^Evolved? In 29 years? Unless evolution works very differently on Jakku, they'd need remarkably short lifespans to evolve that quickly.
First off: always remember it's space fantasy, not science fiction. Secondly, there's plenty of precedent for this kind of thing going all the way back to the dianoga in ANH and the mynocks & space slug in tESB.
The way I look at it, there's been large scale space travel in this galaxy for at least 25,000 years (probably a lot longer) as well as planetary scale cities and probably planet scale factories too out there somewhere. It's not hard to imagine a scenario where such creatures would arise. Maybe some naturally evolved in a heavy metal rich environment and when the miners and refineries came to their world, they thrived. Maybe originally some were genetically engineered as a way of dealing with waste products.
Who knows? The truth may be long lost in the distant past and honestly, nobody really cares to find out.
Either way, their spores/eggs/whatever would have hitched a ride on freighters which in turn would inadvertently spread them all across the galaxy, just as real world trade ships supposedly spread rats, domestic cats and the like all across the globe.
That's one of the unique things about the Star Wars galaxy, it's not a vast empty place full of unknowns and unexplored worlds because almost everywhere you go, someone else has already been there and probably left behind litter. It may have been thousands of years ago and long forgotten, but the ground has been trodden nonetheless. In that sense it's very reminiscent of Tolkien's Middle Earth where you can't walk more than a few days without running into one ruin or another and even the open wastes were the sites of ancient battles and the depths of the ocean home to sunken kingdoms.
