I assume the implication here is that they have belly buttons, because at some point, both Adam and Eve had an umbilical cord attached to them, which in turn would mean that they weren't the first man and woman, but someone else was.

I'm thinking Occam's razor applies here: the artists simply weren't thinking that far ahead when painting those pictures.

Those are of course fake buttons, they had plastic surgery to avoid the crushing social pressure of, hmm... themselves i guess?

Not that I advocate "A" "I" "art" or anything but I've found that Biblically Accurate depictions of (usually) Adam and (sometimes) Eve can be obtained by typing "navel destroyer" in the input prompt of the Hugging Face Stable Diffusion 2.1 thing at huggingface dot co slash spaces slash stabilityai slash stable-diffusion