It has been suggested that the 30K MPs evolved via duplication or horizontal acquisition of the CP gene in a virus that infected an ancestor of vascular plants, followed by exaptation of one of the paralogous CPs.
Some scientists propose that the 30K MPs evolved via the duplication and subsequent integration of the CP gene from a viral infection in a common ancestor of vascular plants. One of these duplicated genes was then repurposed for its current function.