Depression has existed since the dawn of life and is observed in all different types of animals as behavioural patterns.
Mental illness is a complex area - however there are quite amazing advances in research, finally, which suggest bodily inflammation could be the cause of between a quarter to a half of depression cases seen in the western world.
Much like Crohn's disease is the autoimmune system seemingly mistakenly targeting the terminal ileum (in most cases) then it is theoretically plausible and, now, provable that the body's autoimmune system targets certain other specific materials within the body. This could be collagen, tissue or evidently: specific DNA that also is contained within parts of the brain.
This chimes with friends of mine who have other health issues in which depression is a secondary symptom, whereas in the past elevated inflammation levels, such as C Reactive Protein (CRP), was considered to be a symptom of depression. Rather than being caused neurologically it is instead clear that some autoimmune fault is targeting particular areas of the brain and therefore people genuinely appear depressed due to the fact they are severely unwell but current medical provisions don't cater for this and mistreat them with SSRIs in an attempt to resolve a symptom rather than resolve the cause of the problem.
Prior to being diagnosed celiac my ex-partner really did appear very unwell and depressed for two years before further tests were attempted. Hopefully we're moving from an age of trial and error to systematic analysis of the symptoms presented to a GP in order to ensure people who are very unwell stop attempting and succeeding in suicide.
The programme is very interesting and well worth listening to:
BBC Radio 4 - The Inflamed Mind
Edited to add:
In my opinion a major breakthrough would also be to ensure that when a patient visits a GP the GP is able to perform a test that could quickly indicate the various raised inflammatory markers and therefore provide alternative treatment for the depression. Too many people are visiting GPs with health anxiety and receiving anti-depressants. My understanding at this time if the reason less analysis is performed for depression is purely because the cost of providing an individual with a full body health check is prohibitively expensive when you extrapolate the cost to the entire country.