At sites scattered across western North America, long, fluted stone projectile points mark the presence of ancient people from a culture archaeologists now call Clovis. For much of the 20th century, the Clovis people were considered the very first Americans. But more recent data has shown that people arrived in North America several thousand years before the oldest known Clovis projectile points were made. But Clovis appears to be the first widespread culture, and it still represents a key chapter in the story of how people spread across two continents.

Beyond the things they left behind, there’s little trace of the Clovis people themselves. In fact, a lone infant skeleton may be the only known representative of the Clovis culture. Due to some discrepancies in radiocarbon dating, however, archaeologists still aren't sure whether the child's remains are Clovis. Now, a new study adds some evidence to that debate.

Meet Anzick-1

In 1968, construction workers near Anzick, Montana, unearthed the partial skeleton of an infant boy: fragments of his skull, his left collarbone, and a few ribs. At first glance, the boy now known as Anzick-1 seems to be the only member of the Clovis culture found so far. His skeleton was found with Clovis-style artifacts—more than 100 stone and bone objects, all dusted with red ochre. But radiocarbon dating of his remains didn't line up with dates from a pair of antler artifacts from the Clovis materials, raising questions as to how they ended up at Anzick together.

According to paleogenomic studies carried out previously, Anzick-1 is most closely related to modern Native American groups, but he also shares some ancestry with the Mal’ta, a group of people who lived in Siberia during the Upper Paleolithic. And he’s more closely related to Central and South American indigenous groups than to those who settled farther north. If archaeologists could be sure of when Anzick-1 lived, he could help us better understand how and when the first Americans split into different populations and spread across two continents.

When archaeologists dated Anzick-1 in the late 1980s and early 1990s, they used several different methods to prepare the samples for study. Two of them involved pretreating collagen samples from the bones, but two others involved separating single amino acids from the samples and then radiocarbon dating those isolated compounds. The methods that involved testing whole collagen generally showed the skeleton being younger than the Clovis antlers: about 10,575 to 9,005 years old. But when archaeologists dated samples of amino acids glycine and glutamic acid extracted from the collagen, their results lined up with the 13,000- to 12,795-year-old range for the antler artifacts.

Because radiocarbon dating collagen samples produced dates that didn’t match the Clovis artifacts, some archaeologists say they can’t consider Anzick-1 a Clovis burial. Others argue that the collagen samples may have been contaminated by modern carbon sources during sample preparation. If so, then isolating single amino acids should help filter out that contamination and produce a more accurate date.

New methods, really old bones

Most of the arguments about Anzick-1 have centered on which radiocarbon dates were more accurate, so Oxford University archaeologist Lorena A. Becerra-Valdivia and her colleagues decided to try the dating again with more recent sample preparation methods, which she says have improved significantly in the last 20 years. Three of the newer methods involved different ways of decontaminating the collagen samples. But one method, called HYP, extracted and purified a sample of a single amino acid, hydroxyproline, from the collagen sample. Becerra-Valdivia says that this will provide a purer sample with less chance of contamination.

The dates still weren’t consistent; the three collagen methods still produced dates different from the antler samples, but the HYP method yielded an overlapping set of dates: 12,905 to 12,695 years old.

According to Becerra-Valdivia and her colleagues, that means Anzick-1 was buried at the same time as the ochre-dyed Clovis artifacts, which proves that he was a member of the Clovis culture.

“Our data, coupled with Rasmussen et al.’s palaeogenomic evidence on the Anzick-1 individual, tell us that the peopling of the American continent was a more complex process than previously thought,” Becerra-Valdivia told Ars Technica. “It also serves to emphasize the importance of a robust chronology in the study of human dispersal. First Americans research must be anchored in a reliable timeline.”

But it’s probably far from a settled debate, since the dates on the whole collagen samples still don’t line up with the antler artifacts from the Clovis collection. These results lend support to the dates from single amino acids in the earlier studies, but they may not fully resolve the debate over whether to trust dates on single compounds or whole collagen samples.

“Disagreement is good and healthy in academia; it pushes us toward the critical assessment of evidence,” Becerra-Valdivia said. “The Anzick site has always sparked a good amount of debate. I hope for our data to add to the current discussion on a peopling process that is complex and nuanced.”

Anzick-1 was reburied in 2014, with several Native American tribes participating in the ceremony.

PNAS, 2018. DOI: 10.1073/journal.pnas.1803624115 (About DOIs).