Pull tabs were invented in 1959 (patented in 1963). They replaced church keys, which were in use starting 1934.

The tin can was in use by the Dutch navy since at least 1772, and was patented in 1810. The can opener was patented in the UK in 1855, a good 100 years before the ring tab.

The first cans were very beefy, and instructions stated to open them using a chisel and hammer. In practice people used whatever tools they had handy. I've seen illustrations of people using guns when nothing else was available.