I know the Lightning Network (which Electrum wallet experimentally supports) is an implementation of a payment channel, but Mastering Bitcoin (2nd ed.) ch. 12, § "Routed Payment Channels (Lightning Network)" says:
The Lightning Network was first described by Joseph Poon and Thadeus Dryja in February 2015, building on the concept of payment channels as proposed and elaborated upon by many others.
What are the "many others"? Is the Elements Project's "pegged sidechains" "building on the concept of payment channels" too?