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Birds have developed complex brains independently from mammals, studies reveal

Two studies published in the latest issue of Science have revealed that birds, reptiles, and mammals have developed complex brain circuits independently, despite sharing a common ancestor. These findings challenge the traditional view of brain evolution and demonstrate that, while comparable brain functions exist among these groups, embryonic formation mechanisms and cell types have followed divergent evolutionary trajectories.

The pallium is the brain region where the neocortex forms in mammals, the part responsible for cognitive and complex functions that most distinguishes humans from other species. The pallium has traditionally been considered a comparable structure among mammals, birds, and reptiles, varying only in complexity levels. It was assumed that this region housed similar neuronal types, with equivalent circuits for sensory and cognitive processing.

Previous studies had identified the presence of shared excitatory and inhibitory neurons, as well as general connectivity patterns suggesting a similar evolutionary path in these vertebrate species.

However, the new studies reveal that, although the general functions of the pallium are equivalent among these groups, its developmental mechanisms and the molecular identity of its neurons have diverged substantially throughout evolution.

The , conducted by Eneritz Rueda-Alaña and Fernando García-Moreno at Achucarro, with the support of a multidisciplinary team of collaborators from the Basque research centers CICbioGUNE and BCAM, the Madrid-based CNIC, the University of Murcia, Krembil (Canada), and Stockholm University, shows that while birds and mammals have developed circuits with similar functions, the way these circuits form during embryonic development is radically different.

"Their neurons are born in different locations and developmental times in each species," explains Dr. García-Moreno, head of the Brain Development and Evolution laboratory, "indicating that they are not comparable neurons derived from a common ancestor."

Using spatial transcriptomics and mathematical modeling, the researchers found that the neurons responsible for sensory processing in birds and mammals are formed using different sets of genes.

"The genetic tools they use to establish their cellular identity vary from species to species, each exhibiting new and unique cell types." This all indicates that these structures and circuits are not homologous, but rather the result of convergent evolution, meaning that "they have independently developed these essential neural circuits through different evolutionary paths."

The further explores these differences. Conducted at Heidelberg University (Germany) and co-directed by Bastienne Zaremba, Henrik Kaessmann, and Fernando García-Moreno, it provides a detailed cell type atlas of the avian brain and compares it with those of mammals and reptiles.

"We were able to describe the hundreds of genes that each type of neuron uses in these brains, cell by cell, and compare them with bioinformatics tools."

The results show that birds have retained most inhibitory neurons present in all other vertebrates for hundreds of millions of years. However, their excitatory neurons, responsible for transmitting information in the pallium, have evolved in a unique way.

Only a few neuronal types in the avian brain were identified with genetic profiles similar to those found in mammals, such as the claustrum and the hippocampus, suggesting that some neurons are very ancient and shared across species.

"However, most excitatory neurons have evolved in new and different ways in each species," details Dr. García-Moreno.

The studies used advanced techniques in spatial transcriptomics, developmental neurobiology, single-cell analysis, and mathematical modeling to trace the evolution of brain circuits in birds, mammals, and reptiles.

Rewriting the evolutionary history of the brain

"Our studies show that evolution has found multiple solutions for building complex brains," explains Dr. García-Moreno. "Birds have developed sophisticated neural circuits through their own mechanisms, without following the same path as mammals. This changes how we understand brain evolution."

These findings highlight the evolutionary flexibility of brain development, demonstrating that advanced cognitive functions can emerge through vastly different genetic and cellular pathways.

The importance of studying brain evolution

"Our brain makes us human, but it also binds us to other animal species through a shared evolutionary history," explains Dr. García-Moreno.

The discovery that birds and mammals have developed neural circuits independently has major implications for comparative neuroscience. Understanding the different genetic programs that give rise to specific neuronal types could open new avenues for research in neurodevelopment.

Dr. García-Moreno advocates for this type of fundamental research: "Only by understanding how the brain forms, both in its embryonic development and in its evolutionary history, can we truly grasp how it functions."