The earliest records that we have of an instrument bearing the essential characteristics of the hurdy-gurdy are those of the Organistrum in a stone carving on the Portico de la Gloria of the church of Santiago de Compostela in Galacia, Spain at a date believed to be around 1175 a.d.
Although looking quite different to our modern instruments we consider it as being their ancestor because of the presence of multiple strings; a handle turning a wheel; and a key-box through which the strings pass. Presumably the key-box man was the senior musician. Musicological reconstructions of the instrument have been made by the impressively dedicated Senor Poves during the work on his doctoral thesis. Plans of the instrument above are available for sale from Antonio Poves ( Antonio Poves Olivan ) as is his extraordinary, voluminous and estimable thesis. Try: http://www.instrumentsmedievaux.org/icoins/vielroue/organispoves/pag.html
My own belief is that the instrument was developed in order to provide a bass line for choirs. Possibly a variable drone note rather than a counter melody. This opinion is based on the actual instruments that I have heard; on my experience with non-selective choirs (i.e.very mixed ability choirs where it is difficult to exclude even toneless singers); on scholarship regarding early music practice; on all the iconographical contexts; and on musical pragmatism.
Clearly note selection by means of pull handles must have been relatively slow compared to a keyboard for fingers, possibly very slow.
Consider the following quotations from A History of Western Music by Donald J. Grout (Dent & Sons 1973 Revised Ed.).
First, while discussing the plainsong melody that forms the tenore of the piece Haec Dies written by the Notre Dame of Paris musician Leonin during the third quarter of the 12th century "Could this actually have been sung by one soloist? It would seem more likely that it was played on a stringed instrument or on the organ, or at least carried by several singers, who could take breaths at different times." p.85 and bear in mind that Grout writing originally in 1960 probably didn't know about the organistrum which was, perhaps, the monk's solution to this problem.
Second, and again while speaking of 14th and 15th century music he says "We can be fairly sure that certain parts, such as Latin tenors in isorhythmic motets and the textless tenors in Landini's three-part ballate, were instrumental rather than vocal." p.143.
the organistrum basically will not, or cannot, function as a standard drone instrument, it simply sounds hideous, which is one of the reasons why most people have never heard one. (Unless it used a bass melody with a the drone above, a technique often found in organ music.)
the organistrum does not need to breath and consequently playing extremely long notes is easy. Bowed instruments or voices would need to do 'choral breathing (bowing)' i.e. not all breathing (changing bow direction) at the same time in order to properly play the music. Only the pipe-organ is as fortunate as the organistrum.
Southern European climates, such as Spain's, can be very hot and humid. Choristers know that under hot and humid conditions it can be very difficult not to sink (involuntarily slide pitch downwards) during the course of an a cappella (unaccompanied) piece without a musical instrument to provide a frame of reference. The organistrum could provide just such a reference, even if it only needed to play one or two low notes as a real drone.
- Organum, according to Grout, "properly refers only to the style in which the lower voice holds long notes:" p.80. Surely we can be excused for noticing that it is only really the pipe organ and the organistrum that antiquity chose to link to this musical form.
The 'next' development for the design was that the two man instrument became adapted for use by one player. Although, it may possibly be an assumption that the 2 man instrument came first. I say this because the iconography for the single player instrument seems to date from more or less the same time.
This one demonstrates a problem with iconography as evidence, which is that the artist seems to have painted the instrument incorrectly. It would certainly be difficult to play, the way that it looks.
The next style to appear, although it is not necessarily an advancement, was the Sinfonie. This was a simple looking box-shaped instrument, the name of which seems to imply that its musical effect was of more than one instrument playing at the same time. In other words almost certainly a drone and melody combination.
The keys all along the side of the box are a puzzle because they are technically impossible. Therefore they are either: a mistake or indulgence by the artist; or, if they actually existed on the instruments, they were decorative only.