I have no idea about decussation but I am surprised by how the question was posed. There is no purpose nor design in evolution. Natural selection does not guide evolution towards a purpose. Natural selection works on pre-existent diversity so in order to have "absence of decussation" this option must be generated my mutation (if we consider mutation stochastic, nature DOES throw dice) and, in order to be generated it need to be physically possible. If there are constraints as I think the paper from Filippo's comment indicates it might be impossible to evolve "absence of decussation" (I have read only the abstract, as I said, it is not decussation what moves me to write this comment).

In the transition from unicellular to multicellular organisms , allocation of motor/sensory components favored polarity Example :the location of the flagellum for unidirectional movement . for survival and consequent selection. Survival also depends upon sensing light,food,toxins, coming from all directions*, and eliciting balanced directional tropic motor responses . If input was not balanced, and computationally permissive in the earliest transitional organisms,there would not be tropism(survival) .

* the reason for decussation

This is an interesting question, and has come up before. When I studied the medicinal leech, I learned that many of the motor neurons do cross but the sensory neurons don't. In vertebrates, the cerebral hemispheres are concerned mainly with the opposite side, but the cerebellum isn't. Logically, you could move the neuron's soma to either side, but the crossing of the fibers may give more opportunity to interact with others. And doesn't nature throw dice? As long as it works, the arrangement can be kept.

I have no idea about decussation but I am surprised by how the question was posed. There is no purpose nor design in evolution. Natural selection does not guide evolution towards a purpose. Natural selection works on pre-existent diversity so in order to have "absence of decussation" this option must be generated my mutation (if we consider mutation stochastic, nature DOES throw dice) and, in order to be generated it need to be physically possible. If there are constraints as I think the paper from Filippo's comment indicates it might be impossible to evolve "absence of decussation" (I have read only the abstract, as I said, it is not decussation what moves me to write this comment).

This question has arisen many times during medical school teaching ~ so far I have not been able to come up with a rational explanation

The zoologist J.Z. Young lectured to us on this topic when I was a student. His theory, if I remember him correctly, was that the sensory and motor decussation t was consequent upon the vertebrate eye having a convex lens, that crossed over the information from the world on the right to the left retina, etc. The left hemisphere co-ordinated events in the right visual field with the motor control of the limbs, and the motor fibres needed to cross somewhere in the pathway from the left hemisphere to the right arm and leg....

I am not sure I was convinced by this. What is the evidence in lower animals without a convex (reversing) lens in the eye? I am no zoologist.

Really interesting topic however is it not that the evolutionary base is opt for survival and now that we now that we have the neuroplasticity too deal with too that the most important nuclei(locus of control concerning breathing/movement etc) is geographically klocated close by ??

Lynn bindmann's statement is true; I read the same.

@Yvonne Freund-Levi: the reason why the nuclei for vital functions are close to each other is, according to hierarchial theory, due to the fact that they evolved in the early evolution and are therefore close to each other.

But I still see no good explanation for why the right side of the brain controls the opposite side of the body. I thought that this was the main point of the question.

@hossein haghbin ok i buy this but there must be a reason for it as eg after stroke the contraletarl part of the brain can ¨take over¨maladapted functions or ?

Although vast microevolution is readily observed of course within a particular species, there are no examples of observed macroevolution, say from one genus to another. So I cannot answer the question in the context posed. The simpler question, what function does the criss-cross arrangement have, has many possible theoretic answers and any one is about as valid as another.

To Tom, Nature does throw dice when it comes to evolution. Mutations arise largely randomly and then are selected or not. Miguel Navascues is entirely right in what he writes

It takes the Crimson Tide to bring us & the tide back to the question. Thank You, Robin. Half of what strikes the right retina goes to the left optic cortex. I get it...but why? and why the need for the left motor area (precentral gyrus) to control the right half of the body? Nature/Evolution could have done it any way it wanted...but "things are this way because they got this way."

How come, y'all?

Wasn't the transition from one cell to 2 a mutation? Also, a simpler way to answer my original question is "What was so inferior with no decussation that led Nature to devise the cross-over? What got better?"

I think that it is important to realize that the decussation was an early innovation, not a late one. It probably predated the convex lenses in question, and may have been as simple as a bilateral worm needing to swing it's head to the right to move to the left. Thus, making vision out of its left eye important for muscle tension on the right side. Later innovations would require the crossing of the optical nerve to compensate for the convex lenses, etc.

Sir Smith------I think we're closing in on it. This would carry over to higher forms....snakes and lizards, etc. And the worms came out of the water about 70 MYA so they've had quite a long time to perfect the technique....and pass it on. Thank You!

I am not sure that we shouldn't look back even before they came out of the water, since many sea going forms such as eels, and fish have similar requirements for turning in order to move through the water.

partial decussation is easy to understand -- each retina shares with both halves of the brain. Functions of the complete decussion of neural tracts is more mysterious. Here is a recent contribution that offers an answer : http://onlinelibrary.wiley.com/doi/10.1002/ar.20731/pdf

Daniel----you've uncovered a real nugget there and singlehandedly have moved the discusson forward....or sideways, as noted in the article. Thank You! Tom

Dear Tom, it is very nice to see such a question and actually be able to provide a reasonable answer!

I developed a theory which was published last year in Animal Biology (download also available on my research gate account, see link, and on arXiv). The answer is as follows: in the early development of vertebrates, shortly after gastrulation, the anterior forehead region twists anti-clockwise about the body axis (from the perspective of the animal), whereas more caudal body part twist clockwise. As a result the fore head region inclusive forebrain, eyes and olfactory organs, is crossed laterally and inverted dorsoventrally. The heart and bowels do not take part in these axial rotations and thus obtain their asymmetric location inside the body. The evolution origin is unclear, but compatible with, e.g., the dorso-ventral inversion hypothesis. The common ancestor of vertebrates must have turned on its side, and the external body parts regained symmetry over the evolutionary course. These compensations have been retained in the early development of all vertebrates. For more evidence: see the manuscript.

In fact the question after the extensive decussations, the optic chiasm (as well as the chiasm of the trochlear nerve) is an old one and as far as I know the first to have looked into seriously is Ramon y Cajal. I was not convinced by his ideas, and they were indeed already criticized for the logical inconsistencies in the foreword by Paul Flechsig to the first german publication in 1899.

A very different kind of explanation was offered by Shinbrot and Young, as cited above, by Daniel Blackburn. Their theory does not explain the optic and trochlear chiasms, and neither why the cerebellum is not crossed.

Article An ancestral axial twist explains the contralateral forebrai...

This question has piqued my curiosity for years. Although there may not be a definitive answer, here is an interesting possibility: "Decussation evolved as a byproduct of a genetically determined partial inversion of the body plan, which resulted in a 180 degree rotation posterior to the brain and oropharynx." from "Somatic twist: a model for the evolution of decussation" by M. Kinsbourne in the journal, Neuropsychology. The model reminds me of the explanation for dorsoventral axis inversion in deuterostomes vs. protostomes (http://en.wikipedia.org/wiki/File:DV-axis-inversion.png). Cool stuff, indeed!

Hi Christopher, Michael Kinsbourne is one of the many who have been puzzled by this problem over the centuries. Nice that he wrote his ideas up in the end. Unfortunately I do not have access to the pdf. Judging from the abstract I think that my model is more specific. Also, that it is more consistent with the complex twisting patterns of other deuterostomes. I have been trying to motivate some of the developmental specialists to test the model and the connection with the d-v axis inversion, but I am not aware that anyone has taken the initiative for this.

Hello Marc and Christopher-----Thank you both for carrying this discussion further. As far as there being no "crossing fibers" in the cerebellum: (1) perhaps the need hadn't yet arisen for the crossing; (2)since the fibers that do cross have their beginnings, for most, in the cerebral area and since that is much older than the cerebellum evolutionarily speaking, perhaps the need for them to cross came much later?

Hi Tom,

Thanks for your thoughts. (1) Though not a neuroanatomist myself, I needs to stress that are almost always bilaterally. Thus, even if connections between the telencephalon and other regions strongly tend to cross whereas other log-range connections are more frequently ipsilateral, there is almost always a proportion of connections that do otherwise. One reviewer of my paper even went so far as to maintain that there is no general pattern at all! (I definitely do not agree with him!)

(2) All connections to and from the telencephalon tend to cross. There is no difference whether afferent or effent connections, whether cortico-cerebellar, cortico-medullar, cortico-spinal, or whatever you like.

(3) The cerebrum is not "older" than the cerebellum. There is a remarkably persistent 19th-century fairy tale that the mammalian neopallium is more advanced (and "newer") than in any other vertebrate. However, this really is nonsense. Birds do have a neopallium as well. All vertebrates, including sharks, lampreys and myxines have well-developed and highly organized cerebellum and forebrain (if you have access to a university library, see for example Nieuwenhuys et al. 1998, The central nervous system of vertebrates). From the fossils of many of the earliest jawless vertebrates it can be recognized that they had a well-developed cerebellum and forebrain (see, for example Janvier, 1996, Early vertebrates.). All vertebrates also have a majority of crossing afferent and efferent forebrain connections.

Also there is an interesting situation in sharks, which you can find in my paper, which is also freely available on arXiv.

http://arxiv.org/abs/1003.1872

I just got this question and googled and found this interesting discussion.  There are three doubts:

1.  Whether decussation is a way of compensating our limitations of brain processing in the evolution?

2.  Being bipedal and access to all 360 degrees?

3.  The sense of Pain and its importance for body integrity?

This very question seems of extreme interest, not just from an anatomical or physiological standpoint, but from a cognitive and behavioral ultimate resources to explain the evolutionary path that set the early stage for hand preference, hemispheric specialization, language development and neuropathology interpretation.

I am personally intrigued by such questions since 1980. Not found yet a congruent and univocal solution. Answers cannot be simple, nor uni-disciplinary.

I wish there is a moment when and where all these matter could be addressed inter-disciplinarily among those who have really worked in such fields of neuroevolutionary and neurocognitive and clinical research fields.

Dear all,

As anyone, I pondered this question for some time, but now the answer seems quite simple to me: if an organism is attacked from (e.g.) the right side of its body and suffers brain damage to this side, it still needs to be able to spot its attacker and perform immediate defensive or offensive action in the direction of its attacker with its right side (or perform evasive maneuvers). Hence, sensory function should be (largely) intact for the side at which the brain damage occurs and (perhaps more importantly), motor action should be preserved for this side. Despite its (partly) hemiplegic existence, an organism with decussation might just live long enough to reproduce after it has suffered such an attack.

Hope this adds some teleological perspective to the evolutionary debate,

With kind regards,

Rutger Goekoop.

I have no answer! No evolutionary, nor embriologicaly, nor physiologicaly!

have a look at wikipedia:

https://en.wikipedia.org/wiki/Contralateral_brain

https://en.wikipedia.org/wiki/Evolution_of_nervous_systems https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4339819/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4592696/ https://bmcbiol.biomedcentral.com/articles/10.1186/1741-7007-11-53 https://en.wikipedia.org/wiki/Evolution_of_the_opposed_cerebral_hemisphere_control http://www.cell.com/cell/fulltext/S0092-8674(15)00761-8

I suggest that the lens refracting property is the basic reason for the contralateral nervous system organization.

http://mkazarnovsky.com/WEB-ContraUniLateral.htm

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