by DNA decay and contamination. A list of " authenticity criteria " emerged during the 1990s, aimed at preventing similarly bogus claims from entering the literature [2]. This list includes stringent laboratory controls; cloning of products amplifi ed by polymerase chain reaction (PCR); replication of results from a second, independent extract; and, for really new or unexpected results, replication of results by an independent research group. Such requirements have allowed work on aDNA to move on and mature. Now, it's possible to focus on the really interesting questions that aDNA can answer. " What we're able to do with ancient …