The Ecology of Humans (26-17) Evolution


As with all other parts, the human brain is a product of evolutionary descent. As such, its features are found in all other mammals, albeit in different proportions and durations of development. The basic structures of the brain date back much further: before animals lived on land.

The functions of all animal mind-brains are essentially the same: perceive the environment and formulate activities for survival and enjoyment of life. The differences between brains – whether fish, fowl, or fellow – are adaptations to lifestyle.

To coordinate movement and gauge water pressure, fish have a large cerebellum. Befitting a near descendant, the amphibian brain resembles that of a fish except for structures adapted to life on land. These include adaptations allowing better senses of smell, hearing, and vision, as well as coping with gravity: a larger olfactory bulb, larger forebrain, and midbrain changes.

The reptile brain evolved from its amphibian antecedent with a more sophisticated olfactory processor, better adapted to the a more arid environment. Birds are a reptilian offshoot. Hence their olfactory bulb is relatively large, though avian sense of smell, with exceptions (e.g., kiwi), is unexceptional.

The avian cerebellum is well developed to control balance and position in flight. Bird cerebrums are larger; perhaps related to their greater sociality compared to reptiles.

Mammals do not face the agility demands of bird flight, so their cerebellum is overshadowed in size by the cerebrum, which is also more elaborated.

The cerebrum surface (cortex) of reptiles and birds is relatively smooth, as it is for rodents. With descent, primates show progressively larger cerebrums relative to body size. Apes have larger cerebrums and more wrinkly cortices than monkeys.

With neural growth guided by glia, cortex creases arise naturally due to mechanical instability in soft tissue that grows unevenly. Cortex folds afford greater operational volume for the same size. (Similarly, the stacked sheets of the endoplasmic reticulum, a eukaryotic cell organelle, maximize production space within a limited volume.) The folds form ridges (gyri) separated by grooves (sulci).

The mammalian cerebral cortex underlies many higher-order processes, such as perception, memory, language, and advanced motor skills. With its intricate furrows and ridges (i.e., the sulci and gyri), the complexity of the cerebral cortex is evident even at its surface. Beneath the surface, the cerebral cortex is separated into layers of densely packed neurons with axons reaching deep into the white matter. These layers are divided further into functional regions that correspond to the body plan. ~ developmental neurobiologists Lora Sweeney & Liqun Luo

Marine ragworms first appeared well over 500 million years ago. Their mushroom-body brains share many features with the mammalian cerebral cortex. While considerably different in numerous particulars, the genetic bases for development of the ragworm mushroom-body and mammalian cerebral cortex are analogous.