Brain expansion: astrocytes drove enlargement (Evolution)

by David Turell , Friday, July 31, 2020, 22:43 (1574 days ago) @ David Turell

A review article which makes some interesting points about astrocytes and enlargement:

https://www.sciencedirect.com/science/article/pii/S0306987713005884?dgcid=raven_sd_reco...

"Abstract
Cells within the astroglial lineage are proposed as the origin of human brain evolution. It is now widely accepted that they direct mammalian fetal neurogenesis, gliogenesis, laminar cytoarchitectonics, synaptic connectivity and neuronal network formation. Furthermore, genetic, anatomical and functional studies have recently identified multiple astrocyte exaptations that strongly suggest a direct relation to the increased size and complexity of the human brain.

"The enlargement of the human brain is the prima facie example of punctuated equilibrium. “A progressive enlargement of the hominid brain started by about 2–2.5 million years ago, probably from a bipedal, australopithecine form with a brain size comparable to that of a modern chimpanzee”. (my bold)

"Comprehensive surveys of vertebrate brains fail to explain human cognitive abilities based on relative or absolute brain size. Importantly, these studies focus on the density of neurons, excluding glia, which account for approximately 85% of cells of the human neocortex. Additionally, electrical and histological studies show no significant differences in neuronal electrical properties, neural cell types, or depth of cortical lamination among mammals.

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"Protoplasmic astrocytes, the predominant cell in mammalian gray matter, are essential for normal synaptic function and maintenance. Additionally, they are instrumental in expression, storage and consolidation of synaptic information from individual synapse to global neuronal networks

"Recent anatomical and functional exaptations and genetic alterations specific to humans strongly support a critical role of astrocytes in human brain evolution. Additionally, a recent in vivo study infers that human astrocytes independently contribute to cognition.

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"A well demarcated neurogenic zone was recently discovered in the outer subventricular zone of the human fetal neocortex. RGCs in this area “accelerate the expansion of the neuronal population” [28] and contributes to the rapid intrauterine growth of human brains.

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"The extreme magnocellular exaptation of protoplasmic astrocytes in humans and the appearance of three primate and human-specific astrocytes leads to the conclusion “that this astrocytic complexity has permitted the increased functional competence of the adult human brain”

"A recent in vivo study provides evidence to “strongly support the notion that the evolution of human neural processing, and hence the species-specific aspects of human cognition, in part may reflect the course of astrocytic evolution”

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"Two human-specific genes are of particular interest relative to the increased size and cognitive abilities of human brains. These are members of the Thrombospondin group of genes

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"Thrombospondins are astrocyte-secreted extracellular-matrix glycoproteins that control fetal synaptogenesis and neurite outgrowth

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"This is the “first gene expression changes in human evolution that involve specific brain regions, including portions of the cerebral cortex. Increased expression of thrombospondins in human brain evolution could result in changes in synaptic organization and plasticity, and contribute to the distinctive cognitive abilities of humans …”

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"RGCs are instrumental in timing and directing corticogenesis. Novel genetic variations in these cells, through the process of natural selection, may profoundly influence brain size and functions [56]. As discussed earlier, empirical data over the past two decades demonstrates that protoplasmic astrocytes in adult mammals are critical for brain information processing and integration essential for higher cognitive functions.

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"Cells within the astroglial lineage determine the phyletic expression of mammalian brains, including humans. The increased size and complexity of the human brain clearly correlates with astrocyte anatomical and functional exaptations and genetic variations. Furthermore, the earlier concept that neurons and glia are derived from separate progenitor cells has been dispelled. Therefore, there is no compelling evidence supporting a neurocentric role in human brain evolution."

Comment: Our big brain is astroglial driven. "Due to an exaptation". Wow!: exaptations are unexpected changes in function of that which previously existed with a different function. As if God stepped in and added His design. It certainly was 'punctuated equilibrium', although not in the true Gouldian sense. One senses the appearance of a surprising evolutionary event.