It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Friday, February 14, 2025
Birds have developed complex brains independently from mammals
Science publishes two studies led by an Ikerbasque researcher at Achucarro Basque Center for Neuroscience and UPV/EHU that reveal their unique evolution
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 have revealed 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 first study, 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 second study 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, published in Science, 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."
References:
Rueda-Alaña E, Senovilla-Ganzo R, Grillo M, Vázquez E, Marco-Salas S, Gallego-Flores T, Ftara A, Escobar L, Benguría A, Quintas A, Dopazo A, Rábano M, dM Vivanco M, Aransay AM, Garrigos D, Toval A, Ferrán JL, Nilsson M, Encinas JM, De Pitta M, García-Moreno F (2025). Evolutionary convergence of sensory circuits in the pallium of amniotes. Science (in press). doi:10.1126/science.adp3411
Zaremba B, Fallahshahroudi A, Schneider C, Schmidt J, Sarropoulos I, Leushkin E, Berki B, Van Poucke E, Jensen P, Senovilla-Ganzo R, Hervas-Sotomayor F, Trost N, Lamanna F, Sepp M, García-Moreno F, Kaessmann H (2025). Developmental origins and evolution of pallial cell types and structures in birds. Science (in press). doi: 10.1126/science.adp5182
The brains of birds and mammals have diverged substantially during the course of evolution. A research team led by Prof. Dr Henrik Kaessmann at the Center for Molecular Biology of Heidelberg University has investigated how similar cognitive functions could still arise in some bird species. Analyses on the composition, development and evolution of the pallium – the brain region in birds and mammals largely responsible for memory, learning, and thinking – show that some brain cell types remained nearly unchanged over hundreds of millions of years, whereas others evolved quite differently.
Bird brains differ fundamentally in structure from those of reptiles and especially mammals. Yet some bird species possess complex cognitive abilities similar to apes, and the pallium plays a key role in this. This region in the forebrain consists primarily of the folded cerebral cortex in humans but is structured quite differently in birds, despite supporting similar functions. Prof. Kaessmann’s team studied the cellular composition and evolution of the pallium in chickens. Using ultramodern single-cell technologies, the biologists mapped the cell types represented in this brain structure and compared the information with similar datasets in mice and reptiles.
The analyses show that in spite of the dissimilar brain architectures, the neurons that regulate brain activity are remarkably similar across the species studied – unlike the neurons responsible for signal transmission, which followed a more dynamic evolutionary pathway, explains Dr Bastienne Zaremba. Whereas some barely changed, like those in the hippocampus, which is responsible for learning and memory, others evolved in dramatically different ways or reorganized themselves anatomically. What the researchers did not expect: “Certain excitatory neurons probably possess a common evolutionary origin across species. This applies to the neurons in the deeper layers of the neocortex, which is responsible for higher cognitive functions in mammals, and the neurons in the mesopallium in birds. This finding calls into question existing assumptions about the evolution of these brain regions,” states the scientist, a member of Prof. Kaessmann’s “Evolutionary Genomics” research group.
The research also provides new insights into the hyperpallium, a structure unique to birds within the pallium. Until now, researchers assumed that the avian hyperpallium corresponds to the mammalian neocortex. The Heidelberg researchers were able to demonstrate that although some neurons are similar, others are fundamentally different. “Our findings challenge previous theories that proposed a simple one-to-one correspondence between brain regions in birds and mammals based on their location,” explains Dr Zaremba. Instead, what emerges according to the scientist is a considerably more complex evolutionary mix of conservation, divergence, and convergence. Some features have remained remarkably similar, others have changed dramatically, and still others have become more similar over time.
The researchers also found that certain neurons in two distant regions of the bird brain are surprisingly similar, even though they originate from different locations in the embryo. “We need to rethink the idea that a neuron’s final role is strictly determined by where it forms in the embryonic brain,” says Prof. Kaessmann. “To gain a differentiated understanding of brain evolution and the development of complex cognitive abilities in birds and mammals, molecular data that takes developmental processes into account is crucial,” adds the Heidelberg evolutionary biologist.
The research was carried out in close collaboration with Dr Fernando García-Moreno of the University of the Basque Country (Spain). Researchers from Sweden also participated. The European Research Council, the government of the Autonomous Community of the Basque Country, and the Swedish Research Council provided funding. The research results were published in the journal “Science”.
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