Most people (especially in very simple social environments, which is where this whole thing started) hold opinions for approximately good reasons. This is less true of very complex or infrequently occurring issues.
Nevertheless, individual opinions do constitute a certain degree of evidence. It strikes me as very likely that a mechanism for accepting second-hand, unverifiable information would provide a fairly substantial evolutionary benefit to a hunter-gatherer. For that matter, it provides some benefit to us today.
Religion is also a good source of individual motivation, if you don't happen to have an elaborated system of reasoning and ethics and metaethics handy. People tell stories about events (not all of them religious in nature) to contextualize them and predict them. Though that doesn't necessarily explain their tendency to spread in and of itself.
Today's post, A Failed Just-So Story was originally published on 05 January 2008. A summary (taken from the LW wiki):
Discuss the post here (rather than in the comments to the original post).
This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was Rational vs Scientific Ev-Psych, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.