Even though gold is more expensive, it is not needed to survive like water. Water not only keeps you hydrated, it is used to grow crops, which can be sold for money which can buy gold or be more useful than gold or be eaten to once again, survive. Water is used and needed everywhere and by everyone that when it comes down to it, is it more valuable than gold.
I believe the answer is: The sector Model
The sector model was popularized in 1939 in order to create <span>concentric zone </span>model<span> of city development. Using this model for building a city would give the city enough space for important infrastructure to be bulit and enough space for outward expansions of the city.</span>
As humans dispersed from the equator peoples skin got lighter and lighter because of fewer exposure to sun radiation.
They didn't get tanned as much :)
