Wombat poop looks like the squared-off chunks from the larger size Tootsie Rolls of my youth. Tootsie Rolls didn't taste all that great, but they were cheap and didn't taste all that great for a long long time, and I speculate that the same could be said for wombat poop. If all goes well, we shall never know. But why, you might ask, do wombats crank out cubes in the first place?
This theory implies that without boxy poop, all the wombats in a given population trying to find their way home via the process of elimination might wind up bunched together in a ravine, or some other low spot. I see a number of problems with this. For one, I have never noticed that shit in general does a lot of rolling. It's sticky. I have counted on this very property of shit over the years, as one who can only find the direction of a slope in the woods by observing which of my feet gets wet when I pee. Also, you can roll dice. Especially in craps.
Whatever the reason, somebody has found a market in paper made of wombat poop. It is dense and fibrous, and, after all, it's already square, but I suspect that the original innovator got the idea because of the poop's stay-puttedness. "Look," our inventor said, a dim light bulb going off over his head, "it's stationary." This is the kind of thing you get in the spell-check generation, and also explains why so many little asses are running boroughs in New York City.
All of this is a lot to ask natural selection to account for. The alternative is to postulate that God, by the eighteenth or nineteenth day, just flang out a bony-assed fuzzy critter that carries its young in a pocket with a view of its own butt and likes to produce geometrically pleasing turds on rocks, just for the pure hell of it. I'm a science girl, but I'm going with Number Two. That's sure what I'd do.