The concept of a mammalian “lizard brain” can be well and truly put on hold, a new study suggests.
Based on a study examining the brains of bearded dragons (Pogona), a large lizard from the Australian desert, scientists have shown that the brains of mammals and reptiles evolved independently from a common ancestor. This is another nail in the so-called trinity brain concept.
Based on comparative anatomical studies, the concept of the lizard brain first emerged and became popular in the 1960s and 1970s. Neuroscientist Paul MacLean has noticed that some parts of the mammalian brain are very similar to some parts of the reptile brain. This led him to conclude that the brain has evolved in stages after life moved to land.
First, according to MacLean’s model, the reptile brain emerged, defined as the basal ganglia. Then there’s the limbic system — the hippocampus, the amygdala, and the hypothalamus. Finally, the neocortex appears in primates.
Under the Trinity model of the brain, each part is responsible for a different function; for example, the basal part of the brain is thought to be more concerned with primitive responses – such as basic instincts for survival.
However, neuroscientists have denounced this model for decades. The brain doesn’t work that way, playing different roles in various parts. Brain regions, while anatomically distinct, are highly interconnected, a humming network of neural networks. With the advent of new technologies, we can begin to better understand how the brain has evolved.
In a new study, a team of researchers at the Max Planck Institute for Brain Research turned to actual lizard brains to investigate, publishing their findings in a paper led by neuroscience graduate students David Hain and Tatiana Gallego-Flores.
By comparing the molecular features of neurons in modern lizards and mice, the researchers hope to unravel the evolutionary history written into the reptile and mammalian brains.
“Neurons are the most diverse cell type in the body. Their evolutionary diversification reflects changes in the developmental processes that produce them and may drive changes in the neural circuits to which they belong,” said Gilles, a neuroscientist at the Max Planck Institute for Brain Research. Laurent said.
About 320 million years ago was a very important period in the evolution of vertebrates and their brains. It was then that the first tetrapods (tetrapods) emerged from water to land and began to diverge into parental families that would eventually give rise to birds and reptiles, and mammals on the other.
During embryonic development in all tetrapods, some structures are established in the brain: ancestral structures shared by subcortical regions.
However, because traditional anatomical comparisons of developing regions may not be sufficient to fully detail all the differences and similarities between the reptile and mammalian brains, the researchers took a different approach.
They sequenced RNA (a messenger molecule that serves as a template to form proteins) in individual cells from the bearded dragon’s brain to determine the transcriptome (the full range of RNA molecules in a cell) that was present to generate a map of cell types in the lizard’s brain . The atlas was then compared to existing mouse brain datasets.
“We analyzed more than 280,000 cells from Pogona’s brain and identified 233 different types of neurons,” Hain said.
“Computational integration of our data with mouse data suggests that these neurons can be classified into a common family transcriptome, which may represent an ancestral neuron type.”
In other words, both mammals and reptiles share a core set of neuron types with similar transcriptomes, even though they have each evolved over 320 million years.
But these neurons are not restricted to specific “reptile” regions of the brain. The analysis revealed that most areas of the brain had a mix of ancestral and newer types of neurons, challenging the notion that some brain regions are older than others.
In fact, the researchers found that neurons in the thalamus can be divided into two groups based on their connectivity to other areas of the brain. And these connected regions are quite different in mammals and reptiles.
The team found that the transcriptome diverged in a way that matched regions of connectivity, suggesting that the neuron’s transcriptome identity — the complete genetic readout of proteins it may require — is either confirmed or reflected by its connectivity.
“Since we don’t have ancient vertebrate brains, reconstructing brain evolution over the past 500 million years requires wiring together very complex molecular, developmental, anatomical and functional data,” Laurent said.
“We live in very exciting times because this is becoming possible.”
The research has been published in science.