In the 1960s, plastics were just becoming popular. Polyethylene, which today is one of the world's most ubiquitous plastics, had been created in 1898, and then again in 1933. But it wasn't until 1953 that anyone figured out how to make high-density polyethylene—the plastic that's identified in the recycling system as No. 2 and that's generally used to make the type of grocery store plastic bags that California just banned.
In Sweden, a company called Celloplast—that had sold cellulose film—was working out ways to use and sell the stuff. In 1960, the company filed for a U.S. patent for "tubing for packaging purposes," designed by a team of three Celloplast employees. Their idea was that the tube of plastic, laid flat, could be sealed at regular intervals to create the bottom of a bag and left open at the top to insert whatever it was that needed to be packaged.
It was a good idea, but one of the team members, Gustaf Thulin Sten, apparently had a better one: Seal the bottom of the tube, but, on the other end, punch out part of plastic tube to create handles. In 1965 Celloplast obtained a U.S. patent for the idea that was later called "the T-shirt plastic bag," and it's the design, essentially, of every plastic bag you've ever been given in a grocery store check-out line.