Hello and thanks for researching this interesting question! This topic is actually the subject of many weeks of university study, so you would have to investigate books about comparative physiology in order to delve very deeply.
We cannot really separate an animal's brain evolution from its body evolution...the canine brain would not be able to distinguish so many odors if the canine nose were not constructed with so many more receptors than the human nose.
Dogs might have evolved to rely on their eyesight more if their eyes had evolved to be able to distinguish between colors and shades of colors. The sight centers in the canine brain are limited in part by the limitations in canine eye physiology.
Both the brain and body evolve (develop and change over time) together...changes that are advantageous (make an animal more successful) can be passed down to offspring through the genes. Changes that are disadvantageous eventually meet limitations, or even become extinct as time passes.
The human brain is much larger and heavier than the canine brain. The 'thinking' parts of the human brain account for this difference. Dogs do not think critically and are not able to always predict the outcomes of their actions. They do not mull over the past and do not think up elaborate plans for the future (although they do have memories that do affect their behaviors and they do have an ability to plan in the short term, "If I chase the rabbit I can catch it and eat it right now").
This is a good general outline of the issues you are investigating:Read Dog-Human Sense Comparisons by clicking here
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