Corpus callosum

The corpus callosum is part of the brain in humans and other eutherian mammals. It is the largest connective pathway in the brain. Its over 200 million nerve fibers connect the left and right hemispheres of the brain. 
The corpus callosum relates to the lateralization of brain function. Because the two side of the brain communicate so intensively, they can afford to specialise in doing somewhat different things.
Only in higher mammals.
The corpus callosum is found only in placental mammals (the eutherians). It is absent in monotremes and marsupials, and other vertebrates such as birds, reptiles, amphibians and fish. 
Other groups do have brain structures that allow for communication between the two hemispheres. The anterior commissure serves as the main cross-hemisphere communication in marsupials. 
The fibrous bundle of the corpus callosum increases to such an extent in humans that it wedges apart the hippocampal structures.