The origin of plants were Green Algae. The origin of fish, and all resulting invertebrates and vertebrates, were tunicates. And before that Larvacae or Appendicularia. Recently, the most basal Animalia with the smallest genome was (re)discovered. It is called Trichoplax and looks like an amoeba or a slime mold. It contains a few thousand cells in four layers with an absorption digestive system and reproduces asexually, although it is in an evolutionary transition state as putative eggs have been observed. Before this, Choanoflagellates are considered to be the closest living relatives to animals: protozoa with one flagellum. The original ancestor to all plants and animals had to be both the ancestral species that gave rise to green algae and the same ancestral species that gave rise to Choanoflagellates. This would have to be the ancestral Archaeplastida (just plastids surrounded by two membranes that evolved directly from a endosymbiotic Cyanobacteria). Check my math.
