I believe the answer is: . central nervous system.
According to gestalt principle, when an individual observe a certain motion, the neural organization immediately start to give a response as an entire field with differentiated parts and the experience would not connect the physical input in a piecemeal way.
The map attached is a world climate map.
Let's analyze it to forecast which regions would be less populated.
It can be regarded how the areas which are coloured in red represent very arid desertic zones, were there is no vegatation and animal or human life almost do not exist. These zones, together with the two areas depicted in white, the South and North Poles, would be the regions that are likely less populated due to their extreme weather conditions.
This should be the answer you are looking for just make sure to reword it.
There was no unified state that could grow all parts of life equally so they had to depend on trade in order to get goods that they needed from each other since they couldn't develop all parts of production or manufacture. Ones would for example have a developed pottery industry while the others would have mastered making Olive oil so they would trade in order to get what they needed because they couldn't make it on their own.