Saturday, February 1, 2014

117. Evolution of the Human Brain

'. . . our intelligence is not disembodied, but is instantiated in physical objects: our brains. Their structure is due to the long process of evolution, and their operations are governed by the laws of physics. Since they are physical entities, our brains run without being told how to run' (Douglas Hofstadter, Gödel, Escher, Bach).

The human brain is a physical organ, governed by the laws of physics. The mind is what the brain does. Our intelligence is no different from swarm intelligence, the swarm here being that of neurons.

There is a belief that the transition from intelligence to 'consciousness' needs the acquisition of a human language. The ‘society of mind’ (comprising of ‘communities’ of large numbers of interacting neurons) emerged
 as a hierarchical structure, so typical of any complex adaptive system.

Any living entity exploits the existing structure and order of its surroundings to ensure its survival and reproduction. Consider a single-celled organism in a pond. On its surface are molecules which can ‘detect’ (are influenced by) the presence of nutrients. There is usually a gradient of the nutrient concentration, so that it is higher on one side of the organism than on the other. The single-celled organism has chemical sensors which can detect this gradient. Biological evolution has programmed it to propel itself in the direction of increasing concentration of nutrient. An attribute of intelligence is the problem-solving capacity of the system; other important attributes are prediction and memory capabilities. As Hawkins (2004) pointed out, both prediction and memory are involved here. The prediction is that, by moving in the direction of increasing concentration of nutrient, more nutrient will be found. This is not something the organism has ‘learnt’ and ‘remembered’ in its lifetime. The 'memory', evolved over many generations of evolution, is in its DNA.

Plants also exploit the existing order and structure (constancy or sameness over reasonably long time scales) by employing memory and prediction. The memory in the genes of a tree tells it that it will find greater sunshine by sending its branches and leaves towards the sky. And that it will find water and minerals by sending its roots down into the soil. These actions are automatic, and there is no ‘thinking’ involved, just as there is no thinking involved in the actions of a bacterium.

At a certain stage in the evolutionary history of plants, more complex behaviour emerged in the form of communication systems among the various parts of a plant, based mainly on chemical signals. Suppose an insect damaged some part of a tree, and this led to the slow transmittal of a chemical through the vascular system to other parts of the tree. This triggered a defence mechanism; e.g. the making of a toxin for the insect.

Neurons evolved or emerged in due course, as a faster way of communicating information to different parts of an organism. The electrochemical spikes in a neuron travel much faster than the diffusion of chemicals. In due course, the synaptic connections between neurons became modifiable. A neuron may or may not send a signal, depending on what happened in the past. This rudimentary nervous system had elements of both memory and learning.

The evolutionary advantage of this to the creature was qualitatively different. Instead of depending on just ‘genetic memory’ and instinct coded in DNA, the creature could now learn from experience during its own lifetime, and modify its behaviour for achieving better survival and propagation possibilities. In particular, if the environmental structure and order changed rather suddenly, the animal could still make a generally adequate response, instead of having to depend only on the somewhat outdated (and therefore inadequate) genetic memory and instinct. Such plastic nervous systems entailed a huge evolutionary advantage, and there was a burst of new species from fish to snails to mammals, including humans.

Why is it that intelligence evolved mainly in the animal kingdom, but not in the plant kingdom? According to Hans Moravec the difference has arisen because animals are mobile and plants are generally not. The mobility of animals presents to them an ever-changing environment, and therefore intelligence is an important prerequisite for survival and propagation: An animal can survive only if it has a large repertoire of solutions to the continuous stream of problems it faces in a changing environment.

The human brain, like the brain of any other mammal, has something distinctly additional compared to the brain of reptiles from which it evolved, namely the neocortex. The human brain has two main parts: the ‘old brain’ or the reptilian brain or the R-brain or the ‘primitive’ brain, and the neocortex.

Practically everything we associate with conscious memory and intelligence occurs in the neocortex, although the thalamus and the hippocampus also play important roles. In the evolutionary history of life on Earth, sophisticated sensory and actuation organs had evolved in reptiles, and their behaviour was controlled by the old brain, with no cortex. The evolution of the cortex in one of the offshoots of the reptiles, along with the availability of a stream of sensory inputs into it which it could remember and analyse much better than reptiles could, gave the mammals an evolutionary advantage: When they found themselves in situations they remembered to have faced earlier, their much-improved memory and analysis power told them what to expect next, and how to respond effectively.

Here is a useful video (' Evolution of the brain '):