Chemical analysis of the coprolites’ composition was needed to confirm that the specimens containing human DNA had actually been produced by humans rather than by another mammal that had subsequently been exposed to environmental human DNA.

Paul Goldberg and colleagues raised a third category of concern: the stratigraphic integrity of the cave deposits.

If the sediment layers at Paisley had been disturbed by natural processes — frost heaving, bioturbation by burrowing animals, the activities of carnivores, or water movement — then materials from different time periods could have been mixed into the same apparent layer.

The radiocarbon dates associated with a specific layer might not accurately reflect the age of all the materials in that layer.

The stratigraphy of caves is inherently complex.