In many ways this ‘theory’ or better, speculation, either begs the question of origins or is circular in reasoning. Islam from where? From Arabia? From the dhimmiized populations of Spain or North Africa? The point is that Islam in and of itself had very few original ideas. It conquered huge populations of already settled civilizations, Christian, Jewish, Zorastrian, Hindu, Greek, etc. and it is THOSE populations which may have helped in the origin of ideas. Many of those peoples as dhimmi retained, under suppression or oppression, their ideas which may have become part of non-Islamic states and thought. This final comment probably says it all:
“There are clear parallels between Islamic legal history and English law, but unless new historical evidence comes to light, the link remains unproven.”
This is to be expected if dhimmis continued to function in Islamic states and provide the ideas falsely ascribed to Islam itself.
At that moment the Seljuk Turks popped up, began conquering the Middle East, and whatever other input the Arabs might have been prepared to make to the Sharia was suppressed for the next 1003 years.