Not just old but old and cold?

Smith et al. have shown an interesting correlation between the thermal age of Pleistocene–Holocene fossils and ancient DNA retrieval and advocate using this criterion to assess the merit of subjecting ancient bones to destructive analysis, particularly any scientifically valuable bones of Neanderthals and anatomically pre-modern humans. The post-mortem DNA quality is, of course, dependent on the surrounding temperature, but — as the authors point out — many other important factors influence fossil DNA preservation, such as air and soil humidity, soil pH, phosphorus content of the soil, average temperature in different earth layers, and microbial-mediated decay, which also have to be taken into account.