By Talia Ogliore
The silkworm moth, whose caterpillars have been used in silk production since antiquity, may have been brought to southern Central Asia 4,000 years ago — nearly 2,000 years earlier than previously assumed — according to new research from Washington University in St. Louis.
The discovery of three well-preserved cocoons at an archaeological site in Uzbekistan pushes back the earliest verified date for silkworms cultivated outside of China.
Up until now, it was generally accepted that silk products and their accompanying knowledge spread from ancient China to Central Asia and beyond within the context of the Silk Road, the famed route used by traders during the Roman and Han period. While many goods including spices, teas, porcelain and glassware were traded along this route, silk was the most coveted luxury item exported from China to the Roman Empire, giving the expansive route its iconic name.
The new archaeological evidence opens up the possibility of an earlier dispersal of silk and sericulture, according to Xinyi Liu , a professor of archaeology in WashU Arts & Sciences, corresponding author of a study in the journal Science Advances.
"This is the oldest reliable archaeological record of preserved silkworm cocoons with direct radiocarbon dates," Liu said. "Even in China, where the silkworm moth was domesticated, we don't have preserved cocoons as old as this, in part because of challenges with archaeological preservation."
The cocoons were discovered by a Chinese and Uzbek research team conducting excavations at three Bronze Age archaeological sites situated along the Surkhan-Darya River in Uzbekistan, including a site known as Sapalli Tepe. Locally arid conditions at Sepalli Tepe allowed for these cocoons to remain intact and identifiable among fragments of wood charcoal and plant seed remains. Silk proteins extracted from the cocoons were subsequently dated and confirmed to belong to the Bombyx mori silkworm moth species.
"The identification of these cocoons as Bombyx mori is particularly significant because it links them to the mulberry-based silk tradition that originated in ancient China," Liu said. "This differs from the non-Bombyx silk traditions historically prevalent in South Asia, including Tussar, Eri and Muga silks, which relied on other moth species and host plants.
"Our study reveals the presence of domesticated Bombyx silkworms, mulberry trees and other fruits and crops at nearly 2,000 BCE," Liu said. "Taken together, the evidence shows that East Asian Bombyx-mulberry sericulture had reached southern Central Asia by about 4,000 years ago, long before the establishment of the historical Silk Road."
Very hungry caterpillars
The practice of rearing silkworms by feeding them leaves from specially grown mulberry trees is historically important to cultures across Asia.
A story about the origin of silk is often retold. According to Chinese legend, silk was discovered by Empress Leizu, wife of the Yellow Emperor, when a silkworm cocoon fell into her hot cup of tea. As the tight knot of the cocoon loosened in the hot water, its fibers partly unspooled. The empress had a vision for how this filament could be used.
A single silkworm can spin a continuous filament of silk up to 900 meters in length. But to harvest silk in useful amounts, people needed a way to purposefully raise many silkworm caterpillars at once. And that meant feeding those very hungry caterpillars what they wanted to eat: mulberry leaves.
As the food source for Bombyx silkworms, mulberry trees — notably the white mulberry tree — were globally dispersed, but when and how this dispersal unfolded remains obscure, Liu said. At the Sapalli Tepe archaeological site, researchers also found chunks of charcoal that they identified as remnants of mulberry trees.
"The evidence of the mulberry tree is quite important. Ecologically, these trees are not native to Central Asia and could adapt to the southern Himalayan environment much better than the northern arid environment," Liu said. "So that's hinting at a trans-Himalayan connection, rather than an inner-Asian mountain connection. The recovery of citrus wood charcoal at the site further strengthens this interpretation, as cultivated citrus likely originated in South Asia.
"Why this early dispersal of Bombyx-mulberry sericulture, possibly along a Himalayan route, did not lead to a clearly documented and enduring Bombyx silk tradition in the region remains unclear," Liu said. "Historically, South Asian sericulture was characterized with use of non-Bombyx silkworms. Future research investigating archaeological evidence along both the northern and southern fringes of the Tibetan Plateau will therefore be timely."
The study also describes additional evidence for local silk production in the form of a fabric fragment recovered from Sapalli Tepe during an older excavation, completed by Soviet archaeologists in the 1970s. Although a new, direct analysis of this fragment wasn't possible due to conservation restrictions, Liu said existing documentation confirms that this fabric was produced using a traditional plain-weave technique of spun yarn, consistent with the Chinese style of silk manufacture.
"The co-occurrence of Bombyx mori cocoons, mulberry charcoal and silk-like fabrics at Sapalli Tepe indicates an unambiguous connection with East Asia sericultural traditions," Liu said.
Another form of early globalization
The findings from this new study are the latest to challenge the conventional timeline of the Silk Road and of Eurasian interconnectivity. Liu is one of a handful of scholars who have recently uncovered evidence documenting the spread of cereals, fruits, metallurgy and livestock connecting ancient China with the Central and South Asian environments during the third and second millennia BCE — much earlier than the Han dynasty and the Silk Road.
"We know this was a period of time when communities across the Eurasian continent were exchanging goods and learning from one another, in some cases underlined by population movement and in some cases by cultural inspiration," Liu said.
"Before this evidence, we saw the appearance of a globalization focused on domesticated grains — such as wheat, barley and millets — and livestock, including sheep, goat and cattle," he said. "Now it looks like making silk fibers is part of that early globalization."
This alternate path and timeline adds to the momentum for rethinking the Silk Road, Liu said.
"It is likely that dispersals of silk and sericulture were much older than the Silk Road, starting from the Bronze Age," he said.
This study was completed in collaboration with researchers at the Chinese Academy of Sciences in Beijing and the Academy of Sciences of the Republic of Uzbekistan.
