BEIJING, May 14 (Xinhua) -- Proteins extracted from six teeth of early humans residing in what is now China around 400,000 years ago provide new insights into how ancient genetic material may have made its way into modern humans.

An analysis of the tooth proteins from Homo erectus, the first human ancestor to walk out of Africa, was reported this week in the journal Nature. It identifies a genetic marker also found in Denisovans, a little-known early human group first found in a Siberian cave, and in some modern human populations today, suggesting the two ancient populations may have coexisted and interacted in parts of East Asia.

The findings were made possible by a nearly non-destructive method that maintains the fossils' integrity.

Until now, the genetic research on the Homo erectus has been limited because ancient fossils are too precious, and traditional sampling methods require grinding or drilling into the teeth. As a result, compared with other known human species, their genetic characteristics, population diversity, and especially their potential links to modern humans remain unresolved.

A Chinese research team overcame that barrier using a micro-destructive acid etching technique that removes only a tiny superficial layer of tooth enamel without visibly damaging the fossil.

"We use an acid solution to 'wash' the tooth surface, dissolving the superficial mineral layer and releasing the protein fragments hidden inside," said lead researcher Fu Qiaomei, a scientist at the Institute of Vertebrate Paleontology and Paleoanthropology under the Chinese Academy of Sciences. "This way, we can collect those ancient proteins without really damaging the tooth."

This technique, developed in 2011, was first used for sex determination of ancient human teeth from less than 10,000 years ago and subsequently adapted for the examination of other animal teeth.

Fu has been working on paleogenetics, specifically exploring human origins and evolution with ancient DNA. The woman scientist has been nicknamed the "DNA detective" by her colleagues.

From six ancient human teeth uncovered from three sites in China, including Zhoukoudian, where the Peking Man fossils were discovered, researchers extracted enamel proteins dating back at least 400,000 years. These are the oldest ancient human proteins ever obtained in East Asia, pushing the molecular record back from roughly 160,000 years to at least 400,000 years, the study said.

The researchers identified two key mutations in the tooth proteins. One, called AMBN-A253G, was found in all six fossil teeth but has never been seen in other human species, modern humans or apes. That makes it a unique molecular marker for this East Asian population and proves that the physically diverse fossils from the three Chinese sites all belonged to the same genetic group.

The second mutation, AMBN-M273V, had previously been thought to be specific to Denisovans. But the study found the same mutation in the much older 400,000-year-old teeth, indicating that it is shared by Homo erectus.

This suggests the mutation may have originated in the older East Asian population and then flowed into Denisovan groups through interbreeding. From Denisovans, it was passed into the ancestors of some modern humans, particularly in Southeast Asia and Oceania, after a second wave of interbreeding, a process scientists named "introgression."

The findings reveal a previously unknown aspect of human evolution: the East Asian Homo erectus was not only a distinct evolutionary lineage but also one of the sources of ancient genes in the modern human gene pool, Fu said.

Wang Wei, a professor at Shandong University who researches early humans and was not involved in the study, called the work "a startling discovery."

Wang noted that while China has rich records of Homo erectus, the genetic information of this human species had long remained a gap. The new study, he said, bridges Homo erectus and modern humans, providing significant scientific insights into human evolution.

Enrico Cappellini, an expert in ancient proteomics at the University of Copenhagen in Denmark, commented that the conclusions of the study are reliable, especially as the data used were generated using state-of-the-art methodologies.

The study also developed a new method, called protSexInferer, to determine the sex of ancient individuals based on tooth proteins, without relying on fragile DNA. Using it, the research team determined that five of the six ancient individuals were male and one was female.

The researchers say it is still unclear what physical traits these genetic mutations actually affect. But as a starting point, it is like throwing a stone into a pond. "It makes a big splash, and that splash will lead to more research, helping them figure out the rest," Fu told Xinhua.