A Spanish scientist rewrites the history of the brain: those of birds and reptiles evolved independently to that of mammals

The pallium or cerebral cortex is a region of the brain responsible for complex cognitive functions and that more distinguishes the human being from other species. We have more neurons located at this point, which seems that it allows us to process complex information, such as thought, perception, decision making and language production. However, we are not the only ones: all mammals, in addition to birds and reptiles have developed complex brain circuits. To date, it was thought that we had all evolved from a common ancestor. Now, two studies published in the magazine ‘Science’ (here and here) and led by Fernando García-Moreno, researcher Ikerbasque at the Achucarro Basque Center for Neuroscience (Achucarro) Research Center and at the University of the Basque Country (UPV/EHU), they come to refute this belief: in reality in all three cases, The different brains evolved in parallel and divergent.

The first study, developed by Eneritz Rueda-Esalaña and Fernando García Moreno in Aachucarro and supported by a multidisciplinary team of collaborators of the Basque Cicbiogune and BCAM centers, Madrid’s CNIC, the University of Murcia, Krembil (Canada) and the University of Stockholm , shows that although birds and mammals have developed circuits with similar functions, the way in which these circuits are generated during embryonic development is radically different.

“Their neurons are born in different places and moments of development for each species,” explains García Moreno, director of the Cerebral Development and Evolution Laboratory, “indicating that they are not comparable neurons derived from a common ancestor.” Through space transcriptomic analysis and mathematical modeling, researchers found that neurons responsible for sensory processing in birds and mammals are made up using groups of different genes. «The genetic tools they use to cement their cell identity varies from one species to another, each sample new and unique cell types». All this indicates that these structures and circuits are not homologous, but the result of convergent evolution. That is, “they have generated these essential neuronal circuits through different evolutionary paths.”

The second study deepens more in these differences. Carried out at the University of Heidelberg (Germany) and co-direct by Bastienne Zaremba, Henrik Kaessmann and Fernando García Moreno, provides a detailed atlas of cell types in the brain of birds and compares it with that of mammals and reptiles. “We have been able to describe the hundreds of genes used by each type of neuron in these brains, cell to cell, to compare them with bioinformatics tools.”

The results show that birds have preserved most inhibitory neurons present in other vertebrates, for hundreds of millions of years. However, their excitatory neurons responsible for the transmission of information in the Palio have evolved uniquely. Only some cell types were identified in the avian brain with genetic profiles similar to others present in mammals, such as the cloister and the hippocampus, which suggests that some neurons are very old and shared. “However, most excitatory neurons have evolved from new and different ways in each species,” says García-Moreno.

Evolution finds different paths

“Our studies show that evolution has found multiple solutions to build complex brains,” explains García-Moreno. «Birds have developed sophisticated neuronal circuits through their own mechanisms, without following the same path as mammals. This changes the way we understand the evolution of the brain ».

These findings underline the evolutionary flexibility of brain development, showing that advanced cognitive functions can emerge through very different cell and genetic pathways.

The discovery that birds and mammals have developed neuronal circuits independently has important implications for comparative neuroscience. Understanding the different genetic programs that give rise to specific neuronal types could open new ways for research in neurodevelopment. García Moreno is committed to this type of basic research, “only understanding how the brain forms, both in its embryonic development and in its evolutionary history, we can understand how it works.”