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Scientists unearth more clues about mystery American civilization that vanished 600 years ago
Archeologists have unearthed more clues about a Native American tribe of up to 50,000 people that mysteriously vanished without a trace.
The Cahokia people once thrived in what is now Illinois, only to abandon their six-square-mile city more than 600 years ago.
While the leading theory has been that the ancient city had become unlivable after massive crop failure that followed an epic drought, a new study has discovered evidence pointing toward another explanation.
Carbon-dating analysis of deep-cut layers of soil and agricultural remnants, the researcher said, now show that farming practices remained consistent in Cahokia even amid those punishing drought years.
Cahokia's indigenous residents, they now believe, might have more gradually left their city for better opportunities elsewhere or to connect with distant loved ones.
At their apex, this indigenous civilization had constructed an estimated 120 earthen mounds: the largest city north of Mexico prior to the arrival of European settlers (artist impression)
New carbon-dating analysis examining deep-cut layers of soil and agricultural remnants challenges the view that Cahokia fell due to crop failure and drought. Above, US Bureau of Land Management archeologist Caitlin Rankin collects samples from the Cahokia Mounds
'It's possible that they weren't really feeling the impacts of the drought,' said archeologist Dr Caitlin Rankin with the US Bureau of Land Management, who worked with the Washington University in St Louis on the new soil analysis.
The technique involved hunting for traces of differing versions, or isotopes, of carbon atoms, which are present in all living organic matter.
Residue from the plant life that would be key to solving the mystery of how badly these 600-year-old droughts impacted Cahokia all left unique traces of the carbon isotopes Carbon 12 and Carbon 13.
Plants that had adapted to drier arid climates — including prairie grass and maize, the corn-like crop that had only just recently found its way up to North America from Central America — left similar ratios and concentrations of carbon isotopes behind.
But other local crops likely grown by Cahokians, like squash, goosefoot, and sumpweed, left a different radio-carbon signature, as did other native plant species.
The crucial discovery, as published in the journal The Holocene this June, was that farming life didn't change for Cahokia during those drought years.
The archeologists found these isotopes, surprisingly, did not change during or after the drought, suggesting Cahokia's agriculture remained unharmed by the dry spell.
Despite their expectation to find an abandoned city overrun with weeds, the researchers discovered the same mix of crops and wild plants all across the city.
'We saw no evidence that prairie grasses were taking over, which we would expect in a scenario where widespread crop failure was occurring,' said Dr Natalie Mueller, assistant professor of archaeology at Washington University in St Louis.
Distinct from the Maya or Aztec people to the south, the Cahokia emerged in the Mississippi Valley over a thousand years ago, around 700 AD - in what is now the state of Illinois, across the river from present day St. Louis, Missouri (mapped above)
At their apex, the Cahokia had constructed an estimated 120 earthen mounds: the largest city north of Mexico prior to the arrival of Europeans. Many of the mounds might be better described as great pyramids, cornered square at the bottom and smoothed level at the top
Dr Mueller said she now envisions a scenario, not unlike many of today's modern and fast-evolving cities, where new generations sometimes move out to other areas over time for a variety of cultural, familial or economic reasons.
'I don't envision a scene where thousands of people were suddenly streaming out of town,' she said in a press statement.
'People probably just spread out to be near kin or to find different opportunities.'
Cahokia was intentionally built on a flood plain along the Mississippi River, across the river from the modern day city of St Louis, now home to Washington University.
The benefits of this flood plain, according to Dr Mueller, might explain why drought conditions do not appear to have undermined the Cahokian's agricultural projects.
The two researchers also noted that the tribe was a sophisticated society whose food storage systems for grains and other foods would have aided them in riding out most potential droughts.
'This doesn't mean that [the drought] might not have affected other parts of the landscape, including some of the places where they grew food,' as the archeology professor told Newsweek.
'It also may have destabilized the region even if it didn't cause crop failure at Cahokia,' Dr Mueller added — a scenario where more drought-ravaged neighbors might have taken a toll on Cahokia's strategically better equipped food resources.
Above, an artist's conception of the Cahokia Mounds Site. The illustration shows the large Monks Mound at the center of the site with the Grand Plaza to the south. The site includes land across the Mississippi River from present day St. Louis, Missouri in south-western Illinois
A recent aerial survey of the Cahokia Mounds conducted flybys over more than 490 acres of the site, one of the largest single areas ever surveyed by the National Geospatial-Intelligence Agency via drone. This data was used to build a 3D mesh model of Monks Mound (above)
Distinct from the Maya or Aztec people to the south, the Cahokia emerged in the Mississippi Valley over a thousand years ago, around 700 AD — in what is now the state of Illinois, across the river from St. Louis, Missouri.
At their apex, this indigenous civilization had constructed an estimated 120 earthen mounds: the largest city north of Mexico prior to the arrival of European settlers.
Many of the mounds might be better described as great pyramids, cornered square at the bottom and smoothed level at their highest points.
Anthropologists believe these mounds — including the lost city's largest, the 100-foot Monk Mound — served as high ground to elevate, honor and protect homes of the Cahokia's civic leaders.
But by 1350 the society that build these impressive structures had vanished without explanation, just about a century before Columbus sailed into the Americas.
Why they left, with the drought theory less likely, now remains an even bigger mystery than ever before.
'The picture is likely complicated,' Dr Rankin noted. 'They put a lot of effort into building these mounds, but there were probably external pressures that caused them to leave.'
Decades of research into the city of Cahokia's eating habits and agricultural system show a society with a diversified and sophisticated diet, according to Dr Mueller.
'Cahokia's farmers grew at least eight crops,' Dr Mueller said. 'These included both warm and cool season grasses, oil seed crops, and highly nutritious pseudo-cereals, each of which is adapted to slightly different conditions.'
'In addition,' she added, 'Cahokia had access to an enormous freshwater fishery, one of the great flyways of the world, perennial wetland plants, and "food forests" full of nuts and fruits that had been shaped by local communities for millennia.'
The sheer abundance of this Native American society's well-selected urban mecca, she said, 'makes us skeptical that food shortages played a decisive role in the abandonment of the city.'