For today’s children, it must be difficult to imagine a time before backpacks. They’re ubiquitous in classrooms and on school buses around the country and have been for decades. But as late as the 1960s, they weren’t widely available.

Back then, the simple act of carrying stuff to and from school was difficult.

“Students had no choice but to tote their textbooks and notebooks around campus with their hands,” wrote backpack innovator Skip Yowell in his book “The Hippie Guide to Climbing the Corporate Ladder & Other Mountains.” “Some tied a belt around them or clutched them to their chest as they walked. Either way, lugging study material was little more than a glorified juggling act — without the pay.”

What saved students from this avalanche of loose books and papers? The smart retooling of an existing product. Day packs, the smaller, lighter offspring of hefty hiking backpacks, were already popular among recreational climbers. JanSport, the company Yowell began with Murray McCory, formerly Pletz, and Jan Lewis in Seattle in 1967, made its own line. It was when these day packs made their way into university bookstores that the revolution began.

By the early ’70s, the sports shop inside the University of Washington bookstore started selling JanSport packs — and not just to outdoor enthusiasts. According to Yowell’s memoir, the bookstore manager soon suggested improvements that would further enhance the product’s appeal to students, and the day pack’s design began to evolve and become more student-friendly. Yowell and McCory added jam-proof nylon coil zippers and used vinyl — and later leather — to reinforce the improved packs, which sold well and eventually landed in Oregon, Idaho and beyond.

“Towards the end of the ’70s,” Yowell wrote, “many college bookstores were carrying our revised day packs in stock.” Over the past three decades, the company has built a school backpack empire. USA Today reported in 2007 that since 1979 JanSport had sold 25 million of its best-selling SuperBreak pack.

Still, it took awhile for the backpack revolution to reach the East Coast. In 1980, a Harvard Law student sent L.L. Bean product manager Ned Kitchel a suggestion letter. It would be great, the aspiring lawyer wrote, if you designed something that comfortably carried my heavy books.

“The guy has no clue,” Kitchel said, “what impact he had on the day-pack business in the United States.”

Two years after receiving that fateful letter, L.L. Bean introduced the Book Pack, the Maine-based mail-order giant’s first backpack made specifically for students. (It had been selling day packs for years, including, for a time in the ’70s, some manufactured for the company by JanSport.) The one-pound nylon Book Pack, which cost $25 and initially was available in only three colors, looked unremarkable. But since 1992 alone, L.L. Bean has sold more than 12 million Book Packs.

Admittedly, Kitchel didn’t set out to create an icon. In the early ’80s he attended a Las Vegas trade show, where he met designer Marcia Briggs.

By then, Caribou Mountaineering, the Chico, Calif.-based company she had co-founded in 1974, was already in the day-pack business. In the mid-’70s, Caribou co-founder Gary Kirk — at the time also a college student — asked Briggs to build him a pack that held his stack of chemistry books.

“That’s kind of crazy,” she remembers thinking. “Sure, I’ll do that.”

The squared-off bag Briggs came up with, the Cricket, spawned a successful campus day-pack line. The packs first sold at a local community college, then at Chico State, then at universities nationwide. Soon, those day packs made up about 40 to 45 percent of Caribou’s sales. At that point, Briggs was ready to market her company’s products to an even larger audience. To her, teaming up with mighty L.L. Bean was the logical next step.

“If you could put something in the L.L. Bean catalog,” she said, “It would do well.”

Luckily, Kitchel was on the lookout for a book bag. When he looked at the concepts Briggs came up with for L.L. Bean, he was immediately impressed. The design Kitchel picked was roomier than a hiking pack and also squared off to fit books. It was made of water-resistant 420 denier nylon packcloth, and it had a spacious main compartment, adjustable foam shoulder straps, and an organizer pocket for pens and pencils — all features that ended up in the first L.L. Bean Book Pack.

The Book Pack first appeared in L.L. Bean’s fall 1982 catalog. At the outset, it was manufactured by Caribou. In the late ’80s, L.L. Bean took over production, which had already moved overseas. According to “Guaranteed To Last,” L.L. Bean’s 224-page official history, the Book Pack wasn’t an instant smash, but “sales grew steadily as word of mouth and reviews in the media spread the story of the indestructible book bag.”

As the decade came to a close, the Book Pack evolved. L.L. Bean introduced the $32 Deluxe edition in 1989 and added a second big pouch, a waist belt, and a safety-conscious diagonal reflective strip. (The 3M-produced material was also added to the smaller model, which got a new name: the Original Book Pack.) Later, the Deluxe got ergonomic curved shoulder straps, more reflective tape, and a port for headphones.

The rise of the backpack signaled a societal shift. The book bags eventually spread from universities and law schools to elementary schools.

And school at all levels had become serious business. Cultural anthropologist and author Grant McCracken views the book bag as a symbol of the centrality of education. The days of kids carrying small stacks of books to school in their arms are far, far behind us. Children with big backpacks, McCracken said, are “weighed down and kept from levity.”