The piano has developed greatly throughout history, as the creation of the actual piano didn't happen till the 1700s but before that, there were many instruments and inventions that inspired what the piano is today. Together let's look at how it has developed as stated:

14th century – The invention of the clavicytherium which is an instrument with strings arranged in a harp-like triangle

15th century – The clavicytherium was improved, as that led to the invention of the clavichord. improvements made include the addition of more strong and pins to allow the pressure to vibrate the strings. 

16th century – To make clavichord easier to play, some advancement was made to assign individual keys to the strings. As well as the addition of cloth in between the strings for it to act as a damper. 

1503 – The spinet was introduced by Giovanni Spinnetti, similar to a clavichord yet with longer strings and tangents that pricked the strings. Dissimilarly, to the clavichord, the spinet couldn't control the pressure or the strength of the tone.

1521 – The introduction of the harpsichord, which was an instrument that is played by plucking the strings that are set in vibration. 

1710s – the harpsichord was developed and adjusted to create the pianoforte, as hammer actions were added to it.

1720 – Bartolomeo Cristofori introduced the first pianoforte, and is credited by many as the “inventor” of the piano

1760 – Johannes Zumpe invented the square pianos which he introduced in London, England.

1777 – The Grand Pianoforte was developed and introduced to the world.

1780 – Hammer changes were made to the piano in order to improve the tone of these grand pianos as many contemporary composers preferred this more, this was the Stein and Stein-Streicher piano

1811 – Upright pianos were introduced by many European manufacturers, as one of the most popular ones was the Wornums upright piano as many appealed to its improved sound quality compared to others.

1855 – The square piano with a new scale that was developed by the Steinway and Sons, changed the sound quality, that later was used by many in the future.

1880 –  During this time of the year the square piano became extinct and invalid, as it was abandoned by most of Europe and America, and instead, uprights were the trend as they were space saving pianos. 

     So as you can see the piano underwent many developments before the actual piano was created by the hands of Bartolomeo Cristofori.