SAN FRANCISCO — The oldest pair of blue jeans in the Levi's archive are kept in a fire-proof safe to which only two people know the combination.

They are wide and durable enough to wear over long johns while out harvesting timber or mining silver in 1879, the year they were produced. Back then, the relic went by a different, old-timey name: waist overalls, to contrast with competitors' popular bib overalls.

The marketing slogan for the pants at the time: "For men who toil."

One of the two people who has unfettered access to the 136-year-old pair is Tracey Panek, the Levi Strauss & Co. historian.

"I like to think of them as the very first early sustainable garment ... you could wear them out, you could pass them on, you could patch them up," she said in a tour of the Levi's archives in February. They were first made for men who worked in mines, cowboys and farmers, and developed in parallel with the history of the American West, before going global.

"They were built to last," so they tell us volumes about our pioneering predecessors who wore them.

The oldest pair in the Levi's archives date from 1879. Note the whiskering on the back of the knees. Image: Kate Sommers-Dawes/Mashable

More than a century later, consumers worldwide wear blue jeans an average of 3.5 days a week and own 8.6 pairs (based on a 2008 survey). They're arguably the most personal garment a person can wear — they form to the body over time and are egalitarian in the sense that everyone, from babies to presidents, has a pair. With high-end lines costing the better part of a grand, however, they still make a statement about your socio-economic status, as they did a hundred years ago.

And, at least in Levi Strauss & Co.'s case, it started with the metal rivet used to attach straps to horse blankets.

"The secratt of them Pents is the Rivits that I put in those Pockots," the Nevada tailor Jacob Davis wrote to the successful San Francisco merchant Strauss in 1872. "I cannot make them up fast enough ... My nabors are getting yealouse of these success and unless I secure it by patent papers ... everybody will make them up."

The pair from 1879. White gloves are required when handling archival garments to prevent dirt and oils degrading the material. Image: Kate Sommers-Dawes/Mashable

The duo quickly patented the durable pants in 1873 and the oldest known pair, pictured above, date from about six years later.

These waist overalls were called the XX, for the material produced at the Amoskeag Denim Mill in New Hampshire. They only had one pocket in the back, as well as three in the front, including the little one, likely used for a pocket watch. (The fifth pocket wouldn't be added to the back until 1901.)

By 1890, they would become the 501 that Levi's still sells today, and the basics would remain, with a few exceptions.

There are no belt loops on the XX — just a few buttons around the waist to attach suspenders, the preferred method of keeping pants up in the late 19th century, and a cinch in the back for adjustable size. Based on the wear patterns at the knees, Levi's archivists have concluded that at least three people wore the pair. (Another archival pair from 1879, which has one leg cut off, presumably for patching purposes, appears below, with standard cinch in view.)

The cinch, which would remain on the waist of Levi's jeans until World War II, when materials had to be conserved. Image: Kate Sommers-Dawes/Mashable

Another pair of jeans in the archive, nicknamed Calico for the California silver mine where a woman discovered them in the '40s, date to about 1890. The inside pocket features the two-horse trademark, which was introduced in 1886 and was meant to signify the durability of a pant that customers would wear for decades and "couldn't be pulled apart with two horses."

A pair of Levi's jeans from 1890, featuring the two-horse logo that had been introduced four years earlier. Image: Kate Sommers-Dawes/Mashable

"For over 17 years our celebrated XX blue denim, copper riveted overalls have been before the public," the pocket reads. "This is a pair of them."

A pair of lovingly-patched Levi's from circa 1890. The cuffs (not pictured), torn at the back, were likely worn through by a cowboy's spurs.

The company introduced the first pair of jeans for women in 1934 and competitors Lee and OshKosh quickly followed suit after the 'Lady Levi's' were featured in a Vogue magazine summer travel spread in 1935, according to Global Denim, a cultural study of the iconic garment.

A 1938 pair of the ladies' line live in the archive. They belonged to Harriet Atwood, according to the tag she sewed in, who bought them on 5th Avenue in New York City, likely to visit a western dude ranch. They're slimmer cut than the originals, high-waisted, retain the cinch from the earlier pairs, but favor the belt loop over the suspender button.

Ms. Atwood was apparently shy about her measurements — she cut off the trademark leather tag.

A pair of 'Lady Levi's' from 1938. The tag reveals where the owner purchased them: Best & Co. in New York City. Image: Kate Sommers-Dawes/Mashable

Harriet Atwood's 'Lady Levi's.' In those days, the arcuate on the back pocket was hand sewn. Image: Kate Sommers-Dawes/Mashable

Lady dudes at the Quarter Circle U Ranch in Crow Agency, Montana, 1941. Image: Marion Post Wolcott/Library of Congress

Jeans became increasingly popular for women during the Great Depression, alongside an increased emphasis on both the working class and western culture, the garment's target audiences. Even more so during the Second World War, when they signified a patriotic statement about getting to work for the war effort.

The scarcity of textiles, however, affected the design during the war — Levi's jettisoned the extra crotch rivet and the cinch, which were never to return. To save thread, the arcuate on the back pocket was painted on, instead of stitched.

A pair of un-worn women's Levi's from the 1940's, with a painted-on arcuate to save thread. Image: Kate Sommers-Dawes/Mashable

"This is the kind of pant that people are trying to recreate today," Panek said of the oldest pairs, "but it's all just original wear."

She's not exaggerating. The dying, selective fading, whiskering, stonewashing and sandblasting required to treat raw denim to achieve high style are all based on the textile's natural wear over time. As Daniel Miller and Sophie Woodward put it in Global Denim, we're all too busy now to create it naturally. "So commerce provides us with this ‘as if’ scenario in pre-distressed jeans.”

At Levi's, that's achieved at the Eureka Innovation Lab, a workshop in San Francisco where denim scientists tinker with the chemical processes that help a designer make the raw material look, well, cool.

Here, after programming a wear pattern into Adobe Photoshop or Illustrator, a technician uses a giant laser to burn a faded pattern into an indigo pair, which then gets softened by hand with sand paper before it goes in the washer. The high-tech process condenses decades of wear into seconds.

A pair of jeans, after it has been lasered, gets a sand paper treatment. Image: Kate Sommers-Dawes/Mashable

Though more disposable than it was in the late 19th century, the "early sustainable garment" Panek envisioned looms large in the modern landscape.

On Tuesday, Levi's announced it has saved 1 billion liters of water since 2011 through the water-reducing finishing processes it utilizes in its WaterLess line of jeans, while encouraging consumers to track and reduce their own water use.

That's the thing about the history of the blue jean — it reflects who we are even as we change; the more we change, even. The textile that looks lived in shows how we lived in it; farmers, innovators, factory workers, rebels, entrepreneurs and cowboys alike.