Two six-foot-tall mounds on the campus of Louisiana State University (LSU) are the oldest man-made structures ever discovered in North America.
Using radiocarbon dating, the authors of the study published in American Journal of Science, determined that building efforts at the mounds began 11,000 years ago and were completed over several millennia.
Analysis of sediments taken from the two mounds on the LSU campus showed that they were built in layers, the oldest containing ash from burnt reeds and reed plants. In the ash, the researchers discovered microscopic fragments of carbonized mammalian bones called osteons.
“Layers of reed and reed phytoliths, which contain very few osteons, are indicative of extensive fires,” the authors wrote. Such infernos would have been too hot for cooking and are therefore likely to have served a ritual function. “This discovery supports the argument that the fires were used for ceremonial or cremation purposes,” the researchers concluded. according to IFL Science.
The ritual role of mounds
The team could not determine whether the osteons were of human or animal origin. “We requested permission to conduct DNA testing on the microscopic bone material found, but permission was denied by the Native American communities that were contacted,” they revealed.
Known as Mound A and Mound B, the two structures in North America were assembled by ancient indigenous cultures and were only completed 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 giant red star Arcturus, which is among the brightest stars in the sky, would have risen at this very spot.
Commenting on this discovery, study author Brooks Ellwood explained that “the people who built the mounds, about 6,000 years ago, coordinated the orientation of the structures to align with Arcturus, as seen in the night sky at that time.”
Reconstructing the history of the two mounds
Through their analysis, the researchers reconstructed the history of the two mounds, revealing that Mound B was started about 11,000 years ago, while Mound A was started about 9,500 years ago. However, the distribution of tree roots in the mounds suggests that they were abandoned for about a millennium, beginning 8,200 years ago, when the Northern Hemisphere experienced a sudden and drastic drop in temperatures.
“We don’t know why they abandoned the mounds about 8,200 years ago, but we do know that their environment changed suddenly and dramatically, which could have affected many aspects of their daily lives,” says Ellwood. Construction finally resumed about 7,500 years ago, when the two mounds were about half their current height. Over the next 1,500 years, the two mounds were built to their current size, at which point “the mounds on the LSU campus were permanently abandoned by the natives.”
Despite being abandoned, the two small mounds have withstood the test of time, leading the authors to speculate that “the mounds on the LSU campus may represent the oldest known and still intact man-made structures on Earth.”