Curiously modern DNA for a "250 million-year-old" bacterium

Studies of ancient DNA have attracted considerable attention in scientific journals and the popular pre-ss. Several of the more extreme claims for anc ient DNA have been questioned on biochemical grounds (i.e.. DNA surviving l onger than expected) and evolutionary grounds (i.e., nucleotide substitutio n patterns not matching theoretical expectations for ancient DNA). A recent letter to Nature from Vreeland et al. (2000). however. tops all others wit h respect to age and condition of the specimen. These researchers extracted and cultured a bacterium from an inclusion body from what they claim is a 250 million-year (Myr)-old salt crystal. If substantiated, this observation could fundamentally alter views about bacterial physiology, ecology and ev olution. Here we report on molecular evolutionary analyses of the 16S rDNA from this specimen. We find that 2-9-3 differs from a modern halophile, Sal ibacillus marismortui, by just 3 unambiguous bp in 16S rDNA, versus the sim ilar to 59 bp that would be expected if these bacteria evolved at the same rate as other bacteria. We show. using a Poisson distribution, that unless it can be shown that S. marismortui evolves 5 to 10 times more slowly than other bacteria for which 16S rDNA substitution rates have been established. Vreeland et al.'s claim would be rejected at the 0.05 level. Also, a molec ular clock test and a relative rates test fail to substantiate Vreeland et al.'s claim that strain 2-9-3 is a 250-Myr-old bacterium. The report of Vre eland et al. thus falls into a long series of suspect ancient DNA studies.