Programmes that control the production of neurons during brain development determine how the brain folds.
The neocortex is the part of the brain that enables us to speak, dream, or think. The underlying mechanism that led to the expansion of this brain region during evolution, however, is not yet understood. A research team headed by Wieland Huttner, director at the Max Planck Institute of Molecular Cell Biology and Genetics, now reports an important finding that paves the way for further research on brain evolution: The researchers analyzed the gyrencephaly index, indicating the degree of cortical folding, of 100 mammalian brains and identified a threshold value that separates mammalian species into two distinct groups: Those above the threshold have highly folded brains, whereas those below it have only slightly folded or unfolded brains. The research team also found that differences in cortical folding did not evolve linearly across species.
The Dresden researchers examined brain sections from more than 100 different mammalian species with regard to the gyrencephaly index, which indicates the degree of folding of the neocortex. The data indicate that a highly folded neocortex is ancestral – the first mammals that appeared more than 200 million years ago had folded brains. Like brain size, the folding of the brain, too, has increased and decreased along the various mammalian lineages. Life-history traits seem to influence this: For instance, mammals with slightly folded or unfolded brains live in rather small social groups in narrow habitats, whereas those with highly folded brains form rather large social groups spreading across wide habitats.
A threshold value of the folding index at 1.5 separates mammalian species into two distinct groups: Dolphins and foxes, for example, are above this threshold value – their brains are highly folded and consist of several billion neurons. This is so because basal progenitors capable of symmetric proliferative divisions are present in the neurogenic program of these animals. In contrast, basal progenitors in mice and manatees lack this proliferative capacity and thus produce less neurons and less folded or unfolded brains.
Duration and speed of brain development
The highly folded brains of mammals not only contain more neurons, they also grow with greater speed: The brain weight accumulated per gestational day is 14 times greater in species with a high degree of cortical folding. The differences among species between the two groups separated by the threshold value can be explained by longer neurogenic periods rather than different neurogenic programs. The neurogenic period of a human fetus is eight to nine days longer than that of apes. This leads to a brain three times larger than that of a chimpanzee – a fundamental difference that contributes to what makes us human.
Contact: Prof. Dr. Wieland B. Huttner – Max Planck Institute
Source: Max Planck Institute press release
Image Source: The image is credited to PLoS Biology unter Verwendung von Hirnschnitten von, and is adapted from the Max Planck Institute press release
Original Research: Full open access research for “An Adaptive Threshold in Mammalian Neocortical Evolution” by Eric Lewitus, Iva Kelava, Alex T. Kalinka, Pavel Tomancak, and Wieland B. Huttner in PLOS Biology. Published online November 18 2014 doi:10.1371/journal.pbio.1002000
An Adaptive Threshold in Mammalian Neocortical Evolution
Expansion of the neocortex is a hallmark of human evolution. However, determining which adaptive mechanisms facilitated its expansion remains an open question. Here we show, using the gyrencephaly index (GI) and other physiological and life-history data for 102 mammalian species, that gyrencephaly is an ancestral mammalian trait. We find that variation in GI does not evolve linearly across species, but that mammals constitute two principal groups above and below a GI threshold value of 1.5, approximately equal to 109 neurons, which may be characterized by distinct constellations of physiological and life-history traits. By integrating data on neurogenic period, neuroepithelial founder pool size, cell-cycle length, progenitor-type abundances, and cortical neuron number into discrete mathematical models, we identify symmetric proliferative divisions of basal progenitors in the subventricular zone of the developing neocortex as evolutionarily necessary for generating a 14-fold increase in daily prenatal neuron production, traversal of the GI threshold, and thus establishment of two principal groups. We conclude that, despite considerable neuroanatomical differences, changes in the length of the neurogenic period alone, rather than any novel neurogenic progenitor lineage, are sufficient to explain differences in neuron number and neocortical size between species within the same principal group.
“An Adaptive Threshold in Mammalian Neocortical Evolution” by Eric Lewitus, Iva Kelava, Alex T. Kalinka, Pavel Tomancak, and Wieland B. Huttner in PLOS Biology doi:10.1371/journal.pbio.1002000.
