A new study suggests that the concept of the “lizard brain” in mammals can be put to bed.
Based on a study that examined the brains of bearded dragons (Pogona vitticeps), large lizards from the Australian desert, scientists have shown that the brains of mammals and reptiles evolved separately from a common ancestor. This is another twist in the coffin of the so-called concept triune brain.
The idea of a lizard brain first emerged and became popular in the 1960s and 1970s based on comparative anatomical studies. Neuropathologist Paul McLean observed that parts of the mammalian brain are very similar to parts of the reptilian brain. This led him to conclude that the brain evolved in stages after life moved onto land.
First, according to McLean’s model, appeared the brain of reptiles, defined as the basal ganglia. Then the limbic system appeared – hippocampus, amygdala and hypothalamus. Finally, the neocortex arose in primates.
According to the triad model of the brain, each of these sections is responsible for different functions; more basal parts of the brain, for example, were supposedly more concerned with primal responses – such as basic survival instincts.
However, there were neurologists condemning model for decades. The brain just doesn’t work like that, in separate parts, each with a separate role. The regions of the brain, despite the fact that they are anatomically different, are closely interconnected, a web of buzzing neural networks. And with the advent of new techniques, we can begin to better understand how the brain evolved.
In a new study, a team of researchers from the Max Planck Institute for the Study of the Brain turned to real lizard brains, publishing their findings in paper led by neuroscience graduate students David Hein and Tatiana Gallego-Flores.
By comparing the molecular features of neurons in modern lizards and mice, the researchers hoped to understand the evolutionary history recorded in the brains of reptiles and mammals.
“Neurons are the most diverse cell types in the body. Their evolutionary diversification reflects changes in the developmental processes that create them and can cause changes in the neural circuits to which they belong.” – says neurologist Gilles Laurent Max Planck Institute for Brain Research.
About 320 million years ago was a very important time for the evolution of vertebrates and their brains. This was when the first tetrapods (four-legged animals) came out of the water onto land and began to diversify into the parent families that eventually produced birds and reptiles on the one hand, and mammals on the other.
The brain has structures created during the embryonic development of all tetrapods: a common ancestral architecture in subcortical regions.
But because traditional anatomical comparisons of developmental regions may not be sufficient to fully detail all the differences and similarities between the brains of reptiles and mammals, the researchers chose a different approach.
They sequenced the RNA – an information molecule used as a template to make proteins – in individual cells from the brains of bearded dragons to determine transcriptomes – the entire range of RNA molecules in the cell – is present and thus creates an atlas of the cell type of the lizard’s brain. This atlas was then compared to existing mouse brain datasets.
“We profiled more than 280,000 cells from Pogona’s brain and identified 233 different types of neurons,” – says Hayne.
“Computational integration of our data with mouse data showed that these neurons can be clustered transcriptomically into common families that likely represent progenitor types of neurons.”
In other words, there was a core set of neuron types with similar transcripts shared by mammals and reptiles, even though they evolved separately over 320 million years.
But these neurons are not limited to a certain “reptilian” area of the brain. The analysis showed that most brain regions have a mix of ancestral and new types of neurons, challenging the notion that some brain regions are more ancient than others.
In fact, the researchers found that neurons in the thalamus can be divided into two groups based on their connections with other brain regions. And these connected areas are very different in mammals and reptiles.
The team found that transcriptomes diverge in ways that correspond to junctional regions, suggesting that a neuron’s transcriptomic identity—a complete genetic readout of what proteins it might need—results from or reflects its connectivity.
“Because we don’t have the brains of ancient vertebrates, reconstructing brain evolution over the last half billion years will require piecing together very complex molecular, anatomical, functional, and developmental data.” – says Laurent.
“We live in a very exciting time because this is becoming possible.”
The study was published in Science.