Left Brain - Right Side

I asked this question to Cecil, and there may be a column about it, but I was told if one did appear, it would take a long time.

The question is simple: Why does the left side of the brain control the right side of the body (and visa-versa)? It seems rather strange to me, and my understanding it even includes hearing and eyesight. In fact, the eyes are even stranger. The left and right eye show images to both sides of the brain. But, the image in the left field of each eye is controlled by the right side of the brain (and visa-versa).

These leads to further questions:

[li]Do any invertebrates (at least the ones with brains and bilateral body setups) have the same crossover setup?[/li]
[li]Invertebrates tend to have their main nerve transmission line on their front while invertebrates have their spinal cord in their, well, spine. Why do we see this difference between the two types of animals?[/li]
[li] What would cause evolution to create such a setup? I know it might not be superior. (Evolution is about good enough and not perfection). But what would cause such a cross over in the first place?[/li][/ul]

I would be very grateful if anyone can point me to some scientific documentation.


The answer to your second question is probably: founder effect.

At some point, our ancestors and the ancestors of invertebrates diverged and developed a different plan for the nervous system, and everything that happened since is based on a modified version of that (nervous systems being quite critical to survival/proper function, there may never have been an opportunity to discard and do-over).

[quote=“qazwart, post:1, topic:520600”]

[li]Invertebrates tend to have their main nerve transmission line on their front while invertebrates have their spinal cord in their, well, spine. Why do we see this difference between the two types of animals?[/li][/QUOTE]

As I understand it, each line developed in its own direction away from their common ancestor. One line for whatever reason tended to crawl with the cord on top; the other with the cord on the bottom. I don’t recall if its ever been discovered which is the “right side up” compared to the common ancestor. As they evolved more and more features that were asymmetrical, the difference became more and more locked in; the difficulty of redesign Mangetout mentions.

The generally accepted theory was that it was a result of the way vertebrate muscular systems developed.

We were, for long time, essentially just pairs of muscle blocks of muscle suspended from a rod. Fish today retain this same basic pattern. The problem is that muscles can only contract, it can’t expand. So if the right hand muscle blocks react the animal turns right, if the left reacts it turns left.

The reason that’s a problem is because when our ancestral vertebrate sensed a predator with the right side of its head, if the signal passed to the right side of the body, it would actually move the animal towards the source of danger. To overcome that problem the nerves crossed over. As a result danger sensed on the right side of the head will cause the left side muscles to contract, moving the animal out of danger.

Remember all this evolved back when we were still filter feeders, so we didn’t need to respond rapidly to prey in that manner. It was response to predators that needed quick movements. As as Mangetout notes, once the ancestor adopted that system we all got stuck with it.

Other phyla have completely different body plans, mostly of them with movement based upon body appendages. So for them movement of one side of the body or the other was not an issue. We only developed such appendages very, very recently.

Not that I’m aware of.

Different evolutionary history.

Our ancestors were free-swimming filter feeders. The optimum solution for that lifestyle is to have a relatively rigid upper surface for shape, and to then suspend everything else as a floppy bag underneath that. That gives you flexibility combined with rigidity with minimal materials cost. This is the same basic design that you see in fishing nets and jellyfish, which are also ‘filter feeder’ designs.

So our notochords were the rigid rod that our bodies were suspended under, akin to the float line of a fishing net. And our central nervous system just followed right along under that because that was the safe place to put it where it’s protected from damage and won’t get all twisted up in that floppy stuff.

In contrast most other taxa developed as burrowers or bottom crawlers. The most protected place for their CNS was the bottom, where it was supported by a rigid medium and wasn’t exposed to damage from predators.