This downward transport can result in DNA from surface or near-surface environments contaminating deeper, older sediment layers.

The phenomenon is well-documented and is a recognized source of error in the analysis of sediment cores for ancient DNA.

The specific application to Paisley is straightforward to state: if the cave floor had been occupied at multiple time periods, human DNA from more recent occupants would be present in the sediment of the upper layers.

If water had percolated through those upper layers and carried human DNA downward, it could have reached the older, deeper layers where the pre-Clovis coprolites were located.

Those coprolites, lying in sediment enriched with environmental human DNA, could have absorbed that DNA — creating the appearance of ancient human sequences in specimens that might actually have been produced by non-human mammals.