hmmm... few theories i've heard..
New species of gibbon if not a new species of ape altogether... Not Orang, think thats only been suggested due to the semi familiar appearance of print evidence in the past..
The backwards foot thing is BS, heard that on the 'net but not spoken by anyone else2b honest...
Human like features? Gibbons have been known to look like really old wrinkly people, which they can do and its not been unknown for some old wrinkly people to look like gibbons im sure!
Why a gibbon? Well looking into the history of an area and what diverse environmental events may have caused the evolution of such a creature, this is of course if it ISNT a completely new species, and 50-60,000 years ago western Sumatra was witness to some of the largest volcanic activity, decimating hundreds of square milesof jungle etc etc. This is one of the reasons why some species are only found in the north and south and not in the west etc.
Now if you're a gibbon used to hanging out in the trees with arboreal features for moving etc and suddenly you find yourself in No Tree City,Arizona, on the ground, open to attack by remaining predators, youre going to have to become something else to survive... Gibbons have a pelvic structure not too unlike a humans, closest ape in that structure to us, enabling over time more natural upright stance.. So give you a few thousand years and by the time the jungle grows back around you you're actually way bigger and stronger than you were before and living on the ground is natural, who wants to go back up in the trees??? Ladies and gentlemen, i present the Orang Pendek....