Why does the left side of the brain control the right side of the body, and vice versa? Why shouldn’t each side of the brain control its respective side of the body?


Seems a bit counterintuitive to me.

In: Biology

Answering “why” questions is always difficult in cases like this because evolution doesn’t have a “why” or “should”. Often there is an obvious benefit to a characteristic, so it’s easy to reason out “why”, for example, our stomachs are acidic, but in cases like this it’s far more difficult because there’s no obvious benefit to things being this way.

Which is an arsey way of saying “we don’t know”. There’s a characteristic of vertebrates (animals with an enclosed spinal cord) called ‘decussation’, which describes the way nerves “cross over” as they exit the brain. The result of this is that the left hemisphere of the brain controls the right side of the body and vice versa.

All vertebrates exhibit decussation, but the same is not true for invertebrates. So it’s assumed that, at some point in the early evolutional pathway of the spine, it twisted 180°. We don’t really know why this happened or why it was beneficial enough to be a universal characteristic of vertebrates, and while there are a few theories, they are very difficult to prove conclusively.

As others have said, we have no idea *why* this happens, but I’d like to add one thing.

While we often generalize by saying that one side of the brain controls and interprets the contralateral (opposite) side of the body, the truth is actually far more complicated. While some types of sensation do this this, there are others that stay ipsilateral (on the same side), such as proprioception and vibration sense. In vision, the majority of the neurons decussate, but the lateral most portions of the visual fields stay ipsilateral. Even approximately 10% of motor neurons coming from the brain will stay ipsilateral, even though most people generalize and say that motor fibers all deccusate.

Yeah, the brain is complicated.