What is the anatomical reason for the left side of the brain controlling motor function in right side of body?

The best explanation is that our nervous system evolved to deal with the visual image on the retina. Vertebrates and cephalopods (squids, octopi) have inverting lenses in their eyes. This means that the right side of the visual field stimulates the left side of the retina, and the left side of the visual field stimulates the right side of the retina.





The nervous system needs to have the right side of the vision to correspond to the right side of the body. Therefore, the image from the left part of the retina must be directed to the part of the brain that controls the limbs on the right side of the body. This involves crossing brain circuits so the visual information is sent to the correct hemisphere. Both vertebrates and cephalopods have crossing circuits although their nervous systems are very different. This is an example of convergent evolution.What is the anatomical reason for the left side of the brain controlling motor function in right side of body?
The neuronal tracts actually cross sides in the brain somewhere around the area of the midbrain.