A possible explanation for the differences in structure, function, and behavior between chimpanzees' and humans is provided by the
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The theory of evolution by natural selection

Explanation:

Nowadays, it is widely accepted that chimpanzees are primates closest related to us. Darwin's postulated "The theory of evolution by natural selection", in the book "On the Origin of Species" in 1859. It is a theory that explains how species evolve over successive generations by gradual changes and the selection of the most adaptive phenotypes. Anatomical, fossil, and genetic evidence support the idea of an evolutionary link as Darwin predicted between hominids (humans) and other primates.

In regard to genetic evidence, whole-genome sequence analyses have shown that humans and chimpanzees share about 98% of sequence homology. Moreover, according to the phylogenetic tree, both species evolved from one common ancestor that was present approximately 4 million years ago.  

In regard to anatomical and fossil evidence, it has been discovered that the chimp brain has a similar size to human extinct relatives, and thereby also supporting an evolutionary link between both species.