At an ancient settlement in Peru, archaeologists have discovered scraps of cotton fabric with faded blue stripes. These textiles—some of which are more than 6000 years old—are the earliest known examples of indigo dye use in the world, according to a study published today, September 15, in Science Advances.

The cloths came from a site on the north coast of Peru known as Huaca Prieta, notable for its monumental ceremonial mound. First excavated in the 1940s, the site held hundreds of bunched-up, dirt-caked textiles, discovered between layers of construction and buried under concrete-like mixtures of soot, salt, sand, clay, and shell.

Lauren Urana

“This building material was almost waterproof,” study author Jeffrey Splitstoser, an archaeologist and ancient textile expert with George Washington University, tells mental_floss. For the excavators, the tough material meant difficult digging—but also excellent preservation: “It sealed the textiles in. It made an almost oxygen-free environment. There was almost no breakdown of the fibers."

Once the fabrics were cleaned, Splitstoser noticed that many fragments were dyed blue. Using high-performance liquid chromatography and photodiode-array detection, he and his colleagues were able to confirm that the dye was indigo—the same natural dye compound that’s used to make blue jeans today. (Synthetic indigo is also widely used to color blue jeans.)

Making indigo dye from one of the many indigo-producing plants (usually of the genus Indigofera) can be a complicated ordeal—and the fact that it was done by the people of Huaca Prieta, who hadn’t even started making ceramic pottery, is evidence for technological sophistication. “It’s not an intuitive process at all,” says Splitstoser, himself a weaver who grows and produces his own indigo dye. While other plant dyes can be made simply by boiling down certain plants, with indigo-producing plants, you have to first ferment the leaves.

The incorporation of indigo at Huaca Prieta started out simple, with stripes and bands in the earliest examples. But textiles from later periods have more complicated designs, including checkerboards and geometric figures of crabs, snakes, and birds.

It’s not totally clear what these textiles were used for. Splitstoser says the fabric had no signs of the stitching you would usually see on clothing for features like neck slits or arm holes. He suspects the cloths might have been part of ceremonial bundles used in some sort of ritual. “It’s such an early culture,” Splitstoser says. “We know almost nothing about them.”

Lauren Urana

Jenny Balfour-Paul, author of Indigo: Egyptian Mummies to Blue Jeans, was excited by the findings. (Balfour-Paul wasn't involved in the study.)

“Indigo is well known as one of the oldest of dyes discovered and used in different parts of the world—mummy wrappings of Egypt and China as well as ancient Peruvian textiles—but this new date makes its discovery far older than anyone thought," she tells mental_floss. "And it is on cotton too, which is a less absorbent fiber than wool."

While we don't know where the cotton plant used in these fabrics, Gossypium barbadense, was first cultivated, we do know people were weaving with it in the same region as Huaca Prieta at least 7800 years ago. This variety has seeds that are difficult to remove from the fiber (Eli Whitney's cotton gin helped with this), but it also produces fine, silky yarns.

Cotton didn’t used to be such an important crop in Europe, Splitstoser explained, but when the Spanish colonists came to the Americas, they were impressed by the quality of G. barbadense, which has extra-long staple fibers that make it a choice source for fine fabrics. Indigo and cotton production became major industries for the Spanish and the British.

Without the early innovations of ancient South American textile workers like those at Huaca Prieta, our sartorial choices might look different today. “There actually is a relationship between the blue jeans that we wear today and the findings,” Splitstoser says. “Instead of wearing cotton blue jeans, we would probably be wearing linen.”