Plants don't appear to be of a different origin than animals on this planet. They share most of the genetic code* with all other life we know about. The simplest explanation is that we share a common origin, and furthermore that was a common ancestor that likely began from simpler materials on this planet.
*The genetic code is the translation of nucleotide triplets into amino acid sequences
Did only a few of us miss this? Seems like it could have been explained better up front.