Before you read this you should probably look around, to the left, and to the right. Every time you turn your head to one side or another, there is a part of your brain whose job it is to tell another part of your brain that your whole body is not spinning in circles, but only your head.
In other words, inside us is very old, hard-wired circuitry that still thinks our head is fixed to the rest of our bodies. Think about it – Necks are a pretty recent development. The first animals had no necks. Worms, fish – no necks. If their head was turning to the side, it meant their body was turning with it. Even eyes were developed after necks. Nowadays, when you, homo sapiens, turn your head to the side, one part of your brain cross-references with many other parts to check whether it is the entire head-body, or just the head: the eyes give data, the muscles of the neck especially, but also many other muscles of the body give information to help figure out what is actually happening.
You should be asking, why, after millennia of necks, have we not gotten rid of the automatic neural response to head movement which assumes the head and body are one? So much extra work to cancel out the false assumption… How is it that we still hold onto this little bit of circuitry, how is it that we haven’t gotten around to evolving our way out of this?
The answer is phylogenetics. The functioning of a brain is a palimpsest, or a city. Once the foundation is laid, it doesn’t change. You can work-around and retro-fit, but you cannot re-write. This creates all kinds of confusing and embarrassing problems where very old forms and functions end up mismatched with their current context. Smelling, certainly, is one of these things.
[Inspired by a Robert Sapolski lecture on human biology.]