FORAGING AND BRAIN EVOLUTION

The Illustrated Encyclopedia of the Prehistoric World (Marshall Edition, Chartwell Books 2006) has a beautiful rendition of the Vendian sea environment. In it, there are illustrations of Ediacaran life indistinguishable from plant and animal. These transitional creatures were likely the first life forms to evolve a neural system of the kind we find on the most primitive level of neural structure in the contemporary brain. Those structures enabled the detection, capture, and consumption of the ambient nutrients necessary to the energy demands of Ediacaran survival. How, one may wonder, did the consumption of ambient nutrients become as important to early life as photosynthesis had been for billions of years?

Through photosynthesis, early life survived by using sunlight to convert water and carbon into energy. A byproduct of this life sustaining process was the production of oxygen. Billions of year of photosynthesis released enormous quantities of oxygen that forced adaptive changes in existant oxygen sensitive organisms. Photosynthetic organisms, intolerant of oxygen, would have had either to adapt to an oxygen rich atmosphere or retreat to an environment devoid of oxygen.

In their retreat from early earth’s changing environment, oxygen sensitive photosynthetic organisms would have been driven away from the sun’s life-sustaining rays by their efforts to escape the toxicity of early earth’s oxygenation. In doing so, these retreating organisms had to evolve strategies for sustaining their tenuous existence in the increasing absence of sunlight. One of those strategies likely led to foraging.

FORAGING IMPLICATIONS

Evolving from photosynthesis to foraging suggests that early animals had developed the rudiments of a nervous and sensory system. It also suggests that the energy demands of early life were increasing. The detection, capture, and consumption activity of feeding Ediacarans was likely more energetic and required the expenditure of more energy than that demanded by sunlight conversion through photosynthesis. This suggests that as early animals evolved from photosynthesis to foraging, they were becoming increasingly dependent on their ecology.

Becoming more dependent on their ecology meant that early animals would have become more energetic to satisfy their dependency. They would have expended energy to seek nutrients and would have required a means or strategy to store or conserve energy when nutrient sources were not readily available. If human brain structure mirrors the neural evolution of ancestral animals, then its structure should also reflect every significant stage of that evolution beginning with its most primitive components. At the most primitive level of current brain structure, the myelencephalon, we find neural fibers that we can relate to the more energetic foraging behavior of likely progenitor animals. At the next level, we find neural structures that suggest these animals were becoming even more energetic.

METENCELPHALON

The metencephalon of the human brainstem is contiguous with the myelencephalon and has several afferent neural fibers suggesting the increasing dependency of early animals on their ecology and their increasing energy expenditure. Beginning with the first afferent nerve arising in the metencephalon and closest to the myelencephalon, the vestibulocochlear nerve (cranial nerve VIII) is associated with aural sensory and equilibrium. The introduction of aural and equilibrium sensory into brain structure beyond those merely associated with taste and swallowing through the myelencephalon suggests that ancestral animals, at the metencephalic level of brain evolution, were becoming more responsive to their environment and were beginning to engage in directed gross locomotion.

SOUND SENSORY

Using primitive structures in the human brain to diagram the neurological path of antecedent animals, the entry of metencephalic sound sensory suggest that primitive animals at this level of brain evolution had begun to engage in movements generated by the sounds they perceived. As foraging animals, the arrival of sound perception showed that these early life forms were probably more responsive to their environment by directing their movements either away or towards sources of sound. Although the primitive neural systems suggested by contemporary myelencephalic development infer an earlier evolution of sound perception through the entry of ear sensory, that sensory is merely tactile and not aural. Myelencephalic tactile sensory from the ear followed by the development of metencephalic aural sensory may evidence the evolutional path of sound perception from a tactile origin. The sensitivity of the deaf to tactile vibrations is a likely testament to this tactile origin of hearing.

Following hearing, metencephalic neuroanatomy shows the development of additional taste perceptions (anterior tongue) through the facial nerve (cranial nerve VII). Here again, additional taste discriminations support the increased involvement of early animals in the pursuit and distinction of appropriate food sources. Coupled with the sensory information provided by the trigeminal nerve (cranial nerve V), it is conceivable that most of preexistent metencephalic life was devoted to the detection, capture, and consumption of nutrient sources. The trigeminal nerve is the final afferent neural fiber of the metencephalon and it is associated with sensory from the face, sinuses, and teeth. The earliest visible evidence of brain development in preexisting animals supports this view of early life’s preoccupation with the pursuit of sustenance.

In my next session, Cambrain life and its implications in brain evolution.