Donner was summarizing the state of the field in the following terms: Islamic historians Patricia Crone, Michael Cook, John Wansbrough, and archaeologist Yehuda D. Nevo all argue that all the primary Islamic historical sources which exist are from 150 to 300 years after the events which they describe, leaving several generations for events to be forgotten, misinterpreted, distorted, garbled, etc. They contend that Islam was formed gradually over a number of centuries after the Muslim conquests, as the Islamic conquerors elaborated and backdated their beliefs and history in response to Jewish and Christian challenges. In 1999, Cook and Crone argued that "there is no hard evidence for the existence of the Koran in any form before the last decade of the seventh century." Partial Quranic manuscripts such as the Sanaa manuscript and the Birmingham Quran manuscript, which have been carbon-dated back to the early seventh century, were discovered later. However, Marijn van Putten, who has published work on idiosyncratic orthography common to all early manuscripts of the Uthmanic text type has stated and demonstrated with examples that due to a number of these same idiosyncratic spellings present in the Birmingham fragment (Mingana 1572a + Arabe 328c), it is "clearly a descendant of the Uthmanic text type" and that it is "impossible" that it is a pre-Uthmanic copy, despite its early radiocarbon dating. Author, journalist and scholar Toby Lester notes the Quran "assumes a familiarity with language, stories, and events that seem to have been lost even to the earliest of Muslim exegetes". Cook and Christopher Rose note the Quran contains obscure words and phrases, "mystery letters," or Muqattaʿat—groups of between one and four letters that do not form words and begin about one quarter of surahs of the Quran, and the Sabians religious group unknown to historians. Cook argue that "someone must once have known" what these mean, and that their meaning was forgotten now suggests the Quran may have been "off the scene for several decades". There is no mention of the "Quran" nor "Islam", nor "rightly guided caliphs", nor any of the famous futūḥ battles by Christian Byzantines in their historical records describing the Arab invaders advance, leaders or religion; the lack of any surviving documents by those Arabs who "lived through the establishment of the Caliphate"; the fact that coins of the region and era did not use Islamic iconography until sometime after 685 CE. According to