In what ways have plants and animals evolved to support each other throughout history?
There are probably a few cases of this and exactly how animals and plants co-evolve probably differs between different scenarios. One 'famous' example I like is between ants and acacia plants. The acacia plant has evolved to produce food for a particular species of ant and in return the ants protect it from the grazing of herbivores.

Explained better here: http://bioblog.biotunes.org/bioblog/2007/10/02/cool-bug-9-acacia-ants/