Does mankind evolve or devolve spiritually?
According to Mahayana Buddhism there are Three Ages of Buddhism:

The Former Day of the Law, also known as the Age of the Right Dharma, which lasts between 500-1000 years during which the Buddha's disciples are able to uphold the Buddha's teachings.
The Middle Day of the Law, also known as the Age of Semblance Dharma, (second 1,000 years) during which the teaching only resembles the right Dharma.
The Latter Day of the Law, during which the Dharma declines.

So, according to this, there is devolution in the sense that the Dharma declines over time until it disappears and then the true Dharma will only be re-established by the next Buddha.