Evolution of Plants Cellular life forms occurred on earth about 2000 million years ago. Some of these cells had the ability to produce oxygen through reactions similar to photosynthesis. Slowly, single-celled organisms became multicellular. Seaweeds and some plants probably existed around 320 million years ago.
Fossil studies have shown that these major subdivisons of vascular plants living today had been established as early as about 345 mya and are distinguishable from one another on the basis of position of spore producing structures called sporangia:-
for example in Psilopsida (a group of most primitive vascular plants), the sporangia lie at the tip of plants. And in Lycopsida the sporangia lie at the base of their leaves.