By starting from an assumption that a psychiatric diagnosis has an objective existence independent of the observer he sets psychiatry in a world of its own. All other diagnoses are observer dependent.
All reality is observer dependent.
Migraine exists as an illness and may be related to changes in serotonin. It is irrelevant whether Neolithic people suffered with what we would call migraine. The concept of migraine is useful in clinical practice. Its existence is dependent on its utility. In today's society such a view is laughable.