I believe Shanks could be some failed experiment or maybe successful experiment by the WG, ditched on god valley. Perhaps Shanks will be some kind of psychological warfare tool for the WG, and that he was intentionally put on Roger’s bot so that Shanks could learn haki from him, It’s possible Shanks was hypnotised from birth and can do anything at the command of the WG. It’s also possible that he was hypnotised at the time he was going after the Gomu Gomu no mi, thus the reason why he hadn’t eaten it yet.

This also makes sense just because so far every kinda of warfare has been used in One piece by the WG or associated nations, including artillery, chemical and biological. Perhaps Shanks is somewhat aware that he can be controlled at the drop of a hat and that’s why he was allowed in Marijoa, because he couldn’t try anything no matter what it was.