Photosynthesis taking part in the eukaryotic cell marks the most recognized departure point in the evolution of plants versus animals, although the last common ancestor (LCA) was probably much before at about 1.6 billion years ago. In the process of photosynthesis, atmospheric carbon dioxide together with water and sunlight produces oxygen, and carbohydrates. Note this is in compliment to glycolysis outlined above so the plants provide the fuel (carbohydrates) and the fire (oxygen) for animal life.
However note that this sun-supercharged eukaryotic ancestor of plants is quite independent of the precursor photosynthesis-less co-existing eukaryotes that went on a different evolutionary track to become animals. In other words, plant life proliferated and as a result, the oxygen composition of the atmosphere increased to a level that encouraged the development of aerobic organisms such as animals.
Therefore there is a very basic evolutionary reason why animals are fundamentally linked to plants as their source of nutrients.
