The Quran most likely existed in scattered written form during Muhammad's lifetime. Several sources indicate that during Muhammad's lifetime a large number of his companions had memorized the revelations. Early commentaries and Islamic historical sources support the above-mentioned understanding of the Quran's early development. The Quran in its present form is generally considered by academic scholars to record the words spoken by Muhammad because the search for variants has not yielded any differences of great significance.[page needed] University of Chicago professor Fred Donner states that "...there was a very early attempt to establish a uniform consonantal text of the Qurʾān from what was probably a wider and more varied group of related texts in early transmission. [...] After the creation of this standardized canonical text, earlier authoritative texts were suppressed, and all extant manuscripts—despite their numerous variants—seem to date to a time after this standard consonantal text was established." Although most variant readings of the text of the Quran have ceased to be transmitted, some still are. There has been no critical text produced on which a scholarly reconstruction of the Quranic text could be based. Historically, controversy over the Quran's content has rarely become an issue, although debates continue on the subject.