Two six-meter (20-foot) high mounds on the campus of Louisiana State University (LSU) are the oldest human-made structures ever discovered in North America, according to new research in the American Journal of Science. Using radiocarbon dating, the study authors determined that construction of the mounds began around 11,000 years and was completed over several millennia.
Analysis of sediment cores taken from the two LSU Campus Mounds revealed that they were built up in layers, the oldest of which containing ash from burned reeds and cane plants. Amongst this ash, the researchers found microscopic fragments of charred mammal bones called osteons.
“The layers of reed and cane phytoliths, containing very small numbers of osteons, are indicative of very hot fires,” write the authors. Such infernos would have been too hot for cooking, and are therefore likely to have served a ritual function. “This finding supports the argument that the fires were used for ceremonials or cremations,” continue the researchers.
Tantalizingly, the team was unable to determine whether the osteons were of a human or animal origin. “We did request permission to perform DNA tests on the microscopic bone material found, but permission was denied by the Native American tribal communities that were contacted,” they reveal.
Known as Mound A and Mound B, the two hillocks were assembled by ancient Indigenous cultures and were not completed until about 6,000 years ago.
According to the researchers, the structures are aligned with a point on the horizon that is 8.5 degrees east of true north. Six millennia ago, the red giant star Arcturus – which is among the brightest stars in the sky – would have risen at this very point.
Commenting on this finding in a statement, study author Brooks Ellwood explained that “the people who constructed the mounds, at about 6,000 years ago, coordinated the structures’ orientation to align with Arcturus, seen in the night sky at that time.”
Through their analysis, the researchers reconstructed the history of the two mounds, revealing that Mound B was initiated around 11,000 ago while Mound A was started some 9,500 years ago. However, the distribution of tree roots within the mounds suggests that they were abandoned for about a millennium, beginning 8,200 years ago when the northern hemisphere experienced an abrupt and drastic drop in temperatures.
“We don’t know why they abandoned the mounds around 8,200 years ago, but we do know their environment changed suddenly and dramatically, which may have affected many aspects of their daily life,” says Ellwood.
Construction eventually restarted roughly 7,500 years ago, when the two knolls were about half of their current height. Over the next 1,500 years, the two mounds were built up to their present size, at which point “the LSU Campus Mounds were abandoned by the Indigenous people for good.”
Despite being forsaken, though, the two small hills have withstood the test of time, leading the authors to speculate that “the LSU Campus Mounds may represent the oldest known and still intact, man-made structures on Earth.”