It is degraded into short fragments — typically 50 to 150 base pairs in length, compared to the millions or billions of base pairs of intact chromosomes.

These fragments are mixed in with vast quantities of bacterial and environmental DNA that contaminated the sample after death.

They carry specific patterns of chemical damage — particular changes to the bases at the ends of fragments — that are the signature of genuine ancient DNA as opposed to modern contamination.

The methods that Svante Pääbo and colleagues at the Max Planck Institute developed over the past three decades allow researchers to extract these short, damaged, rare ancient DNA fragments from a bone sample, sequence them using high-throughput DNA sequencing technology, and computationally assemble the fragments into something that represents the original genome.

This requires solving several related challenges simultaneously.