The specific concern at Paisley was that if the cave had been occupied by people at multiple time periods — which it clearly had been, given the archaeological evidence from multiple sediment layers — then DNA from more recent occupants could potentially have moved downward through the sediment and contaminated the older layers.

The coprolites from the oldest layers, lying in sediment that also contained DNA from later periods of occupation, might have absorbed some of that later DNA, making ancient non-human coprolites appear to contain human sequences.

This concern was sharpened by a second category of objection: the question of whether the coprolites identified as human were in fact human.

Coprolite identification is not always straightforward.

Human feces and the feces of other omnivorous mammals can be morphologically similar, particularly after thousands of years of desiccation and mineralization have altered their original character.