Why do tracts cross in the spinal cord

Most sensory and motor tracts in the spinal cord cross to the contralateral side (either in the spinal cord or brain stem).

What is the evolutionary advantage of this? Or is there any other logical reason

Not just nerves in the spinal cord either. The optic nerves cross over, for example.

I have heard the case made that there must be some advantage to long tracts because so much of the brain is arranged sub-optimally – most things are contralateral, vision is processed at the back, the homunculus is upside-down etc.

But even this observation is disputable, let alone there being good hypotheses for why it is the case.

What do you mean upside down? and why would it be “sub-optimally”?

The feet are at the top, and then the body is represented down the lateral surface, with the head at the bottom.

I acknowledge that calling it upside down is a simplification however. As I’ve said, the observation that brain areas tend to be placed as far away as possible from the corresponding muscle / sensory organ is disputed.

Shorter nerves would give better reaction times, less vision lag etc.

Of course there are likely many considerations of what is an optimal brain layout. I’m just talking optimal in terms of this one consideration.

In my experience, the organization of the nervous system has less to do with evolutionary advantage than it does with the way human embryos develop.

If you spend some time researching embryology, you see that all structures in the human body are related to each other in ways that you wouldn’t normally expect by gross examination of an adult body. For instance, the relationship between branchial arches, aortic arches, cranial nerves, and musculature is highly conserved, and as such disorders of primitive embryological structures tend to strike at multiple sites that we wouldn’t normally expect.

But yeah, it all makes sense when you look at the embryology.

It may be a byproduct of the sum of effects which did provide an advantage, very early on in the story.

Say, suppose a freak adaptation that turned the head around 180 degrees provided an edge against predation (or some other benefit) and was passed on. Miscegenations upon miscegenations later, another adaptation which turned the head around 180 degrees provided an advantaging in foraging (or some other benefit) and was passed on - except instead of “untwisting” the original twist, it went in the same direction.

No direct benefit in having the spinal cord do a 360, but an incidental consequence earlier adaptations.

This is at the limits of my understanding of biology, but I believe vertebrates (including humans) are “upside-down” compared to non-vertebrates. That is, the front and back of the basic body plan of vertebrates match the back and front of non-vertebrates, respectively. This flip can explain a lot of the crossing in the internal plumbing.

Embryology is part of evolution as much as anything

Does not count for the fact that the tracts have a tendency to cross to the other side in rather well defined tracts, there is a clear pattern, for example the spino-thalamic tract (giving sensory to the cortex of pain and temperature) or the Dorsal column which has a “decussation” in the midbrain". This is found in most vertebrates. Why is this crossing all about? (not a biologist)

There is crossing over in the optic chiasm, but the eyes themselves do no cross over. It is instead the visual fields that project contralaterally. So your right visual cortex gets input from the left visual field, which itself consists of both left and right eye input.

The only(?) function that projects ipsilaterally is smell. That is a more primitive function, although that may just be a coincidence.

This is a ridiculous notion, cerebellum is ipsilateral, sympathetic innervation is ipsi.
Whatever. The question was Why do many of the tracts CROSS, namely Cortico-spinal, Dorsal column and spinothalamic, There has to be some evolutionary benefit.

conclusion: your left side of the brain gets a smack - the right side of the body is affected, because your nerves cross on the way down, why?

Shinbrot’s and Young’s “Why Decussate? Topological Constraints on 3D Wiring” (Anat. Rec. 291: 1277-1292, 2008) will probably interest you, scamartistry.

Full article (PDF) is available here,

That is an awesome paper.