TY - JOUR T1 - An Adaptive Threshold in Mammalian Neocortical Evolution JF - bioRxiv DO - 10.1101/001289 SP - 001289 AU - Eric Lewitus AU - Iva Kelava AU - Alex T. Kalinka AU - Pavel Tomancak AU - Wieland B Huttner Y1 - 2013/01/01 UR - http://biorxiv.org/content/early/2013/12/16/001289.abstract N2 - Expansion of the neocortex is a hallmark of human evolution. However, it remains an open question what adaptive mechanisms facilitated its expansion. Here we show, using gyrencephaly index (GI) and other physiological and life-history data for 102 mammalian species, that gyrencephaly is an ancestral mammalian trait. We provide evidence that the evolution of a highly folded neocortex, as observed in humans, requires the traversal of a threshold of ∼109 neurons, and that species above and below the threshold exhibit a bimodal distribution of physiological and life-history traits, establishing two phenotypic groups. We identify, using discrete mathematical models, proliferative divisions of pro-genitors in the basal compartment of the developing neocortex as evolutionarily necessary and sufficient for generating a fourteen-fold increase in daily prenatal neuron production and thus traversal of the neuronal threshold. Finally, using RNA-seq data from fetal human neocortical germinal zones, we show a genomic correlate to the neuron threshold in the differential conservation of long intergenic non-coding RNA. (see arXiv:1304.5412) ER -