The Corpus Callosum (Latin for 'tough body') is a large bundle of nerve fibres which serves to 'connect' the right and left hemispheres of the mammalian brain - except in marsupials (e.g. Kangarooo) and monotremes (e.g. Platypus).

Put another way, it's only found in 'placental' mammals (e.g. Humans)

Given that it's a crucial part of the brain for placental mammals (damage leads to a wide variety of very debilitating mental and physical symptoms) it remains unexplained why the marsupials and monotremes don't have it. Or why it has only evolved in placental mammals.

Further info. Wikipedia