You Don't Actually Have A 'Lizard Brain', Evolutionary Study Reveals

The idea of the mammalian "lizard brain" may be definitively disproved, according to a recent study.

Based on a study that looked at the giant lizards from the Australian desert known as bearded dragons (Pogona vitticeps), scientists have demonstrated that mammal and reptile brains evolved independently from a common ancestor. The idea of the so-called triune brain has still another fatal flaw.                                     

Based on comparative anatomical research, the theory of the lizard brain first appeared and gained favor in the 1960s and 1970s. Neuroscientist Paul MacLean discovered that the mammalian brain shares many similarities with the brains of reptiles. He came to the conclusion as a result that once life moved to the land, the brain evolved gradually.

According to MacLean's concept, the basal ganglia, which represent the reptilian brain, came first. The limbic system, which consists of the hippocampus, amygdala, and hypothalamus, thereafter emerged. The neocortex finally developed in primates.

According to the triune brain concept, each of these regions is in charge of distinct tasks. For instance, the more primitive regions of the brain were thought to be more preoccupied with fundamental reactions, such as the most basic impulses for survival.

Neuroscientists have been criticizing the idea for decades, though. Simply put, the brain does not function in discrete regions that each have a distinct role. Despite their physical differences, brain regions are intricately connected and form a web of active neural networks. And when new methods emerge, we can begin to comprehend the evolution of the brain.

In a recent investigation, a group of scientists from the Max Planck Institute for Brain Research used actual lizard brains to learn more. David Hain and Tatiana Gallego-Flores, two graduate students in neuroscience, spearheaded the publication of their findings.

The researchers intended to unravel the evolutionary histories encoded in the brains of reptilian and mammalian species by comparing the molecular characteristics of neurons in contemporary lizards and mice.

"The body's neuronal cells are the most varied cell kinds. Their evolutionary diversification reflects changes in the mechanisms that give rise to them and may lead to modifications in the brain circuits they are a part of "says Max Planck Institute for Brain Research neuroscientist Gilles Laurent.

An very significant period in the development of vertebrates and their brains occurred around 320 million years ago. The earliest tetrapods—animals with four legs—emerged from the water onto land at that time and began branching into the parent families that would later give rise to mammals and birds, respectively.

All tetrapods have features in their brains that were developed during their embryonic development: a subcortical region's shared ancestral architecture.

But the researchers adopted a new strategy since conventional anatomical comparisons of developmental regions might not be sufficient to completely detail all the differences and similarities between reptile and mammal brains.

In order to ascertain the transcriptomes—the complete range of RNA molecules in the cell—present and create a cell-type atlas of the lizard's brain, researchers sequenced the RNA—a messenger molecule used as a template to build proteins—in individual cells from the brains of bearded dragons. Then, this map was contrasted with pre-existing mouse brain datasets.

Over 280,000 cells from the brain of a pogona were analyzed, and 233 different types of neurons were found, according to Hain.

These neurons can be organized transcriptomically into common groups that likely reflect ancestor neuron types, according to computational integration of our data with mouse data.

In other words, even though mammals and reptiles have evolved independently for more than 320 million years, they share a core set of neuron types and transcriptomes.

However, these neurons are not confined to a particular'reptilian' area of the brain. The investigation disproved the idea that some brain regions are more old than others, showing that most brain regions contain a mixture of ancestral and more modern types of neurons.

In fact, the scientists discovered that the thalamus's neurons may be divided into two groups based on how connected they are to other parts of the brain. And in reptiles and mammals, these interconnected zones differ greatly.

The researchers discovered that the transcriptomes diverged in a way that matched the connecting regions, indicating that a neuron's connection is what gives rise to or reflects its transcriptomic identity, which is the complete genetic readout of what proteins it may require.

The molecular, developmental, anatomical, and functional data that must be connected in order to recreate the evolution of the brain over the past half billion years will be extremely complex, according to Laurent.

"This is becoming conceivable," we are living in really exciting times.

Science has published the research.