That has been answered before, here, from two points of view:
Naturally, the religious wackos were all over that dancing on pin-heads question. I don't recall, and could care less, what the consensus was.
I offer the alternative that the Annunaki needed something to hook up their test subjects to the equipment while they were doing their gene splicing experiments to create "The Adam" from their genes and hominid genes.
But seriously, the Annunaki used surrogate mothers, so yes, their creations and offspring had belly buttons.
BTW, look up gene (chromosome) differences between us and our nearest relatives, such as chimps. You will see some interesting discrepancies.