The corpus callosum is a broad flat structure in the brain that connects the two cerebral hemispheres. It allows the two halfs to communicate with each other.
In cases of severe seizures, the treatment of last resort is to cut the corpus callosum to break this connection. This was the treatment Scott has undergone in Both Sides Now.
In rare cases, the lack of a corpus callosum can be a congenital defect. This gives rise to a condition resembling autism.
Humans and all other mammals with a placenta also have a corpus callosum, but it is absent in marsupials, birds, reptiles, amphibians and fish despite the fact they also have brain hemispheres.