Sounds to me more like someone just wanted an excuse to look at a bunch of bellies.
"Aki Sinkkonen at the University of Helsinki in Finland thinks the belly button, aka the umbilicus, serves a greater purpose than mere cosmetics: It may be an indicator of mating potential in fertile women.
'I propose that umbilicus, together with the surrounding skin area, is an honest signal of individual vigor,' Sinkkonen wrote in the latest issue of The FASEB Journal. 'More precisely, I suggest that the symmetry, shape, and position of umbilicus can be used to estimate the reproductive potential of fertile females, including risks of certain genetically and maternally inherited fetal anomalies.'"
If it is linked to women's fertility, then way to men have them??
It also does not address whether other species which only have one offspring per birth (multiples are rare in humans after all) have belly-buttons or not ... I don't know about apes but you have to wonder.
I think belly buttons are just 'there' ... like the appendix ... and they don't really have any real purpose to themselves, they just "are".
So why do they vary? Perhaps it has to deal more with where the embilical cord is located in the uterus ... but I highly doubt it has anything to so with fertility ... otherwise when women hit menopause the belly-buttons would disappear - and they don't.
If anything, they probably hide a scent gland ... now-a-days, humans shower and bath a LOT more than most other animals. If you have ever been sick for an extended length of time, be it for the flu or for bed-rest while pregnant, then you would realize that the less bathing you do the more goopy and stinky it gets in there.
Scent is a very powerful thing for wild beasts - perhaps it is a pheramone factory, like the spots behind your ears or at your wrists.
Well it makes a heck of a lot more sense than this gals theory that its linked to 'fertility' ....