How were the biographies of the narrators documented, even though biographical books appeared long after their deaths, and what is the reliability of the information contained therein?

The science of al-Jarh wal-Ta’deel (discrediting and accrediting narrators) emerged with the beginning of the era of narration. It was considered a form of enjoining good and forbidding evil, and an obligatory piece of advice. Its practice began during the era of the Companions, as Ibn Sireen said: "They did not used to ask about the Isnaad (chain of narration). But when the Fitna (strife) occurred, they said: 'Name your men for us.' So, the people of Sunnah would be looked at and their hadeeth taken, and the people of innovation would be looked at and their hadeeth not taken." The people of Hadeeth continued to examine the conditions of narrators and distinguish them, and they transmitted the sayings of those who accredited and discredited with Isnaads, just as specialized books were authored in this science. They relied on studying the narrations of a narrator and comparing them with the narrations of trustworthy individuals. Those who came after relied on these authored books. They consider a narrator for whom they do not find accreditation or discreditation from the preceding Imams to be maj-hool (unknown), and they reject his narration.