Fossilised faeces found in a US cave may help solve the riddle of when and how humans came to the Americas.
The samples date back just over 14,000 years, before the time of the Clovis culture.
Clovis people dominated North and Central America around 13,000 years ago, and whether any groups came before them has been controversial.
In the journal Science, the researchers describe how their conclusion hinged on modern genetic analysis.
The 14 faecal fragments were discovered in caves near a lake in the north-western US state of Oregon, among other signs of ancient human occupation.
These included threads made from animal sinew and plant fibre, baskets, animal hides and wooden pegs.
The presence of these artefacts at various depths in the cave floor indicated it was populated for extensive periods - but by whom?
"We found a little pit in the bottom of a cave," related Dennis Jenkins from the University of Oregon, whose team excavated the Paisley Caves in 2002 and 2003.
"It was full of camel, horse and mountain sheep bones, and in there we found a human coprolite."
This and 13 other coprolites - fossilised faeces - proved the star attraction, because they contained tiny quantities of human mitochondrial DNA - genetic material found outside the nuclei of cells which is passed down from each mother to her children.
Several kinds of genetic analysis performed at several different laboratories confirmed that the DNA was human, and suggested the ancient cave residents were closely related to ethnic groups indigenous to Siberia and East Asia.
This adds to other strands of evidence suggesting that the Americas were settled from Siberia - and the age of the samples indicates the migration happened before the emergence of the Clovis culture with its distinctive fluted stone blades.
"If this doesn't convince what's left of the 'Clovis first' people, it should," University of California scholar David Smith, who was not involved in the study, told Science journal.
In an era when the north of the Americas were heavily glaciated, the question then is: how did the pre-Clovis people make the journey?
"The first humans either had to walk or sail along the American west coast to get around the ice cap," contended Eske Willerslev, director of the Centre for Ancient Genetics at Copenhagen University, who led the DNA work on the new study.
"That is, unless they arrived so long before the last ice age that the land passage wasn't yet blocked by ice."