Pottery and grains traditionally found in northern China were recently unearthed in the southwestern province of Yunnan, renewing a long-standing debate about how Chinese civilisation evolved in the region and offering clues about an ancient migration route.
Beijing and Kunming researchers found charred remains of wheat and millet nearly 4,000 years old in Yunnan. The crops are typically cultivated in the Yellow River drainage basin, but not further south. The finding suggests the early settlers in Yunnan cultivated the crop far earlier than previously thought.
"Nobody expected wheat or millet this old from Yunnan," said Peking University researcher Jin Hetian after presenting the findings at an archaeology conference in Fukuoka, Japan, in June.
The findings have not been published in any journals, pending further data analysis. But they could shed new light on Yunnan's Dian people, who are credited with creating one of the four distinct Bronze Age civilisations, a society one researcher described as "almost as mysterious as the Mayans".
More than 1,000 grains of wheat were identified in dirt samples taken from the Haimenkou excavation site in northwest Yunnan, and carbon-dating tests showed the wheat samples were from 1,600BC to 400BC. A few hundred charred millet grains were also found, some dating back to a much earlier era.
Scientists hope that the archaeobotany study - examining plant remains from archaeological sites to understand a certain society's lifestyle and economy - may confirm a long-held, but unproven, theory that millet entered the Indochina peninsula through what is now Yunnan.
The Haimenkou site also offers clues about an ancient migration route across China. Pottery and bronze artefacts unearthed there are strikingly similar to those in the Qijia culture (2400-1900BC) that existed from the Neolithic to the Bronze Age in the north in present-day Gansu province.
The new evidence suggests a connection between the southern and northern regions, which are some 1,500 kilometres apart, the study says.
"Painted pottery and jars with single or double handles are similar to those typical of the Qijia culture," Jin said as she read the findings at the Fifth World Conference of the Society for East Asian Archaeology.
Jin speculates that some northerners such as the Qijia people, who grew crops, and made pottery and bronze, could have migrated south in search of warmer climes. "This most likely has to do with climate changes some 5,000 years ago," she said. "Temperature dips in northwest China around 3,000BC may have driven the local inhabitants to warmer places."
One possible scenario is that early migrants completed a daunting journey to Yunnan that spanned hundreds of years, on a trail that covered mountain ranges and waterways, according to Jin. She theorises that the Qijia people followed this trail, which eventually led to Haimenkou.
The earliest wheat grains found at the excavation site date to 200 years after the Qijia culture died out in Gansu, suggesting that it took many generations before these prehistoric refugees reached their destination.
However, some researchers cast doubts on the theory.
Zhao Jianlong, of the Gansu Province Institute of Archaeology in Lanzhou, called the idea of a mass migration through the southwest's forbidding terrain implausible.
"Can you imagine these sedentary crop growers - not nomads - traversing hundreds, even thousands of kilometres? I just don't see it," said Zhao, who was not part of the study.
Zhao is leading a group of archaeologists in Gansu in a project to save valuable artefacts from excavation sites in Yongjing county that are in the way of a high-speed rail line being built to link Lanzhou and Urumqi. Pottery fragments attributed to the Qijia have been found at these sites.
Early human migration may have been more common and frequent than Zhao is prepared to believe. Artefacts and plant remains found in valleys leading all the way to Yunnan mark a possible route for some of these movements.
"The eastern edge of the Tibetan Plateau was buzzing with traffic in the Neolithic [period] to the Bronze Age," said Professor Zhao Zhijun, director of archaeobotanical studies at the Chinese Academy of Social Sciences, who was also not involved in the Peking University study.
Zhao led an expedition in November 2010 down this ancient route, starting in eastern Gansu, skirting the Sichuan Basin and ending in Chuxiong city in Yunnan, about 300 kilometres from Haimenkou. His team found resemblant pottery, bronze and millet grain that suggested a common origin in the northwest. Still, Zhao noted that they found no wheat samples. "So while it is very likely that some people brought the Qijia craft down to Yunnan, we need more evidence to figure out exactly where wheat came from," he said.
As to what drove the migration, Zhao theorises it was due to population pressure. "Early human colonies expanded as their population grew," he said. "In fact, the settlers of today's northwest China attempted to explore every way they could. When their eastbound march was thwarted by the early Central Plain dwellers, they simply found their way to the sparsely populated southern territory [because it was] easier and less hostile."
Whoever brought wheat into Yunnan settled down and interacted with the local rice growers, the Peking University study suggested. The new findings also show that wheat eventually replaced rice as the dominant crop of the region in the Bronze Age.
Min Rui, of the Yunnan Provincial Institute of Cultural Relics and Archaeology in Kunming, who led the most recent excavation of Haimenkou in 2008, says wheat farming could have spread either through wars or intermarriage among tribes.
Min also said more evidence, particularly DNA proof, was needed to establish whether newcomers conquered the region and drove out indigenous people, or if a new ethnic group emerged.
But according to Min, one thing is certain: the diverse geography of northwestern Yunnan, with soaring mountains and deep valleys, means many colonies must have settled there, perhaps speaking distinct languages but still interacting with each other.
The Peking University study is part of a broader, ongoing collaboration with archaeologists in Yunnan to learn more about ancient humans in the region, which is apparently the halfway point on Neolithic- and Bronze-Age communication routes within and beyond China.
"When you think about it, Yunnan was at the crossroads of the inter-regional communication. Whoever travelled to [present-day] Vietnam or Myanmar must have passed through there, as did the ancient travellers to Tibet," Jin said. "Yunnan is promising in terms of providing more insights into the early inhabitants of the region and how they helped spread technology like agriculture and bronze-making further to the south and west," she said.
Haimenkou may hold the key to further understanding the spread of crops, said Zhao Zhijun, including the answer to the puzzle of which ethnic group introduced millet to Southeast Asia.
"Now that we have evidence of wheat and millet in Bronze Age Yunnan, we really have more questions to answer like where they came from and where they went," he said.
This intercontinental pattern also applies to wheat. Although wheat was grown in northwest China about 1,000 years before its emergence in Haimenkou, Min said one theory was that the crop was brought to Yunnan from India.
Scientists are currently trying to unlock the secrets of these seemingly ordinary grains.
"The plant was, and still is, everywhere in our life," said Zhao Zhijun. "We eat it, wear it, make furniture with it, and use it in so many other ways. If archaeology is about reconstructing the way of life of an early community - what they ate, what they wore, what gods they worshipped and what they did with their leisure time - then I can't think of a single archaeological question that archaeobotany cannot help to answer."
Jiang Zhilong, a researcher on ancient Yunnan civilisation at the Yunnan Provincial Institute of Cultural Relics and Archaeology, said: "The Dian people created one of the four distinct Bronze Age civilisations in the world, along with the ancient Greeks and Romans, the Central Plain Chinese and the Eurasian steppe societies."
It was a war-like society. Bronze sculptures attributed to the Dian people depict them fighting, hunting, herding horses and cattle and growing crops, forging a prosperous civilisation, many parts of which are still shrouded in the unknown, Jiang said.
"This is a society almost as mysterious as the Mayans," he said. "We have just started to understand it and many questions are still waiting to be answered."