By proving that chunks of prehistoric poop were produced by humans, researchers have helped to affirm when people first arrived in the Americas. 

Most scientists agree that humans first arrived in North America towards the end of the last ice age some time before 13,000 years ago (or perhaps long before this time). When and how they arrived, however, has been hotly debated. Until recently, the consensus was that the earliest inhabitants in the Americas were a single group known as the "Clovis culture” that set foot on the continent around 13,000 years ago. However, most now believe there were several groups present on the continent long before the Clovis culture, known as pre-Clovis populations.

To weigh into the debate, archeologists from the UK took a deep look at prehistoric poop found within the Paisley Caves of present-day Oregon in the US.

Previous radiocarbon dating estimated the poop to be at least 14,000 years old (or ~12,400 radiocarbon years before the present). Although this analysis claimed the poop contained human mitochondrial DNA, some were skeptical and argued the specimens were susceptible to contamination. This ambiguity meant there was some dispute around whether the pre-Clovis populations were present in the area at the time. 

This new study has found another way to prove the poop is human. Reported in the journal Science Advances, the team increased the certainty about the ancient poops' identity by looking at lipid biomarkers, which are less prone to contamination than DNA. They found that 13 out of 21 fossilized feces contained lipids that strongly suggest they were created by humans, indicating that people occupied the Paisley Caves since at least 14,000 years ago during the “pre-Clovis” period. 

“The question of when and how people first settled the Americas has been a subject of intense debate. By using a different approach, we have been able to demonstrate that there were pre-Clovis populations present in the area of the Great Basin and resolve this debate once and for all,” Dr Lisa-Marie Shillito, study author and senior lecturer in landscape archaeology at Newcastle University, said in a statement.

However, there was one point of confusion in the discovery: the fossilized poop didn’t just contain lipids you’d associate with a human, it also had hints of dog-related lipids. Although this might add some more haziness to the debate, the researchers argue that dogs most likely were consuming human feces (as, you know, they tend to do). If that's accurate, it could provide some further insights into the mysterious pre-Clovis people and their apparent dog-loving lifestyle. 

“We know dogs do this [eat poop] today, and the fact that we have dogs doing this at Paisley is really strong evidence that these were domesticated and living alongside people,” explained Dr Shillito. 

“So far, there has been a lot of focus on answering the when and how of how people arrived on the continent,” added Shillito. “As a result, the nature of early occupation has received relatively little attention, in terms of understanding the relationship between these early human populations and their environment.”

“We want to know more about the people themselves.”