According to Joseph Banowetz (in The Pianist’s Guide to Pedaling, which is freely downloadable online to anyone who registers at the Indiana University Press website), the knee lever actually dates back to around 1765 on German pianos. He states that a foot-controlled mechanism with a cleft pedal was introduced in 1777 by Adam Beyer in London, providing separate control of bass and treble groups of dampers. Various split-pedal systems were superseded by our familiar single-pedal damper control around 1830, though Banowetz doesn’t elaborate on which maker(s) first introduced it.