the MetMuseum has a series of short films on Art, mostly about pictures, but this short film combines science and art and music...
the harpsicord plays strings by plucking them: in constrast the piano hits the strings with a hammar, as in a hammared dulcimer.
history of the harpsicord HERE.
plucking strings with fingers or feathers goes way back, but who decided to use a keyboard to pluck the string?
the keyboard started with the organ type musical instruments,
the "water organ" dates to 300 BC but again no information on who invented the keyboard to play them.
link keys were shaped like right angles. The top of the angle hooked onto a wooden slider, and the bottom was pressed to pull the slider horizontally off a valve, which allowed the pressurized air to escape into a sounding pipe.
more HERE about ancient organs and keyboards...keys were originally the same size as the pipes and allowed the water or air to flow through the desired pipe. presumably when air allowed a greater range, the keyboards evolved, but this is not stated.
the HurdyGurdy dates to the 14th century... and is related to the organistrum, which dates to the 10th century.
so the "keys" were used on the bridge to press the string as a fret while a rotating wheel powered the bow.
The organistrum is the earliest known stringed instrument to be played with keys (learn the oldest known keyboard ever). It originated in Spain as a simple, one-octave cousin of the medieval fiddle, with ties to the bowed Byzantine lyra.
when I feel more alert maybe I'll find a book about all of this, or google more details.