Why Are Humans Smarter Than Chimps if We Have the Same Common Ancestor?

(Find this image here)

Evolution is a complicated process, but it basically happens like this. When one species separates into two groups and eventually turn into two different species is what is called speciation. As these two groups acquire different traits from each other, they are going through what is called natural selection which they go through in order to adapt so they would be best fit to live in their environment. When these two groups have finally become different species, they have evolved. A perfect example of this is from looking at a certain kind of bird that has two different kinds of traits. One group of this bird lives in the west and is white. The other group of this bird lives in the east and is black. Right now, they are a part of the same species, but since they do not mate with each other and live in different locations, they will eventually become two different species because they will be passing different kinds of traits on to their offspring.

(This image gives an explanation to natural selection. Find it here)

This is what happened with humans and chimps. What I want to know is if humans and chimps evolved from the same species, why did humans become so much smarter than chimps? A long time ago, humans and chimpanzees were the same species. Lets call this species A. It is likely that species A had more chimp like traits than human like traits. Nobody knows for sure, but it is probable that the reason why humans have become so much smarter than chimps is because humans have had more changes in traits from species A than chimps have. It is likely that when species A was separated, the group that evolved into chimps did not move to a highly different environment from where they already lived and the group that became humans moved far away and they were required to have bigger brains in order to survive in their new environment.

Sources:

http://news.nationalgeographic.com/news/2005/08/0831_050831_chimp_genes.html

http://www.christiananswers.net/q-aig/aig-c018.html

http://www.scientificamerican.com/article.cfm?id=what-makes-us-human

http://www.daviddarling.info/encyclopedia/N/natselect.html