Though there is no direct evidence for Adam and Eve having existed, this does not necessarily make the belief irrational. In principle, it may be supported by indirect evidence (if one holds to a religious belief, itself well supported by evidence, that implies the historicity of Adam and Eve). Either way, current science tells us little about whether Adam and Eve plausibly existed.
Science and Faith
Some may wonder why I have chosen to address this topic prior to engaging with other topics (such as the meaning of the ‘days’ in Genesis 1). I have done so because many young earth creationists consider this to be the more pressing concern, and perhaps even the strongest argument for their position from Scripture.
The “young earth” teaching introduces an unnecessary and erroneous tension between Scripture and science that has confused many and I have personally met many people who have turned aside from their faith due to what they see as the impossible challenge of trying to reconcile that understanding of Scripture with the clear scientific evidence. They end up being neither ‘old’ or ‘young’ earth and worst of all not believing that God is the Creator at all.