Rethinking brain evolution: Birds and mammals took different paths

The human brain is a marvel of evolution, enabling language, reasoning, and imagination. To uncover the roots of this complexity, scientists have studied brain structures across different species.

The pallium, a brain region forming the neocortex in mammals, has been central to this type of research. It plays a crucial role in sensory processing and higher cognition, making it a defining feature of mammalian intelligence.

Traditionally, experts believed the pallium was a shared structure among mammals, birds, and reptiles, varying only in complexity.

They assumed that this brain region contained similar neuronal types with equivalent circuits for sensory and cognitive processing. Previous studies supported this view, identifying shared excitatory and inhibitory neurons and general connectivity patterns across these vertebrate groups.

However, recent findings have upended this long-standing idea, revealing that while the pallium serves similar functions across species, its development follows different genetic blueprints.

The evolution of the brain has followed multiple paths, revealing surprising differences in how birds and mammals developed their neural circuits.

Distinct developmental processes

The pallium has long been considered the seat of higher cognition in vertebrates. In mammals, it forms the neocortex, the part of the brain responsible for abstract thought and decision-making.

In birds, the pallium plays a comparable role in complex behaviors such as tool use, problem-solving, and vocal learning. Despite these similarities, scientists have discovered that the developmental origins of the pallium differ significantly across species.

New research suggests that while birds and mammals have evolved functionally similar brain circuits, these circuits arise through distinct developmental processes.

The neurons that perform cognitive tasks in birds and mammals originate in different locations and at different times during embryonic development. This indicates that they are not directly inherited from a common ancestor but have instead evolved independently.

Dr. Fernando García-Moreno and Eneritz Rueda-Alaña of the Achucarro Basque Center for Neuroscience, in collaboration with multiple research centers, set out to investigate the developmental differences in bird and mammal brains.

“Our studies show that evolution has found multiple solutions for building complex brains,” noted 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.”

New perspective on evolutionary paths

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

“Their neurons are born in different locations and developmental times in each species,” said Dr. García-Moreno. “This indicates that they are not comparable neurons derived from a common ancestor.”

Instead, each species has developed new and unique cell types using distinct genetic programs. This finding suggests that the neural structures supporting cognitive functions in birds and mammals are not homologous but are instead the product of convergent evolution.

Convergent evolution occurs when different species independently develop similar biological traits due to comparable environmental pressures. In this case, both birds and mammals evolved sophisticated neural circuits but through entirely separate genetic pathways.

This discovery challenges the assumption that complex cognition in vertebrates emerged from a single evolutionary trajectory.

Mapping the neurons of the avian brain

A second study, conducted at Heidelberg University, was co-directed by Bastienne Zaremba, Henrik Kaessmann, and Dr. García-Moreno.

The researchers created a detailed cell-type atlas of the avian brain and compared it with mammalian and reptilian brains. Their findings further revealed the fundamental divergence in brain evolution among these groups.

“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,” said Dr. García-Moreno.

Neurons shared across species

The analysis showed that birds have retained most inhibitory neurons present in vertebrates for hundreds of millions of years. However, their excitatory neurons, which transmit information and drive higher cognitive functions, have evolved uniquely.

Interestingly, only a few types of excitatory neurons in birds shared genetic similarities with those in mammals. These included neurons linked to the claustrum and the hippocampus, suggesting that some neuronal types are ancient and widely shared across species.

“However, most excitatory neurons have evolved in new and different ways in each species,” said Dr. García-Moreno.

This study illustrates how evolution can shape the brain in different ways, even when species face similar cognitive challenges. Birds and mammals both require advanced neural circuits to interact with their environments, yet they have taken distinct genetic routes to build these capabilities.

Brain circuits and evolution’s flexibility

Both studies used cutting-edge techniques to trace the evolution of brain circuits in vertebrates. The researchers combined spatial transcriptomics, developmental neurobiology, single-cell analysis, and mathematical modeling to compare how neurons develop and function across species.

The results provide strong evidence that advanced cognitive functions do not require a single evolutionary blueprint. Instead, evolution has repeatedly found different ways to construct complex brains.

These findings demonstrate that the brain is remarkably flexible in its development. Birds and mammals have independently acquired the ability to process information, solve problems, and engage in social behaviors.

The diversity of their neural pathways challenges traditional views of brain evolution, showing that intelligence can emerge from a wide range of genetic and cellular processes.

Significance of studying brain evolution

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

This shared history means that studying the brains of other animals can provide crucial insights into human cognition. Understanding the genetic programs that shape different neuronal types could open new research directions in neurodevelopment and brain disorders.

“Only by understanding how the brain forms, both in its embryonic development and in its evolutionary history, can we truly grasp how it functions,” said Dr. García-Moreno.

These discoveries pave the way for future investigations into how the brain evolves and adapts, shedding light on the genetic and molecular mechanisms underlying intelligence.

As scientists continue to explore brain evolution, they may uncover even more unexpected pathways leading to cognitive complexity. The story of the brain is far from complete, and each new discovery adds another layer to our understanding of how intelligence has emerged in the natural world.

The research is published in the journal Science.

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