The challenge of working with ancient DNA is fundamental: DNA is not a stable molecule.

After an organism dies, the DNA in its cells begins to break down.

Bacteria, fungi, and chemical processes attack the DNA strands, breaking them into increasingly short fragments, modifying the chemical bases that carry the genetic information, and eventually degrading the molecule to the point where it cannot be read at all.

The rate of this degradation depends on conditions — temperature, humidity, pH, exposure to light and oxygen — but even under ideal preservation conditions, DNA from ancient samples is fragmented, chemically damaged, and contaminated with the DNA of microorganisms that colonized the remains after death.

For most of the history of ancient DNA research, the technical limitations were severe.