The Evolution of Thought

“Of all animals man has the largest brain in proportion to his size.”
Parts of Animals, Aristotle


Do you ever wake up sensing nearness?

Unlike the Hindu and Buddhist cultures the mythic cosmologies of the West lack a sense of nearness of the divine. In the East the common greeting is in Anjali; the hands are brought to the heart with a small bow in recognition of the divine in the person you are greeting. In this worldview every sentient being carries a spark of the cosmic, the ultimate.

I find this interesting because in my view the most astonishing features of existence are not the far reaches of space or the mind numbing expanse of the universe as a whole. The most astonishing thing is that all this enormous planetary and stellar matter is so very simple in structure compared to organic chemistries. We find the most complex organization of molecules, if you will, anywhere in the universe, to be right here on planet earth. An interesting contemplation is to sense how all the voluminous yet simple matter in our solar system through its gravitational participation is a necessary support of the rich bounty of earth’s fruitfulness. After all, if any one thing was different, all things would be different. That is what it is to be in a universe of total interdependence, a universe like ours.

If we then ask where we find the pinnacle of organizational complexity among organic matter, the brain is said to have no equal. As ‘possessors’ of said brains we are also familiar with their ability to share their thoughts, like we are doing right now.  In Anjali the greeting ritual lifts social interactions to a sacred place, one recognizing something like the uniqueness we find in the grey matter. ‘Hello expression of complexity flowering in the universe, I greet you.’ I think it better captures the dignity of an encounter in which the mystery of my awareness touches the mystery of your awareness. It sees in individuals something more than social status and bank accounts.

The contemplative has taken their vehicle to be the mind, the psyche, the soul. In Tibetan Buddhism there is the tradition of the three year retreat. Like the traditional Anjali  this too is interesting. It is as if this culture has a tradition of putting the human mind on the altar in recognition of its actual cosmic value. In three years, three months, and three days it is said the yogi can map the mind, familiarize themselves with its outer reaches and in some ultimately inexpressible way, master non-dual awareness. As children of the space age it is easy to feel the real action in the universe is happening somewhere else. With Anjali and retreat traditions of both the East and West we are being asked to reconsider that, invited to consider that we may be looking through the wrong end of the telescope.

Where did these brains come from? How were they made? How do they work? The first and second questions get a rather satisfying answer from evolutionary studies and we will examine them a bit momentarily. The third question is less amicable to the desire for settled answers, though I will suggest modern neuroscience and cognitive science have advanced far enough to send a few popular yet troublingly misconceived ideas out to pasture.

So what is this mind? To begin the story of where brains come from we need to start with the evolution of the nervous system. Whatever else nervous tissue might be it is certainly a means of sending signals. With signals comes information. What is the selective advantage of additional signal processing?

Learning to adapt to the environment through the accumulation of beneficial genetic mutations is a very slow process by our standards. It takes millions of years for speciation to develop and perhaps hundreds of thousands of years for smaller adaptations to establish themselves. In this environment a premium is placed on any developments that enhance the ability of an organism to respond to their specific environment more skillfully.

Early animals evolved numerous sensory mechanisms for reading their environment. The senses that could pick up environmental clues from a distance carried the highest premium of all. To avoid a mortal threat or find a mate, sight and hearing encompass far more of the environment than taste and smell. This was an environment in which the dynamics of predator and prey drove the selection of the fittest for many, though not all, animal species. It is not hard to understand how even the slightest insight into the state of your world would give you an edge; one percent of an eye, when it is one hundred percent more eye than your neighbors have, is of revolutionary importance.

The earliest animals did not have nervous systems. This limited the amount of information about themselves and their environments they were able to make use of. The development of the nervous system took the ball game to a whole new level. Both the speed by which information travels increases and the information density increases by orders of magnitude. The speed comes because the nervous tissue is able to use electrical impulses instead of chemical diffusion for communications. The density comes due to the many connections possible between neurons due to their dendrite structures.

In Dragons of Eden Carl Sagan presented the following graphic as a way of capturing the information explosion that occurred with the evolution of the nervous system. Genetic information tops out (10 to the 10th) but the human brain information content continues an estimated three more orders of magnitude (10 to the 13th).

BrainGeneticInfo2The evolution of nervous tissue would prove to be wildly successful. “Somewhere in the steaming jungles of the Carboniferous Period there emerged an organism that for the first time in the history of the world had more information in its brains than in its genes.”

Another defining characteristic of the human brain is that it has more mass for its body weight than any other animal on earth. Next in line are the dolphins.  This is captured in the brain to body weight ratio. When this is charted Sagan observes the emergence of mammals and primates “was accompanied by major bursts in brain evolution.”

BodyBrainMassRatioSteven Pinker points out the human brain was considerably re-engineered from the primate brain. “Our brains are about three times too big for a generic monkey or ape of our body size. The inflation is accompanied by prolonging fetal brain growth for a year after birth. If our bodies grew proportionally during that period, we would be ten feet tall and weigh half a ton.”

So where did this path to the brain begin? Last week’s post ended with a look at the segmented nerve cord budding into three distinct areas which become the hindbrain, midbrain and forebrain. Remember, we have this DNA expressing itself through the use of its own regulatory genes. The topological mapping of this evo-devo expression in the nerve cord is segmented at one end into these three distinct regions. We can track these regions throughout the vertebrates’ exploration of the DNA’s design space.

The brain of a fish, which isn’t much, is chiefly midbrain with a tiny forebrain. Reptiles and amphibians are the other way around. Another graphic from the same work shows this by illustrating a perch and a toad. Dr. Sagan also included the development of the nerve cord in a reptile, bird and mammal.

SaganSchematicIt is easy to see how the tinkering with the fundamental components has taken place. Can you hear the three part chord echoing through the long centuries?

What we did not know at the time the Sagan book was written is how the regulatory genes actually work. Today there is a rather clear comprehension of exactly which genes are involved. It is an impressive example of information being conserved across a wide spectrum of phenotypes.  In From DNA to Diversity (first edition) the authors summarize the role of the three regulatory genes across the three regions this way:

DNAToDiversityVertBrainFrom the trail of clues left in the DNA evidence the subdivision of the vertebrate brain into fore, mid and hindbrains came before the mammalian neocortex. The last common ancestor had localized, discrete domains of these regulatory gene expressions.

From observation such as these a model of the brain as a triune structure was developed. In this model of the human brain the later parts are added to those that had evolved earlier. The argument for the model relies on the fact that we find some biological elements conserved through evolutionary changes. It stands to reason that mutations that tinker with some of the deepest features of life are likely to be lethal. In this model of brain evolution the early brain parts are left basically undisturbed while the later parts are added.

The oldest strata is said to be the reptilian brain or the R-Complex, to use Paul MacLean’s term. He also refers to a neural chassis consisting of the midbrain and hindbrain. These contain the neural circuitry for reproduction, respiration, heart rate, blood circulation and other systems necessary for self-preservation. They evolved several hundred million years ago. Surrounding that is the limbic cortex which is said to be the seat of the emotions and is associated with the caregiving of the young we find in the mammals. It likely evolved more than one hundred and fifty million years ago. Finally there is the outermost layer consisting of the neocortex which becomes ever more elaborate in the more advanced mammals. It is believed to have evolved tens of millions of years ago, though a leap in its complexity occurred just a few million years ago with the emergence of humans.

In this model the brain consists of three very different modalities, only one of which is graced with speech. Each has its own mentality, intelligence, sense of space and time and its own memory and motor functions. The distribution of the neurotransmitters dopamine and cholinesterase are also found to be strikingly different in each section. We could say the R-Complex is performing dinosaur functions and the limbic cortex is “thinking the thoughts of pumas and ground sloths.”

Culturally we recognize our lower animal nature as reptile-like when we speak of the ‘cold blooded killer’ or when Machiavelli advises his Prince “knowingly to adopt the beast.” It is not hard to recognize the aggressive, territorial, ritualistic behaviors and rigid social hierarchies of the reptiles within our politics of empires and armies.

Care of the young is mostly found among birds and mammals though there are exceptions among the social insects. The development of the limbic cortex is thought to have brought with it these altruistic behaviors. In this model love was invented by the mammals.

With the arrival of the neocortex comes the ability to plan, to think ahead and to consider options. Instead of a potential meal relying only on instinctual jumping or zigging and zagging to escape their predator threat, an animal equipped with a neocortex can also look for an escape route unique to the exact environment in which the confrontation is occurring.

There is something very familiar to western ways of thinking about our humanity in this triune brain model. It is not hard to see Freud’s id, ego and superego in its threefold classification scheme. More anciently, in Phaedrus Plato likened a human being to a chariot being drawn by a set of black and white horses. Popularizers of the triune brain theory pointed out it mapped well to this Platonic insight when the chariot is likened to the neural chassis, the horses to the R-Complex and limbic systems and the neocortex to the charioteer.

The triune brain seems a fairly good model of both brain physiology and human experience. The model is quite popular, quite well known, almost as if it was a theory made to order, one that confirms our expectations. This should make us wary.

Advances in neuroscience have shown the triune model of the brain to be of limited usefulness, even misleading. Just why and what we have found to be the case instead is what we will look at next week.

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