When we legitimately expect evidence and do not find it, that could be evidence of absence.
Sometimes we just assume there will be evidence of an event, but that is not necessarily the case. It takes careful thought to sort out what is expected in the evidence. Sometimes (often) the reality of what we should expect is nothing.
On Adam and Eve, as far as I know, there is zero positive scientific evidence. However, scientific evidence is not why people think they exist, nor do we expect evidence for the existence (if they are in a larger population). It just comes down to whether you trust Scripture, and how you read it. If you find Scripture trustworthy, and feel it gives reliable information that Adam was real, this need not be in conflict with the scientific evidence.
This should not be such a big deal. It is nearly obvious. However, for a long time people have said the opposite. You can even see in the wikipedia page:
There is no physical evidence that Adam and Eve ever literally existed, and their literal existence is incompatible with human evolutionary genetics.
Adam and Eve - Wikipedia
That is just a totally false statement. It misrepresents science and creates a ton of unnecessary conflict. Someone, eventually, needs to get that straight on wikipedia. Maybe some friendly atheists can step up to it =).