First, the extraction must be done in a controlled environment to prevent contamination with modern human DNA — the researchers themselves are the greatest source of potential contamination, since modern DNA is abundant, intact, and much easier to sequence than the ancient material.

Second, the sequencing must be sensitive enough to detect the rare authentic ancient sequences among the much more abundant bacterial and environmental contamination.

Third, the authenticity of the ancient sequences must be verified through analysis of their characteristic chemical damage patterns.

The computational challenge of assembling a complete or near-complete genome from millions of short, overlapping fragments is also formidable — equivalent to reconstructing a book from millions of torn confetti pieces, each a few words long, with the added complication that many of the confetti pieces come from other books (contamination) that must be identified and excluded.

The fact that this is now routinely done — that researchers can sequence ancient genomes from bones tens or hundreds of thousands of years old — represents one of the great technical achievements of modern science.