The specific chemistry of the burial environment determines how quickly and how thoroughly this degradation occurs.

Cold, dry conditions slow degradation.

The permafrost and near-permafrost conditions of subarctic Alaska — where ground temperatures remain low even in summer and where frozen ground can preserve organic material for thousands of years — might seem ideal for ancient DNA preservation.

And in some respects they are: many extraordinary ancient specimens have been recovered from permafrost across Siberia and Alaska, including woolly mammoths with intact hair, skin, and even stomach contents.

But human skeletal material presents specific challenges.