Placebo

Why did the placebo effect evolve? What biophysiological function does it serve?

A key evolutionary selective aspect is the speed of response to stimuli. The quicker an organism reacts to seeing a lion, the higher its rate of survival and its genes perpetuate through that specific species. You can imagine this development much like an arms race, the mutating genes come up with new ways to quickly respond to stimuli. The initial messenger systems from abiogenesis were simple chemical messengers. i.e. there was stimuli, a chemical (such as an endogenous hormone) was released which caused some sort of response.

Over time chemical messaging was improved upon, a mechanism which could communicate at the speed of light was invented called the neuron. The neuron was innervated at one end and could send an electrical pulse to the other allowing the organism to respond in near real time. However, even real-time wasn't good enough. With the development of the neuron came higher thought, and the brain developed to become a predicting machine. The organism understood time in such a way it could extrapolate where a stimulus was going to interact with it even before it came near to it. This massive competitive advantage pushed species with larger brains to the top of the food chain and resulted in us.

Where placebo comes in is almost a hack of this predictive machine. To make a prediction and act on it the subconscious needs the brain needs to believe the stimulus is real. If the interpretation of this stimulus entered conscious thought it would then go through processing cycles which would end up wasting time, time in which you are being eaten by the lion. So the selective force kept this mechanism in the subconscious and tethered it to bypassing the ruminating brain.

So the placebo effect does not have a biophysiological function it is an offshoot of psychosomatism brought about by the brain's need for predictive behaviour.