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Let me start by conceding something, and I'll flog this point hard before I get to answering your main point. Yes, I missed a key phrase "great circle" in your post and didn't stare hard enough at the picture. Thus, I took the fairly standard sized wall of upturned crust around the large impact crater as being your wall.

So I absolutely was looking at the wrong thing. I'm not dodging or brazening that in the slightest way.

So, do I think you have a great case for Iapetus (as most people spell it) as the place from which humans arrived here?

I think it fails on certain Occam's Razor grounds.

For one thing--no doubt this will disgust you no end--I suspect (no peeking, either) that the great circle ridge is still a maximum case of an impact crater ridge, the same kind of feature as the one I was mistakenly looking at on the more obvious crater. There isn't much else going to reshape moons of that size except certain funny period orbital resonances, etc. But I'll admit in this case that I may be the only one who would think so and that all informed speculation is following other lines.

The problem is more with your idea. For one, you propose it as the alternative to thinking humans originated here on Earth along with other DNA life including our genetic sister species, the chimpanzees, who seem more related to us than to other outwardly similar apes (gorillas, organutans). We actually fit well into the kind of nested hierarchy of relatedness a phylogenetic tree of common descent would produce, a fact which would be the bizarrest possible coincidence if we alone were somehow unrelated to the other life and delivered here by Raelian genetic engineers.

Then, there's the question of what humans building a ring around cold, dead, little Iapetus were eating, drinking, and breathing. Then there's the question of what humans were doing building a ring around cold, dead, little Iapetus.

Yes, humans planted a flag on our own little cold, dead, moon, while they ate, drank, and breathed stored resources. Call me when we build the Great Wall of Luna.

Which is to say we don't have the technology right now to do that kind of thing. But we had it once and somehow lost every tiny bit of it getting here. Why?

Placemarker.

I'm not scoring very high on guessing what real astronomers think. For one thing, I didn't know all the elements of the problem, such as the symmetry the ridge exhibits with the light-dark pattern on Iapetus.

http://www.universetoday.com/am/publish/iapetus_consume_saturn_ring.html

Anyway, the "rindge" as the author calls it (evidently a combination of "rind" and "ridge") is 12 miles high.