Popular Science: Are we closer to revealing the origins of mammalian neocortex?
How can we learn more about the processes leading to the evolution of the mammalian neocortex? An American-Spanish-Czech team, including Dr. Pavel Němec from the Zoology Department of the Faculty of Science, the Charles University in Prague, has reviewed the development and neural organization in the adults of different species and the relationship between the number of neurons in the neocortex and their precursors, migration and proliferation during the individual development. The common ancestor of mammals and reptiles most likely had one layer of dorsal cortex cells like today's sauropsid lineage of vertebrates, represented by reptiles and birds (whose cortex has evolved convergently to the mammalian brain in some aspects).
Unfortunately, fossil records cannot tell us as much about brain structure as the study of current life. However, the morphology of an early mammal skull suggests they had a relatively large olfactory bulb and cortical areas responsible for olfactory stimuli. The neocortex likely played a primary role in guiding behavior even then. Early primates, whose visual stimuli had become increasingly more important, already had an increased neocortex volume and density of synapses. The number of neurons in present-day primates varies largely. The marmoset brain contains roughly a billion neurons. The human brain, more than four times more massive, has around 86 billion neurons. The neocortex occupies an incredible 80% of its volume and is divided into approximately two hundred areas in each hemisphere, some of which have functional specificity for the hemisphere. Why did our brain undergo such rapid growth throughout its evolution? Looking at its development could help us answer this question.
In early intrauterine development, there are several waves of neuronal proliferation from the ventricular surface of the telencephalon. These cells then migrate to their final destination in the neocortex. Two mechanisms enable this: radial and tangential migration. In radial migration, the cells follow the protrusions of radial glia, a “construction scaffold” of the cortex. The waves of migration lead to the formation of cortical layers. They form from inside out, which means that the youngest cells are located on the surface from our point of view. In contrast, tangentially migrating neurons move parallel to the germinal area, perpendicularly to the radial glia – they don't use their scaffolding. This path especially produces interneurons – neural cells interconnecting sensory and motor pathways. The differences in the method and timing of the migration enable a divergence of neural populations and therefore indirectly brain functions as well.
However, both radial and tangential migrations have been observed in many taxa. What then is responsible for the fact that some branches of the phylogenetic tree contain organisms with much more developed brains than others? How does the formation of the human neocortex differ from other mammals? The authors argue that the answer most likely lies in differential gene expression. We all know the process: Information contained in the DNA is transcribed into a “working copy” of the RNA, from where it's translated into proteins (or functions directly as the RNA, though we do not need to go there right now). Different loci of DNA are transcribed with a different intensity and the resulting RNAs also have different lifetimes. The result? Different amounts of protein! And we should not forget about processes modifying the RNA before translation or the resulting protein. The same locus of DNA can therefore lead to many different phenotypes depending on the rate of transcription and intermediate steps before the final protein. And this variability is likely the proximate reason for the mystery of the human brain. Its transcriptional activity is much higher than, for example, in chimpanzees. Large differences in gene expression are also present between the areas of an individual brain – for example the visual cortex differs a lot in the transcription rates from the rest of the cortex.
And this leads us back to the question of the origins of the mammalian neocortex: mammals share a number of common unique mechanisms playing a role in brain development. A comparative review suggests that a divergence of the germinal zone, changes in the proportion of different cell populations, glial architecture and tangential migration – all of that guided by a complex molecular signalization – contributed to the development of the neocortex. Its further evolution in different mammalian taxa is then directed especially by gene expression modulation, which is also likely what we can be grateful to for our unusually big and powerful brains.