Question

What is the first plant on earth?

Answer 1

Plant development has created a tremendous diversity of complexity, from the first algal mats to multicellular marine and freshwater green algae, terrestrial bryophytes, lycopods, and ferns, to today’s complex gymnosperms and angiosperms. While many of the earlier groups continue to survive, more recently derived species have displaced previously ecologically prominent ones.

Complete answer:
Plants are multicellular, photosynthetic eukaryotes that belong to the Plantae kingdom. Plants were previously divided into two kingdoms, one containing all non-animal type living organisms and the other containing algae and fungi.
During the Ordovician epoch, when life was rapidly evolving, the first terrestrial plants appeared some $470$ million years ago. Non-vascular plants with shallow roots included mosses and liverworts.
The first terrestrial plants are found in lower middle Ordovician rocks from Saudi Arabia and Gondwana around $470$ million years ago, in the form of spores with decay-resistant walls. Cryptospores were made one at a time (monads), in pairs (dyads), or in fours (tetrads), and their microstructure resembled that of modern liverwort spores, hinting that they have a similar level of organization. They are embryophytes because their walls contain sporopollenin. It’s possible that air ‘poisoning’ prevented eukaryotes from colonizing the land before then, or that the necessary complexity took a long time to evolve.
The thalloid creatures that lived in fluvial marshes and covered much of an early Silurian floodplain were the earliest land plant megafossils. They could only survive if the land was flooded. There were also microbial mats.

Note :
Cyanobacteria and multicellular photosynthetic eukaryotes were found in freshwater communities on land as early as $1$billion years ago, while colonies of complex, multicellular photosynthesizing organisms flourished on land around $850$ million years ago. The first evidence of embryophyte land plants appears in the mid-Ordovician, and by the middle of the Devonian, many of the current traits of land plants, such as roots and leaves, were present.