It's spooky to look at it backwards. So crop circles became more complex as we learned geometry and proportion? But there does seem to be a link somehow. Most crop circles occur in Wiltshire (eg) - it's not short of prehistoric monuments either!
Has there ever been an exact date as to when CCs became so complex?
I would venture that there have been several (hundred??) examples quoted on this site alone about "The Megalithic Yard"...so would the ability to calculate this come under geometry and proportion(as well as rational ratio understanding) so presumably paleo/meso/neolithic man would have an understanding of complexity, so any visiting intelligence could leave coded messages with impunity in the secure knowledge the message would be understood(a feat in itself as no-one these days can agree what the "message" is supposed to mean).
Why just use crops?..Surely a more abiding "message" could be left carved in a rock face or gouged into the earth's pliable soil...hard to imagine that any visiting intelligence, with the ability to traverse space only has tools which are effective on crops...and if the message, if indeed message it is, is so important....why not duplicate it time and time again until we mere homosapiens eventually get it...before shaping up another new and more complex one?