Researchers at the Queensland Brain Institute (QBI) at The University of Queensland have shed new light on the evolution of the mammalian brain.

The study, published in the journal the Proceedings of the National Academy of Sciences, reveals that the corpus callosum, which connects the left and right brain hemispheres in placental mammals such as humans, shares features with more ancient connections in the brains of monotremes and marsupials.

“We discovered that the main patterns of connections between both hemispheres of the cortex arose very early in mammalian evolution such that they're conserved between platypus, marsupials, and placental mammals like us,” said lead author Dr Rodrigo Suárez.

The findings not only help explain how our brain wiring program may have evolved, but could also help inform treatment strategies for people who are born without a corpus callosum, he said.

Connecting the brain's hemispheres

Mammalian brains are divided into a left and right hemisphere. In humans, and all other placental mammals, the left and right hemispheres communicate via a central structure called the corpus callosum.

“The connections between the left and right hemispheres are very important for integrating everyday functions such as sensory or motor functions,” said Suárez, explaining that it's important that both hemispheres are connected so that they can coordinate actions.

“Imagine yourself walking in a dark room and you're touching your surroundings,” he said. “You need to be comparing what you're touching with your left hand and your right hand to get an idea of the whole.”

The corpus callosum also facilitates cross-talk between regions of the brain that are critical to cognitive tasks such as language and emotional processing.

However, other mammals such as pouched marsupials and egg-laying monotremes, which diverged early in mammalian evolution, did not evolve a corpus callosum. Nevertheless, their left and right hemispheres are able to communicate via other bundles of nerve fibres that run between them.

Moreover, a small number of humans are born without a corpus callosum. This can result in mild to severe disabilities, but some of these individuals are high-funtioning. MRI studies have shown this is because their hemispheres may be able to communicate in a way that resembles the connections found in marsupials and monotremes.

Thus, it’s not clear whether our corpus callosum evolved as a unique brain structure, or whether a more ancient principle of brain connectivity is actually involved.

Looking inside platypus brains

To find out, Dr Suárez and Professor Linda Richards at QBI decided to take a closer look at the brain connectivity patterns in marsupials and monotremes, which had never been extensively mapped before.

They used magnetic resonance imaging (MRI) to examine patterns of connections in the fat-tailed dunnart (a marsupial) and the duckbilled platypus (a monotreme).

In this way Suárez, Richards, and their colleagues discovered that the nerve fibres that run between the hemispheres in marsupials and monotremes are arranged in patterns that share similarities with those found in the corpus callosum in placental mammals.

“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 in the dunnart,” said Dr Suárez.

In other words, there appears to be very specific ways that the hemispheres will try to connect with one another in all mammals, and these principles of connectivity must have originated at least 80 million years before the evolution of the corpus callosum.

So rather than evolving independently of these patterns, it seems the corpus callosum has built upon this ancient foundation.

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

This suggests the corpus callosum arose in placental mammals as a way to expand this existing territory of hemisphere-to-hemisphere wiring, he explained.

The findings provide an important basis for further research into the evolution of brain circuits in mammals and other vertebrates.

The new insight could also help researchers better understand conditions where brain connectivity is abnormal, said Suárez.

“If we can understand the evolution of brain wiring, then we may be able to predict and develop therapeutic strategies to minimise the burden on humans born with abnormalities in brain connectivity, such as the absence of a corpus callosum.”