How to identify silk from unearthed cultural relics? The answer is... "nucleic acid test"? !

On July 5, 2023, the Science Popularization China Starry Sky Forum, with the theme of "Empowering Cultural Relics with Science and Technology to Bring Them to Life", invited 5 cultural and historical experts from different fields to talk about how science and technology can empower cultural relics protection and make cultural heritage timeless from the aspects of excavation, restoration, display, protection and exchange of cultural relics.

Zhou Yang, deputy director of the China National Silk Museum, gave a speech: "Searching for the Origin of Silk"

The following is an excerpt from Zhou Yang’s speech:

China has been known as the "Silk Country" since ancient times, and silk is undoubtedly the symbol of China. Silk was the driving force behind the opening of the Silk Road, and through the Silk Road it became a global trade commodity.

Sericulture or sericulture is called Sericulture in English. Ser refers to silkworms and culture means culture. By extension, it means domestication of silkworms through artificial cultivation or breeding. Seres refers to the country that raises silkworms, which is the China in the minds of foreigners.

01

The history of silk is also the history of silkworms

Did silk originate in China? When and where did it originate? What kind of material elements and cultural opportunities were needed for the origin of silk? The origin of silk involves many important time nodes, but the most critical one is the domestication of wild silkworms to domestic silkworms .

Silkworm, an insect of the family Bombyx mori. Photo courtesy: Zhou Yang

Domestication is the process of modifying the genes of wild silkworms through human intervention. This is a profound change in nature that cannot be reversed. All changes are made to meet human’s preset goals.

This should have been an extremely long and particularly difficult domestication process. According to research by the team of Academician Xiang Zhonghuai from Southwest University, during this process, the genes of wild silkworms mutated at 354 sites , bringing about a series of changes in biological traits.

So, which kind of silk produced by silkworms and textiles are the silk we are looking for? We can answer this question with certainty: only the silk produced by silkworms that eat mulberry leaves can be considered as real silk, and the silk woven from mulberry silk is the real silk.

02

Why is China called the “Silk Country”?

Why did the Chinese go to such great lengths to domesticate wild silkworms? What was the original motivation? This involves the cultural opportunity of the origin of silk, which fully reflects the Chinese people's thinking about life and death . It is this unique Chinese view of life and death that gives the Chinese people endless motivation and enthusiasm to domesticate wild silkworms.

The life of a silkworm & China's view of life and death. Photo courtesy: Zhou Yang

The silkworm is an insect with complete metamorphosis. A tiny silkworm egg hatches into a larva, which eats and sleeps and gradually grows up. Finally, it spins silk to make a cocoon and pupates into a moth. Although its short life span is only 20 to 30 days, it goes through four very different morphological changes, which makes people full of curiosity. The transformation between movement and stillness aroused the ancient people's associations and thoughts on major issues such as life and death, heaven and earth, etc.

The egg is the source of life. The hatching of the egg into a larva is like the birth of life. The sleeping and waking are like the stages of life. The pupa can be seen as the end of life. The transformation of the pupa into a moth is what the ancients thought the soul would go after death. Therefore, our ancients associated the silkworm with life and death, and with heaven and earth. The silkworm is the heavenly insect, connecting heaven and earth, and enlightening life and death. (1996, Zhao Feng, "The Cultural Opportunity of the Origin of Silk")

Perhaps, Chinese silk was born in this early cultural background of harmony between man and nature . Therefore, the original motivation for the origin of silk was not to wear gorgeous silk clothes, but mainly to serve ghosts and gods, which has a special meaning.

Silkworms are very delicate creatures, and are easily affected by the environment in nature. In order to ensure smooth communication between heaven and earth, people carefully raised silkworms indoors. The invention of sericulture also began in the unique cultural background of China, and Chinese silk originated from this.

03

The history of silk research

The early physical evidence that can really explain the origin of silk usually refers to three:

Half a silk cocoon unearthed in Xiyin Village, Xia County, Shanxi Province in 1926, dating back 5,000 years;

The silk fragment unearthed in Qianshanyang, Huzhou, Zhejiang in 1958 is 4,200 years old;

The silk unearthed in Qingtai Village, Xingyang, Henan Province in 1983 is 5,630 years old.

Photo courtesy: Zhou Yang

Since then, research on the origin of silk seems to have fallen into silence, with few exciting new discoveries, as if everyone has forgotten this important academic topic.

In fact, it is normal if you think about it. The reason is nothing more than two words, that is **“cold”**.

First of all, the origin of silk is a "cold" topic . Compared with the large number of unearthed gold, silver, ceramic and bronze wares, the unearthed early silk is very rare. The origin of silk is a niche and unpopular topic, and it is difficult to become a hot topic of large-scale and wide-ranging research.

Secondly, searching for the origin of silk requires "high" technology . Silk is a protein composed of 18 kinds of amino acids, which is very complex and very easy to degrade. It is not difficult to imagine that in the long course of history, the silk buried in tombs or ruins has long lost its physical appearance and is completely unrecognizable to the naked eye. Turning into dust and mud, it is difficult to find the trace of silk in an invisible place, which requires relatively advanced scientific and technological means to find it.

Mudification, ashification, mineralization, carbonization (from left to right, from top to bottom)

The origin of silk is too far away from today. After years, the bright and beautiful silk has shown the ultimate state of mud, mineralization, carbonization, and ash. We call it residue or traces. Strictly speaking, the silk we are looking for is no longer a textile, so the traditional methods before are difficult to work.

04

How can technology break the deadlock?

The origin of silk is buried in the mist of history. To penetrate the mist, we need a beam of light. So, what should this beam of technological light in our minds look like?

Solutions to detection problems. Photo courtesy: Zhou Yang

First of all, it must be reliable and cannot be wrong , because the origin of silk is a very important academic proposition, and any conclusion must stand the test;

Secondly, the operation cannot be too complicated , because our battlefield is an archaeological site and our comrades are archaeologists. We hope that ordinary archaeologists will be able to conduct self-detection and obtain results after simple training.

Finally, we have to consider the cost , because if the cost of screening samples on a large scale in the site is too high, the research will be unsustainable. So many requirements, in short, are "profound and simple to understand", which means that no matter how profound the research behind us is, the method presented at the archaeological site must be simple and easy to use.

At this point, the principles of immunology come into our research field of vision. The basic principle of immunology is the antigen-antibody reaction, which is as precise as a key opening a lock. First, we need to lock the search target, which is the "antigen" in the immunology discourse system.

We found that silk protein has its own characteristic fragments, which are the molecular markers we identified. If we find this group of characteristic fragments that survived the catastrophe, we have found silk.

The most rewarding thing is that we have achieved our initial small goal and developed a low-cost, easy-to-use test strip that can complete the test in 15 minutes. This test strip is very similar to the test strip for the new coronavirus. When used, a positive result means there are filaments, and a negative result means there are no filaments.

Research and development results. Photo provided by: Zhou Yang

05

The earliest silk fabric ever found

Our technology was first used in some urn coffins unearthed from the Yangshao site, which is about 5,000 years old. The so-called urn coffin is a burial tool used by prehistoric humans to bury infants who died prematurely, and it exists in large quantities in many sites.

Hard work pays off, and the biggest surprise came from the urn coffin numbered W12 at the Wanggou site .

Cleaning the urn coffin. Photo provided by: Zhou Yang

I remember very clearly that on July 10, 2017, the weather was extremely hot and the urn was full of dirt. We cleaned it layer by layer, taking samples step by step according to scientific research standards, taking photos after taking samples, and cleaning it again after taking photos. Sampling, taking photos, and cleaning again. Finally, the small brush came to the skull. The dirt was a bit hard, and a piece of something smaller than the nail of the little finger appeared. The color was slightly different from the surrounding soil samples . We collected it very carefully.

The soil sample shattered at the slightest touch, so we carefully removed the residue for testing, trying to save the sample as much as possible. We all knew that if this soil sample could be proven to be silk, then it would be the earliest silk, and we must preserve this rare archaeological evidence.

Urn coffin W12 from Wanggou site in Henan, Yangshao period. Photo courtesy: Zhou Yang

Fortunately, our laboratory's methods are non-destructive or minimally destructive, and we did not consume too many samples. We conducted a set of tests on it. We can see that it has tenaciously retained the morphology of silk fibers under an electron microscope, but we still need to further confirm whether it is silk or hemp. We sent out a sharp tool, antigen-antibody test, and the result came out positive very quickly. This shows that this is the earliest silk object discovered in the world.

The earliest silk ever discovered. Photo courtesy: Zhou Yang

We cleaned 7 urns and found positive signals in 5 of them.

At the Shuanghuaishu site in Gongyi, not far from the Qingtai site, we used the same technology to discover silk. At the birthplace of modern Chinese archaeology, the Mianchi site in Sanmenxia, ​​Henan, silk was found in tombs from the late Yangshao and late Longshan periods , expanding the time and space scope of the origin of silk.

We also found a lot of silk in Sanxingdui , which is the earliest silk in the upper reaches of the Yangtze River and belongs to the late Shang Dynasty. This technology has also been well applied in Nanhai No. 1 , breaking the situation of "no silk in the Maritime Silk Road" and providing the latest archaeological evidence for the application of the Maritime Silk Road for World Heritage.

A large amount of silk was discovered in Sanxingdui. Photo courtesy: Zhou Yang

We are constantly refining our technology in the laboratory, and now we can say with some confidence that as long as there has been silk in this place, as long as it has not eventually entered the elemental cycle of carbon, hydrogen, oxygen and nitrogen, we can find it.

If Chinese silk is a 5,000-year-old symphony, then origin, development, dissemination and communication are the four movements. Among them , the movement of silk origin is the most oriental and Chinese wisdom . We hope that through our research, this topic will no longer be a cold topic. Let us work together to continuously push the origin of silk into the depth of history!