The science of ancient DNA — the extraction and analysis of genetic material from ancient biological specimens — has undergone a revolution in the past two decades that has transformed what archaeologists and human geneticists can know about the past.

The basic principle has been understood since the 1980s: cells contain DNA, and under the right conditions, some of that DNA can survive for very long periods of time.

The challenge has always been recovery and interpretation.

Ancient DNA is not intact, readable sequence.

It is fragmented — broken into short pieces over thousands of years by chemical processes, by hydrolysis, by the action of microorganisms, by the slow work of time on molecular bonds.

It is also contaminated: any ancient specimen has been exposed to modern DNA from the environment, from the microorganisms that inhabit every surface, from the hands of the researchers who handle it.