On OB (Oracle Bones), the character YI looked very different from the one we use now. It was a picture of two hands, holding a vase, and pouring the contents over into another vase. After some time, the hands were left away, only the two containers were left, with the drops indicating the liquid. On bronze texts, one of the vases disappeared, and the one left over was drawn simpler and simpler.. until only the drops and the side with handle were still in the picture. It is a convicing sequence - but the part about the haqndle being left over is probably not correct.
This simple character seems to be the oldest one. In Great Seal Script it slowly changes into the 'lizard', or according to others, the sun with moon (yue) or "nothing" (wu) or whatever this part is. The last picture shows the modern character.
The OB meaning was: to give, to accord, to offer, calm down (of disease). Yi ri, 'gift of sun': the sun reappearing again (thanks to an ancestor or spirit).
Bronze character: give, gift (to recompense for..), give (happiness). Yong Yi, 'avail of Yi': use so (the ancestor will) grant (a certain favor). Bu Yi, 'not change': with constancy. The meaning 'easy' comes from: (ex-)change, no-constancy, and thus easy.
Nowadays it is a very commonly used character: in the names of trading companies.
In many languages the words for pour and give are the same, like in German 'schenken'.
But then, it seems it is all different than I thought. New findings often change things. Read here!! Article by Harmen Mesker.
From Harmen's article: "...possible that refers to an offering to the moon, or at least in the dark, to get the sun back. Yi 易 could have been a sacrifice to the ancestors or spirits (indicated by ) during the evening or in the night (indicated by ) to make sure that the sun is returning, that it is 'given' by the ancestors or spirits."
Inquiring the Yi is a way to barter (trading) with spirits or gods. In old times in the sense of "will this happen - or will this please not happen", trying to persuade them to be nice to you. Nowadays we ask "something" whatever we think is answering, what the best course of action is. Usually we ask our spirit, thinking it sits inside us. In old times they asked "the spirits" thinking they were outside. Who is right? Probably both.
I always thought the Yi could not answer yes-no questions. It is true that it is slippery ice - but if quantum mechanics is right, and time is not just the linear thing we can understand, then why could the future not exist already? Problem is, is there one future in this future, or innumerable possibilities and the oracle just shows the one you probably cause yourself to come into existence? Then it is better to ask for the best course of action...