The evolution of the mammal brain

Ancient neural connections found in the brains of marsupials and monotremes provide clues to the evolution of the human corpus callosum – the nerve tract that enables communication between the left and right hemispheres – new research shows.

The study, published in the journal The Proceedings of the National Academy of Sciences, sheds new light on how the hemispheres of the brain communicate in all mammals.

In placental mammals, such as humans, hemispheres do so via the corpus callosum. The 10 centimetre-long tract comprises 300 million nerve fibres and is the largest white matter structure in the brain.

In essence, it functions as a communications bridge, enabling comparison and combination of sensory inputs from the left and right sides of the body. In so doing, it plays an important role in sensory and motor function.

The corpus callosum also facilitates interaction between regions of the brain involved in cognitive tasks such as language and emotional processing.

Yet, not all mammals developed the structure — pouched marsupials and egg-laying monotremes don’t have one. Instead, their hemispheres communicate using a simpler network of nerve fibres.

To date, it’s not entirely clear whether the corpus callosum evolved as a unique structure, or whether a more ancient principle of brain connectivity is at play.

To further complicate matters, in rare cases some humans are born without one. While this is often associated with mild to severe disabilities, some of these individuals are high functioning with few neurological deficits. Magnetic resonance imaging (MRI) analyses suggest that, in these cases, the hemispheres communicate in a way that resembles the neural connections found in marsupials and monotremes.

To get a better understanding of the evolution of brain connectivity in mammals, Rodrigo Suárez and Linda Richards of Australia’s Queensland Brain Institute used MRI scans to examine the brains of a tiny species of marsupial called a fat-tailed dunnart (Sminthopsis crassicaudata), as well as the platypus (Ornithorhynchus anatinus), which is a monotreme.

They discovered that in monotremes and marsupials nerve fibres connecting the hemispheres are arranged in patterns that share similarities to those found in the corpus callosum.

“We see that long range connections are present and precisely organised, not only in animals that have a corpus callosum, but also in marsupials such as the dunnart,” says Suárez.

It appears that, rather than evolving independently, the structure arose in placental mammals as a way to expand existing hemisphere-to-hemisphere wiring.

“If you look at the corpus callosum,” says Suárez, “it’s actually an expansion in the number of axons that are being connected between hemispheres.”

He says the findings suggest that there are very specific ways the hemispheres prefer to connect in all mammals, and the principles guiding this must have originated very early in mammalian evolution, at least 80 million years before the evolution of the corpus callosum itself.

This new insight may also improve understanding of conditions where brain connectivity is abnormal, he says.