Brain folding study defines two distinct groups of mammals

A program which controls the production of neurons in the process of brain development shows how the brain folds.  The researchers analysed the gyrencephaly index which indicates the degree of cortical folding of 100 mammalian brains. They also identified a threshold value that separates mammalian species into two distinct groups. One which are above the threshold, they have highly folded brains, another are those which are below it, they have only slightly folded or unfolded brains.  Increased or reduced folding of the brain is possible at each stage in evolution. A crucial threshold value of 1.5 divides mammals into two groups which are Dolphins and foxes.  Species which 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 neuron and less folded or unfolded brains. The research team also found that the differences in cortical folding did not evolve linearly across species.