Not exactly an answer to your inquiry but more a direction where I would look: beliefs in all manner of golden ages, when everything and everyone were better than later. If someone had maintained that belief, they might have come up with the mechanism of how selfishness could have arisen in a community of ideal human beings - e.g. through corruption by some evil forces.
Maybe you could look among philosophers that idealize man's "state of nature" as opposed to the alleged corruption through civilization. (Not that I've found anyone on linked wikipedia pages, it's just the hypothesis of where such sentiment could have arisen).