Recent research published in the journal Science has uncovered that birds, reptiles, and mammals have independently evolved sophisticated brain circuits, despite originating from a shared ancestor. These insights challenge traditional beliefs regarding brain evolution and highlight that although similar functions are present among these groups, the processes of embryonic development and the types of cells involved have distinctly diverged over time.

The pallium represents the brain region in mammals where the neocortex develops, playing a crucial role in complex cognitive functions that set humans apart from other species. Historically, scientists viewed this area as a comparable structure among mammals, birds, and reptiles, varying only by intricacy. It was presumed to contain similar types of neurons executing equivalent sensory and cognitive tasks.

Previous research had indicated the existence of shared excitatory and inhibitory neurons, alongside general connectivity patterns pointing to a common evolutionary journey among these vertebrate lineages.

However, the latest studies reveal that, despite the pallium serving equivalent functions across these groups, its developmental mechanisms and the molecular characteristics of its neurons have evolved significantly apart over time.

The first study, led by Eneritz Rueda-Alaña and Fernando García-Moreno from Achucarro, along with a diverse team from the Basque research institutes CICbioGUNE, BCAM, the Madrid-based CNIC, the University of Murcia, Krembil (Canada), and Stockholm University, illustrates that although birds and mammals created circuits with similar functions, the formation of these circuits during embryonic development is fundamentally different.

“Neurons arise in distinct locations and at different times during the developmental phases in each species,” notes Dr. García-Moreno, head of the Brain Development and Evolution laboratory. “This indicates that they are not comparable neurons that evolved from a common ancestor.”

Employing spatial transcriptomics and mathematical modeling, researchers concluded that the neurons responsible for sensory processing in birds and mammals are generated using different genetic pathways.

“The genetic mechanisms employed to define cellular identity are different across species, each exhibiting unique new cell types.” This suggests that these structures and circuits are not homologous, but instead represent convergent evolution, where “they have developed these vital neural circuits independently through distinct evolutionary pathways.”

The second study, conducted at Heidelberg University (Germany) and co-directed by Bastienne Zaremba, Henrik Kaessmann, and Fernando García-Moreno, delves deeper into these variations, offering a comprehensive cellular atlas of the avian brain and its comparison to mammals and reptiles.

“We documented the hundreds of genes utilized by each neuron type in these brains, analyzing the data with bioinformatics tools,” they report.

The findings suggest that while birds have preserved most inhibitory neurons common to all vertebrates for hundreds of millions of years, their excitatory neurons, which are key for transmitting information in the pallium, have evolved uniquely.

Only a handful of neuron types in the avian brain displayed genetic profiles akin to those seen in mammals, like the claustrum and hippocampus, indicating that some neurons are ancient and may be shared across species.

“However, the majority of excitatory neurons have uniquely evolved in different ways in each species,” Dr. García-Moreno elaborates.

The studies employed cutting-edge techniques such as spatial transcriptomics, developmental neurobiology, single-cell analysis, and mathematical modeling to trace the evolution of brain circuits in birds, mammals, and reptiles.

Redefining the Evolutionary Pathway of the Brain

“Our research demonstrates that evolution has devised diverse strategies for constructing intricate brains,” explains Dr. García-Moreno. “Birds have established advanced neural circuits through their own distinct mechanisms, separate from mammals. This transforms our understanding of brain evolution.”

These results underscore the evolving adaptability of brain development, illustrating that advanced cognitive abilities can arise from vastly different genetic and cellular pathways.

The Significance of Brain Evolution Research

“Our brain defines our humanity, but it also connects us to other animal species through our shared evolutionary background,” explains Dr. García-Moreno.

The revelation that birds and mammals have developed their neural circuits separately holds significant implications for comparative neuroscience. Grasping the distinct genetic frameworks that lead to various neuronal types could pave the way for new research opportunities in neurodevelopment.

Dr. García-Moreno advocates the importance of this foundational research: “By comprehending the developmental processes of the brain—both during embryonic growth and within an evolutionary context—we can truly understand its functions.”