One additional finding — the one that became the most consequential to revisit — was the apparent presence of Steppe Herder ancestry. The Eastern European Steppe Herders were a population associated with the Yamnaya culture and related groups who expanded into Europe from the Pontic-Caspian steppe roughly 4,900 years ago, bringing with them the genetic, linguistic, and technological contributions that shaped much of modern European and South Asian ancestry. Finding their genetic signature in a man who died around 3300 BCE — roughly six centuries before the Steppe Herder expansion into Europe is thought to have occurred — was puzzling. It was noted in the literature, discussed, and attributed to various possible explanations.
The 2012 genome operated at 7.63-fold average coverage — a measurement expressing how many times, on average, each position in the genome was read during sequencing. This was low enough to leave substantial uncertainty in some positions and to be more susceptible to errors, including errors introduced by contamination with more-recent DNA.
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## The Contamination Problem in Ancient DNA
Ancient DNA research operates under a fundamental challenge that sets it apart from modern genomics: the DNA being studied has degraded over centuries or millennia. The older the sample, the more fragmented the DNA strands, the more chemical modifications have accumulated in the bases, and the smaller the proportion of the extracted DNA that actually comes from the ancient individual rather than from bacteria, fungi, or modern humans who have handled the material.
Modern human contamination — human DNA from researchers, excavators, visitors, or handling at any point in the chain from discovery to analysis — is a particular concern because it is almost indistinguishable from the ancient DNA being studied at the level of sequence composition. Both are human DNA. The techniques used to minimize contamination include using protective equipment during all handling, working in specialized clean-room facilities designed to exclude airborne DNA, treating samples with enzymes that damage modern DNA (which lacks the chemical modifications that accumulate in ancient DNA over time, allowing the two to be distinguished biochemically), and sequencing protocols designed to identify and remove reads that lack the characteristic damage patterns of ancient DNA.