The piano is a string percussion keyboard instrument with. To become what we call today a modern piano, the first piano instrument went through some changes. To understand how and why these changes occurred we must look back into the past and mention them. But how far should we look back? Ancient Greece? Ancient Egypt? Clearly no. The first instrument that can be called the first ancestor of the piano is more likely to be the clavichord.
Invented in the early fourteenth century, the clavichord (from the Latin word clavis meaning key and chorda from Greek meaning string) was described (at that time) as one of the best instruments to accompany melodies. Pressing the keyboard's keys followed up with strikes of the brass or iron strings with small metal blades called tangents thus, producing sound. Vibrations were then transmitted through the bridge(s) to the soundboard. Bellow you can see a picture with this instrument.
Next in line is the harpsichord, which produces sound by plucking a string (unlike the clavichord which produces sound by striking a string) when a key is pressed. The harpsichord was most probably invented in the late Middle Ages. By the 16th century, harpsichord makers in Italy were making lightweight instruments with low string tension. This instrument was widely used in Renaissance and Baroque music until the piano forte gained popularity. Bellow you can see how an 18th century harpsichord looks like.
Other variants of this instrument: Virginal, Spinet, Archicembalo, Clavycimbalum, Clavicytherium, Lautenwerk, Ottavino. Although these are variations of the harpsichord, these instruments have specific characteristics which give them an unique timbre.
The early version of the piano was the Fortepiano invented by the Italian instrument maker Bartolomeo Christofori around 1700. This instrument could provide dynamism to music, unlike its close ancestor the harpsichord which didn't offer the possibility of dynamics in sound (basically, all notes had the same volume regardless of the pressure you applied to any key). Bellow you can see how a 19th century fortepiano looks like.
Notice that it is a smaller instrument than the modern piano we know and that the black keys are white and the white keys are black?
It was Christoph Gottlieb Schroter (1721) who later improved the fortepiano by adding a hammer-based mechanic and Gottfried Silbermann (1730) who added the damper pedal (the modern sustain pedal) which lifts all the dampers from the strings simultaneously. Cristofori, Schroter and Silbermann's ideas were developed by Robert Stodart, John Broadwood and Americanus Backers into the foundations of the English mechanics where the hammers were fixed on a wooden rod being directed to the strings using levers; they also use the “echapement” system which means that the key retracts to its original position.
Johann Andreas Stein played an important role in the development of the piano's mechanics, he created a system in which the hammers are placed directly at the end of each lever. Later on Sébastien Érard who invented the double escapement action, which incorporated a repetition lever (also called the balancier) that permitted repeating a note even if the key had not yet risen to its maximum vertical position. Other important manufacturers that played a significant role in the development of the modern piano are: H.E.Steinway, I.Pleyel, C.E.Friederici, W.Southwell, Vicenyo del Mela, J.Merlin, A.Beyer, etc.
Note that the development of pianos continue even as you read this. Bellow you can see a picture of the modern grand piano.