The response to this concern required demonstrating that the DNA found in the Paisley coprolites was genuinely ancient — that it showed the chemical characteristics of aged material rather than fresh contamination — and that the coprolites themselves were genuinely human rather than non-human specimens that had absorbed environmental human DNA.

Ancient DNA has specific chemical signatures that distinguish it from modern DNA.

The most important is a pattern of chemical damage called cytosine deamination, in which cytosine bases at the ends of DNA fragments are chemically modified to uracil.

This damage pattern accumulates predictably with time and is present in genuinely ancient sequences but not in modern contamination.

Demonstrating this damage pattern in the Paisley coprolite sequences was one way to establish that the DNA was genuinely ancient rather than modern contamination.

But even ancient DNA could potentially have moved through the sediment from more recent ancient layers rather than from the very oldest ones.