Take a good look at the piece of cloth pictured at the top of this story. That was made 6,000 years ago by people living on the coast of Peru.
Now check out those faint blue lines running through it. Yes, they're washed out, but you can still see them.
This square of striped cotton, and a few others like it, represents the first known instance of people using indigo to dye a textile blue.
The ancient Peruvian fabric is more than 1,500 years older than the earliest known Egyptian fabrics with indigo-dyed borders and 3,000 years older than the first blue-dyed textiles in China, according to a study published this week in the journal Science Advances.
"It is possible it is the earliest known example of cloth dyeing in the world," said Jeffrey Splitstoser, a textile expert in the department of anthropology at George Washington University. "I don't know of anything older."
The blue-tinged pieces of cloth were unearthed at Huaca Prieta, an ancient ceremonial mound on the north coast of Peru that was occupied between 14,500 and 4,000 years ago. Thousands of squares of the prehistoric textiles have been found at the site. Splitstoser said he has personally examined 800 of them.
The swatches were mostly square, ranging in size from 1 to 3 feet in length, although the larger squares were usually two pieces of textile that had been stitched together.
Not all the squares were made of the same weave, but oddly, Splitstoser said, all the samples he worked on were fragments of cloth that had been cut, torn or ripped from a larger piece of cloth.
"The preservation at the site is excellent, so their fragmentary nature is due to the fact that prior to being discarded, they were in that condition," he said.
The cloth pieces were not used for clothing because they had no arm, leg or head holes, and the edges were not treated or hemmed the way you would expect for even a simple item of clothing like a poncho, he said. Instead, he suspects that they may have been used to carry items to the site.
"If you got to the Andes today people will take a square of fabric about the same size as what we saw, put whatever they want to carry in the center and then wrap it up," he said. "I think they were carrying things in the bag to the temple and then ritually depositing or using them there and leaving the textiles there as well."
In addition, many of the prehistoric squares of fabric look like they had been wet and were discovered twisted and scrunched up, indicating that they had been dipped in liquid and then wrung out.
Splitstoser said many of the cloth fragments were found on a ramp that led to the top of what may have been a ceremonial temple at the time. There were also many smashed-up gourds on the ramp.
"I don't think it is too big of a leap of faith to think the gourds were carrying liquid and the textiles were carrying the gourds," he said. "Perhaps when the people got to the ramp, they poured the liquid in the gourd on the textiles and whatever else, then squeezed the liquid out of the fabric."
When he first started examining the swatches, Splitstoser couldn't tell they were dyed at all because they were so dirty. However, after they were cleaned in 2011, he started to notice a few faint traces of color.
"That's when I could see they were blue, and that's when I started asking around to see if I could get them analyzed," he said.
It turns out it is not easy to definitely detect ancient indigo. Indigo molecules break down over time and can get washed out of fabrics. It takes extremely sensitive equipment to detect it.
After a few failed tries, Jan Wouters, a chemist at the University College London, was able determine that the blue in the fabrics was indeed indigo and, further, that it was probably made from Indigofera, a genus of plant that has been widely used to produce blue dye across the world.
"It's interesting to see how long people have been using that particular plant," Splitstoser said.
He added that the find is a little surprising because indigo is not the most intuitive dye. Indigotin, the blue component in indigo, is not soluble in water, so it's not like you can just throw some Indigofera flowers in a vat of boiling water and extract the dye. Instead, you have to ferment the leaves, which turns the indigotin into another chemical that is soluble in water, but is not blue.
"It's actually kind of a yellowish color," he said. "In order to get the blue, you dip the clothes in the water with the dissolved indigo molecule, then when you pull it out it oxidizes, and that's when it turns blue."
That means that these ancient people living 6,000 years ago not only knew how to turn plant fibers into thread, and weave that thread into cloth, but also how to use complicated dye processes to stain the cloth new colors.
"In the modern world, we sometimes think of ancient people as primitive with a lack of understanding about the world," Splitstoser said. "But really, you had to be pretty smart to live back then."
MORE IN SCIENCE