See the link below for a discussion on this fascinating question:

https://neuroscience.stanford.edu/news/ask-neuroscientist-why-does-nervous-system-decussate

That said, not all tracts cross-over (e.g., the dorsal spinocerebellar tract).

If we follow the principal of Occam's Razor it would be logical to conclude that the corpus callosum functions much more effectively, and efficiently, if the left and right hemispheres are opposed to each other.

http://brainmadesimple.com/corpus-callosum.html

One theory suggests that it occurred as a result of the evolution of the visual pathway including the retina and the optic chiasm, with the benefit of hemispheric independence (as is well known that in larger brains efficiency is critical).

1.https://en.wikipedia.org/wiki/Evolution_of_the_opposed_cerebral_hemisphere_control

2. Article The optic chiasm: A turning point in the evolution of eye/ha...

3. Introduction to anatomy and physiology. Evolution of the Cerebral Cortex. Brain Mapping, An Encyclopaedic Reference.

@MikyTimothy: Thanks for the link; it is quite interesting. The answer given by Stanford grad student K. Zalocusky (or at least his/her favorite theory among several on this issue) indicates that there is actually little or no evolutionary advantage, but just a quirk (idiosyncrasy) of nature as to how it evolved. We know that evolution is largely stochastic so it could have evolved by a different path and it would have not made much difference. Do you agree with my interpretation? Thanks!

Vicente,

Without reading on this topic, my first instinct was to see this to be one of glorious accidents of evolution. For animals to have eyes is not a glorious accident, hundred of different eyes were independently created in evolution and it is so because exploiting the information from light is crucial for moving organisms. The bilateral architecture of the fishes and all the vertebrae probably evolved because of the move of bilateral propulsion in the water of the fishes and all animals that evolved from fished evolved over this bilateral body structure, including their central nervous system. Why is the left and right side of the nervous system cross over might have been functional in fishes but maybe this feature was a glorious accident of evolution of the fishes bilateral central nervous system of the first fishes.

Dr. Reyes, I absolutely do agree with your interpretation and think that Zalocusky's explanation, or one similar is probably what is going on. I personally am skeptical of an a-priori 'adaptationist' approach to thinking about evolutionary questions, since alternative explanations not based on selection (genetic drift, etc.) often seem more parsimonious.

That is not to say that natural selection is not the driving force behind evolution, just that there is the troubling potential for the propagation of unfalsifiable 'just so stories' if non-adaptive or stochastic mechanisms aren't considered.

By the way, the link below defines and goes into some details regarding what I mean by 'adaptationist'.

https://plato.stanford.edu/entries/adaptationism/

The presence of a blind spot in the vertebrate eye and its absence in the cephalopod eye is often cited as an example of a relatively inferior design (vertebrates) becoming fixed in a lineage. This despite the convergent evolution of an alternative design (cephalopods) that does not possess a blind spot. Ultimately there are structural constraints and considerations in animal body plans that may explain morphological diversity across species. See below:

https://doi.org/10.1016/j.tree.2006.06.003

Hopefully I haven't strayed too much from your initial question, and the above is relevant to our discussion. Your thoughts?

I find this question as one among group of common assumptions about the workings of brain. Like, lateralization/localization vs unitary brain hypothesis, one local site responsible for one single aspect of behavior, one gene-one symptom expression etc. These debates are controversial, and science as yet has not reached to any definite conclusion. So I believe we lack theoretical models to explain such phenomena and this right-left hemisphere problem is unexplainable as of now. Much of what evolutionary theory suggests seems to be a confabulatory behavior of science, where missing links are attributed to some grand explanation (This happens in physics and chemistry too). Evolutionary theory seems to provide answers to all incomprehensible questions in neuroscience. I do not agree to such claims that evolution made our brains that way. Its true that long evolutionary chain of events made us who we are, but without having any definitive answer in other non-evolutionary theoretical ideas, jumping to evolution-attribution would be a mistake.

Let us assume that brain would have connected nerves from same side of the body, what differences could be possible ? How convenient it would have been for the brain to function in that form ? Similarly, lets say there were no two hemispheres, but only one single mass of neurons (i.e. no need of corpus callosum). What if there were no gyri or sulci ? Ample number of such questions can be asked. The answer can be found either through comparative anatomy (somatic twist) or through many other methodologies.

maybe our two hemisphere are just a simple suggestion of duality. in order for neurons to fire they must have some place to go. With this being said, we must understand that there is no place for communication to happen if there is no where for the message to start/end. Duality suggests that one aspect bounces off of the other creating a new life. One cannot live without the other.

As an aside, I believe there is some decent, recent, hand-waving regarding isolated lateralization (e.g., sensory, motor and even memory apparently) in the nervous systems of some invertebrates as well.

Tarun Verma Only the great apes, most especially humans, possess appreciable gyri and sulci; lower animals don't need them as they are not endowed with rational capabilities. Gyri and sulci are necessary to increase the area of the brain cortex without increasing the volume of the brain, which would increase the volume of the skull. This would make vaginal childbirth ..... how could I say it? ....... very difficult indeed.

Dear Dr Reyes,

I have not had time to read all of the answers to this question.

Frankly, I would be quite surprised if this explanation had not already been suggested. In short, we have two hemispheres so that we can rub our tummies and tap our heads at the same time.

Many theories implicate the structures implicit in stimulus as being the primary factor that structures the brain. Because of the high degree of symmetry between the structures on the left and right sides of our bodies, there would be a tendency for these symmetries to be 'connected' in direct. but perhaps, inflexible ways.

By separating the areas that control the structures on the left and right side of our bodies ensures that these structures are 'integrated' separately during the development phase allowing for more complex levels of integration subsequently.

Any of you who can play the piano remembers the anguish of trying to get one hand to do something quite different from the other. I believe that such separation is great facilitated by having two hemispheres.

We know have an evolutionary imperative: - The survival of bi-laterally structured animals would be greatly enhanced if they could achieve asymmetric behaviours within the context of environments which are often asymmetric spatially.

Having evolved two hemispheres the consequence of such a split might well have very interesting and unexpected consequences. Dr Gary Goldberg drew my attention to the fact that certain functions seemed to correlate with the hemisphere associated with the dominant eye.Functions may well 'migrate' away - or toward the hemisphere controlled by the dominant eye because of some sort of cognitive resonance or conflict - as when we turn the volume on the television down when we want to concentrate on an idea.

I imagine that all sorts of complex dynamic possibilities open up once the two hemisphere system has evolved. One idea that was suggested to me by Dr David Williams, was that one hemisphere might 'use' the other to test or simulate cognitive states. The question of how a two hemisphere brain fundamentally alters the structure of our psych is a fascinating one.

Chris

Dr Reyes,

I can't remember - was it Kant? - pointed out that if you take topological pairs like left and right hands, although they appear to us to be different, in fact, in terms of their internal relationships, they are actually identical.

This can be demonstrated quite simply by considering a table covered with right handed 'L's.. We can pick one of the right handed 'L's up and rotate it through the extra dimension and when we place it back on the table - we have a left-handed letter rather than a right-handed one.

The implications for this are quite interesting. If the instructions were sent to another planet to build a motor car and that car was eventually built. Without any other common point of reference by which left and right could be distinguished it would be a matter of chance as to whether it was a right or left handed drive that was eventually built!

Depending upon how information is sent to the brain and in what form may make Kant's observation significant. If the brain can't, in-principle, tell the difference between left and right hands from the information provided then separating the areas of control may be the only 'operational' solution to ensure the distinction is made explicit.

Following on from the above - perhaps the right controlling the left and vice-versa - chance? - a difference that actually makes no difference at all?

Chris

Dr Reyes,

Further to the argument presented above:-

https://www.youtube.com/watch?v=V8AyIaiNeyc

I think that what has happened here is that the octopus evolved from an animal with a more pronounced bi-lateral symmetry - hence the bilaterally structured central nervous system(indeed basically the animal is still bi-laterally structured). However, evolutionary pressure favoured a move towards the eightfold multi-dextrous symmetry that we see today.

This presented the evolving animal with a problem. The evolution of eight highly complex tentacles exhibiting a high degree of symmetry meant that each hemisphere had to 'integrate' individually 4 tentacles within the same neural 'space'. For the reasons stated in the previous posts, I think that this would have been impossible.

Hence the solution.

In order to test this idea then we might expect the animal to exhibit two very different behavioural modes. When the animal is feeding and searching for food, I would expect the central nervous system would become less active and the multi-dextrous mode would be predominant. This is because exercising executive control from a Bi-lateral brain would be in direct conflict with an eight fold system that is most effective when each component is acting independently.

However, during moments of stress or when the animal is fixated upon a goal and it is necessary for 'executive' control to take over, I would expect that the animal would exhibit behaviour more consistent with a bi-lateral symmetry.

Chris