While researching material for my second and most recent book (Neuropsychology of the Dreaming Brain), I realized that my work would be incomplete without a cogent foundation in how the brain evolved to dream. I understood from the beginning that my quest to uncover the origin of the dreaming brain should be inclusive of early life forms other than those exclusively hominid—after all, life didn’t begin with exclusively human ancestry.
If dreaming has an origin in brain evolution, that evolution began with creatures far older than humanity’s apelike ancestors. Several hundred million years before the first hominid and billions of years before evidence of brain structure in the fossil record, there existed creatures whose progeny led to the first animals with the simple neural structures that eventually became the hominid brain.
The record preserved in 3.7 billion-year-old rock suggests that life on earth began with tiny photosynthetic organisms called photoautotrophs. From these organisms emerged a kind of blue-green algae whose primitive existence is evident in 3.5 billion-year-old fossil deposits called stromatolites. Although these early life forms did not leave evidence suggestive of the neural developments leading to brain evolution, we can conceivably perceive in their photosynthetic existence the origin of humanity’s general preference for nocturnal dormancy.
3.1 billion years before the emergence of creatures that might have had some primitive neural structure, the predominant form of life on earth (photosynthetic) experienced a state of dormancy in the absence of sunlight. Photosynthesis in contemporary species (plant and bacterium) requires sunlight to generate energy and necessitates inactivity in the absence of sunlight; therefore, the earliest photosynthetic life forms likely experienced a kind of sleep at sunset. This is important to our perspective of human evolution because the creatures that likely evolved into the animals that became human ancestry appeared to both plant and animal.
It is conceivable that animals with the ability to survive on both sunlight and organic material would have had a survival advantage over those solely dependent on sunlight. Neither plant nor animal, the curious creatures (Ediacarans) of the Vendian era (about 620 million years ago) appear to be a combination of the two. The Ediacarans were soft-bodied creatures that did not leave a fossil record of the kind we find with hard-shelled or boney animal. However, their fossilized contours suggest that they may have been foraging animals (akin to jellyfish and annelid worms) that subsisted partly on sunlight but primarily on nutrients sifted from their primal sea environment.
Although the fossil record contains no evidence of their internal structure, the possibility that the Ediacarans may have been foraging creatures suggests a crucial stage in evolution precursor to current brain structure. Foraging requires an ability to detect and distinguish nutrient rich substances. As a foraging and more complex animal than those that left stromatolite remains, the Ediacaran may have required some neural structure that would have allow them to detect, capture, and consume the nutrients they needed to survive. Although contemporary insectivorous plants—like the Dionaea muscipula (Venus flytrap)—belie the need of a nervous system to detect, capture, and consume nutrient sources, the comparison of Ediacaran life to existing jellyfish and annelid worms infers the evolution of the first neurological structures associated with animal life.
If the human brain evolved from Ediacaran type creatures, then we should find in the primitive structures of our central nervous system (CNS) some reflection of the ancient neurophysiology that allowed the behavior of these early animals. Indeed, we find in the earliest component (myelencephalon) of the most primitive structure in the human brain (brainstem) the presence of neural fibers that we can associate with Ediacaran type food consumption.
MYELENCEPHALON
In the myelencephalon of the human brainstem, we find entry of the first afferent nerve fibers associated with the detection of sensory information. Emerging from a lateral groove near the olive, the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X) arise as the only neural fibers of the myelencephalon that appear to deliver information into brain structure form sensory sources. The glossopharyngeal nerve is associated with taste (posterior tongue), tonsil, pharynx, and middle ear sensory. The vagus nerve is linked to heart, lungs, trachea, bronchi, larynx, pharynx, GI tract (thoracic and abdominal viscera), and external ear sensory. Overall, these neural fibers appear to be ostensibly associated with food distinction and consumption. Although Ediacaran life was far less evolved and their neural structure probably not as sophisticated, their appearance millions of years ago suggest the stage at which ancestral animals may have begun to develop the simple neural structures that ultimately led to the structures we find in the myelencephalon of the human brain. These simple structures probably enabled the ability of early animals to make gustatory distinctions about their sensory environment.
In my next session, I will attempt to explain the implication of forging on the emergence of brain structure and function.
Wednesday, February 21, 2007
Monday, February 19, 2007
BRAIN EVOLUTION
Just the other day, I sent a letter to Dr. P. Thomas Schoenemann who is a professor of anthropology at the University of Pennsylvania. He is studying brain evolution and has published a review on the size and functional areas of the human brain (Annu. Rev. Anthropol. 2006. 35:379–406). In Prof. Schoenemann review, he outlined several criteria for assessing brain evolution focusing primarily on the hominid brain. In my letter, I described my thoughts on the fallacy of seeking clues to the origin of the human brain by focusing solely on hominid brain structure and lineage.
As I see it, the fallacy of most ideas and all research on the nature and function of the contemporary brain is where the science for those ideas and research begin. Invariably, the science of brain research begins with the emergence of cortical structure and function. The error in this beginning is that the brain is not just the cortex and it did not evolve from cortical structure or function.
It is incredibly naïve to think that the totality of the human brain only encompasses cortical structure when the cortex is entirely dependent on subcortical relays and functions. Dr. Michel Jouvet proved as much in his early 1960’s experiments with decerebrate cats. Jouvet showed that the cortex does not engage in any spontaneous activity when it is isolated from subcortical structure (Jouvet, M., & Jouvet, D.,1963: A study of the neurophysiological mechanisms of dreaming. Electroenceph Clin Neurophysiol., Suppl. 24.). Further still, the cortex is not the most primitive constituent of our central nervous systems (CNS), which in itself represent the totality of brain structure and function.
Clearly, the totality of brain structure and function involves a concert of neural processes and activity between the subcortical and cortical components of our CNS. The neural processes and activity these components produce enable the perceptual and behavioral abilities essential to normal life. Of these components, our brainstem is the most primitive. As such, it evolved before cortical structure. Rather than replace the primitive, the evidence in evolution suggests that nature builds upon the successes of the primitive to create modern versions that are more robust and adaptive. As such evidence suggests, the cortex—as a more recent neural development—likely evolved from the success of primitive neural structures. Therefore, if our goal is to determine how the cortex evolved to its current size and function, we must begin with the structure that came before—the brainstem.
As a primer to future entries, consider the shape of the thalamus, the similarity of its form to current cortical structure, and why it has relays for every sensory system of the body. Now consider if it is likely that the thalamus, a brainstem component, was a prototype for current cortical structure and function.
As I see it, the fallacy of most ideas and all research on the nature and function of the contemporary brain is where the science for those ideas and research begin. Invariably, the science of brain research begins with the emergence of cortical structure and function. The error in this beginning is that the brain is not just the cortex and it did not evolve from cortical structure or function.
It is incredibly naïve to think that the totality of the human brain only encompasses cortical structure when the cortex is entirely dependent on subcortical relays and functions. Dr. Michel Jouvet proved as much in his early 1960’s experiments with decerebrate cats. Jouvet showed that the cortex does not engage in any spontaneous activity when it is isolated from subcortical structure (Jouvet, M., & Jouvet, D.,1963: A study of the neurophysiological mechanisms of dreaming. Electroenceph Clin Neurophysiol., Suppl. 24.). Further still, the cortex is not the most primitive constituent of our central nervous systems (CNS), which in itself represent the totality of brain structure and function.
Clearly, the totality of brain structure and function involves a concert of neural processes and activity between the subcortical and cortical components of our CNS. The neural processes and activity these components produce enable the perceptual and behavioral abilities essential to normal life. Of these components, our brainstem is the most primitive. As such, it evolved before cortical structure. Rather than replace the primitive, the evidence in evolution suggests that nature builds upon the successes of the primitive to create modern versions that are more robust and adaptive. As such evidence suggests, the cortex—as a more recent neural development—likely evolved from the success of primitive neural structures. Therefore, if our goal is to determine how the cortex evolved to its current size and function, we must begin with the structure that came before—the brainstem.
As a primer to future entries, consider the shape of the thalamus, the similarity of its form to current cortical structure, and why it has relays for every sensory system of the body. Now consider if it is likely that the thalamus, a brainstem component, was a prototype for current cortical structure and function.
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