"Evidence of tooth-picking was also found on fossils on Peking Man, who was estimated to be between 580,000 and 280,000 years old. But scientists also doubted that the wear on their teeth was caused by tooth diseases," Xing says.

"The Yiyuan's tooth-picking behavior could be the earliest evidence for early human's tooth-picking as a habit in eastern Asia," he says.

The fossils belonged to the Yiyuan hominids, which refers to some skull, supraorbital torus and teeth remains of at least two ancient humans that were discovered in Shandong province in 1981 and 1982.

Earlier research on the Yiyuan hominids showed that they were the earliest people living in the middle and the lower reaches of the Yellow River. One of the Yiyuan hominids was believed to be 40 years old, the other was in its 20s.

"Earlier researchers had carried out detailed morphologic research on the Yiyuan hominids. However, we are restudying these fossils as the researching range of paleoanthropology was extended and the research techniques were enhanced," says Sun Chengkai, a researcher of the Shandong Museum and an author of the article.

Scientists from the Institute of Vertebrate Paleontology and Paleoanthropology found the tooth fossils worth studying again during a visit in 2009. The fossils are still preserved at the Shandong Museum.