For archaeologists in the far-off future, two-legged cotton garments dyed with indigo will be classified as one of the most common human adornments of the 20th and 21st centuries. In typical archaeological fashion, these "jeans" will be assigned various subcategories, such as "skinny" or "bootcut," and their geographic ubiquity will be held up as evidence for rapid globalization of trade and culture during the time period.

These future "jeanaeologists" will also point back to an intriguing report, published today in Science Advances, that shows that humans were already dyeing cotton with indigo at least 6,000 years ago.

This discovery came as a surprise to the researchers who analyzed eight fragments of cotton textiles excavated at Huaca Prieta, a site in northern Peru that was occupied between 14,500 and 4,000 years ago. The torn and cut textiles appeared to have belonged to bags or containers, and most were no more than a foot square.

"The textiles were originally very, very dirty," says archaeologist Jeffrey Splitstoser, an expert in textile structures and co-author of the research paper. "You could see blue in some of the samples but they were mostly grey. You know how your blue jeans fade over time? Well, these were like 6,000-year-old blue jeans." (Read how ancient Peruvians were making popcorn 6,700 years ago.)

View Images This cotton band or belt from Huaca Prieta is between 4100 and 3500 years old, and has an indigo-blue center stripe accented with yellow ochre. Photograph courtesy Lauren A. Badams

Researchers used high-performance liquid chromatography to identify the presence of plant-based indigo dye in five of the eight fabric samples, which ranged in age from 6,200 to around 1,500 years old.

Until this discovery, the earliest-known indigo-dyed textiles were from Egypt's Fifth Dynasty and dated to approximately 2400 B.C.

Researchers believe that cotton was domesticated independently in several parts of the world, including South Asia, the Middle East, and Mesoamerica. Peru's northern coast, however, is believed to be the earliest area of cotton domestication. The region's native cotton, Gossypium barbadense, is also known as Pima cotton, and the strain became a backbone of modern cotton industry hybrids, says Splitstoser.