The Ecology of Humans (26-20) Brain: Evolutionary Perspective

 Evolutionary Perspective

Vision is one of the key factors in triggering evolutionary changes. ~ German zoologist Brigitte Schoenemann et al

An evolutionary perspective highlights the importance of glia. If glia function as the physical correlate to the mental library, then species with greater cognitive facility should have proportionally more glia; and so it is. A leech has 1 glial cell for every 30 neurons. The widely researched earthworm has 1 glia for about every 6 neurons. Glia comprise 16% of a worm’s nervous system cells. Vinegar flies: 20%. Rodents, such as mice and rats: 60%. Chimpanzees: 80%. Humans: 90%. The ratio of glia to neurons increases with what is broadly considered cognitive capacity.

Not only does the ratio of glia grow, but so does their size. Astroglia in humans have 27 times the volume of mouse astrocytes.

Complex behavior in both invertebrates and vertebrates increases along with the compartmentalization of neurons by glia. Glia organize insect brain areas. Glia comprise 57% of bee retina cells. The more processing required for functioning, the greater the glia.

Coleoid cephalopods – octopi, cuttlefish, and squid – are the most intelligent invertebrates, and have the most complex invertebrate intelligence system. Astrocyte-like cells predominate in their large brains.

Coleoids contemplate stimulus, demonstrated by selectively changing skin coloring in response. They make thoughtful decisions based on vision and touch.

Coleoids express emotive behavior in how to handle stressful situations, deciding whether to run or fight. They have complex courtship behaviors. Coleoids are social: when isolated from their own they shoal with fish.

Invertebrates don’t seem to need to sleep. Sleep first appeared with fish.

Fish have less behavioral variation and sophistication than coleoids. They also have fewer glia.

Amphibians exhibit a form of sleep. They are not as behaviorally complex as some of the brighter fish species, but they do learn spatially and develop specific techniques for nabbing prey. Amphibians also have more astrocytes than fish.

Though of a distinctive design, avian brain differentiation approaches that of mammals. Birds are highly skilled manipulators of spatial information.

Birds see color in a different way from humans. ~ Swedish zoological ecologist Anders Ödeen

Birds visually process and categorize colors the same way that humans do, even though their color vision is distinctive, with 4 color receptors compared to 3 in humans.

Birds are also prodigies of temporal processing. Songbirds learn new songs. Some create sonic productions worthy of the finest human composers. Birds raised in isolation develop less elaborate songs. As with other gregarious animals, bird brains need conspecific stimulation to achieve their potential.

Songbirds develop signature songs by which they are recognized as individuals by others. Birds are natural tool users. Bird’s nests, which are often surprisingly complex, are exemplary.

Cortical folding increases with later-evolved species, such as cats, dolphins, and primates. Humans have 35% more cortex glial cells than chimpanzees.

◊ ◊ ◊

Glia are at the hub of brain health and disease. ~ Douglas Fields

The state of someone’s glia, including the genes used to produce them, is so consistent through the years that it can be used to predict someone’s age. The same cannot be said of neurons.