Critics argued that this combination was insufficient — that morphology alone is unreliable for distinguishing human from non-human coprolites, and that using DNA evidence to support the identification of specimens whose DNA was itself the main evidence being presented created a circular argument.

What was needed was an independent line of evidence that could confirm the human origin of the coprolites without relying on the DNA results.

The answer, which would not arrive until 2020, was fecal lipid biomarkers.

Fecal lipid biomarkers are specific compounds produced during the metabolism of cholesterol and other sterols in the digestive system.

Different species of mammals have different sterol metabolic pathways, and the specific compounds they produce in their intestines — and subsequently excrete — differ in ways that can be identified by chemical analysis even after thousands of years in the archaeological record.

The most important distinction is between the fecal biomarker profile of humans and the fecal biomarker profile of other mammals.