The denial over the role of gut microbiome in producing and modulating neurotransmitters seems like one of the biggest misses in modern science. People are in sheer and utter
denial over the idea that some bacteria in the gut might actually affect what's going on in the brain, but now there are potential links to parkinsons and several other neurological disorders.
It's entirely possible gut microbiome is a causative factor (even partial) in the obesity epidemic too. Everyone is obsessed with the CICO idea, but humans are not a blast calorimeter and the actual absorption, satiation, or metabolic paths all have obvious links to the gut microbiome (much more intuitive than the links to neurotransmitters). Maybe we are fat because of some additive or ingredient that has changed our microbiomes, or the relative availability of different types food/energy to that microbiome has changed the microbiome.
You are your microbiome, at a fine enough level you are just cells doing a fixed role, and the other fauna do theirs too. There is no magic "sentience cell" or magic spark, it's just the synergistic effect of all these systems powering the most advanced biological machine known to exist, each doing their own individual thing.
But people find this idea of biologicalism inherently troubling in many respects and tend to downplay it because it leads to uncomfortable conclusions about the nature of consciousness and sentience. Maybe people are actually just reacting to stimuli in accordance with their past training - so that nobel prize winning scientist could have ended up in jail for selling crack if they had been placed in different circumstances. Everyone knows this intellectually but I think it is going to become uncomfortable the degree to which our actions are biological and not conscious etc, and what that means about how we've handled the least of us for the past centuries.
This isn't to throw away the "nature vs nurture" debate entirely but it very clearly is passed through a biological lens on both sides - the nature and nurture are just training set and fine-tuning for a net, there's no inherent "ghost in the machine" but just a series of chemical reactions according to some trained weights and biological machinery. Change the machinery, and you change the person (Phineas Gage, lobotomies, psychiatric drugs, etc). And people really don't like that idea, that what they think of as "consciousness" might just be a low-dimensional projection of the unconscious processing, that the brain has developed as this adjunct facility for language and conceptual processing. Surely the consciousness must be the feature, not the projection, right?
People dug in their heels against this with antidepressants and other things, and the microbiome is an equally uncomfortable topic because of what it says about how our hardware works. It's not even just some chemicals floating around inside our brain that tell it how to work... it involves more or less every part of the body (or at least the metabolic pathway).