Viral DNA in the human genome linked to major psychiatric disorders

There are perfectly plausible reasons why the conditions we regard as mental illness are adaptations that help evolutionary fitness:

- Depression puts one into a low-energy, contemplative state that enables reflection, healing and formulation of reformed ways of thinking and being ("dark night of the soul");

- Bipiolar is a pattern of swinging between high-energy bursts of inspiration and creation, and low-energy states of recuperation and reflection;

- Schizophrenia is a way of disassociating from real-world experiences that are too painful to experience with normal consciousness, and is a preferable alternative (from an evolutionary perspective) to suicide, buying time for processing and healing to take place, given the right kind of support.

Of course, that could be seen as a "just so" story too. Except that evolutionary theory says that only genes that promote evolutionary fitness should survive and spread through the genome, particularly given that replication of any given gene carries a significant cost. We can also easily observe that conditions like depression and schizophrenia normally develop in response to a trigger - i.e., a traumatic life event or extended period of abuse.

So, it's far less of a confected story to just accept that these conditions have been retained in the genome for the reason that makes most sense according to evolutionary theory: it's evolutionarily beneficial for them to be there.