Fact is we really never saw them so what we have to rely on is the fossil record. For example, the illustration of this trilobite shows the fossil embedded in stone, we know areas that were once underwater are now on land. Scientists diligently dug up these facts for us. Plants were really the first pioneers of the land.
Scientists know that stromatelites mad a vital difference to the atmosphere of the planet.
They had to develop features that would allow them to live out of the water. First they needed to have a thick waxy covering to keep from losing water and protect themselves from ultraviolet light. Then they needed skin pores to allow for the exchange of gas that would let photosynthesis take place. Photosynthesis lets the pigments of the plant such as chlorophyl, trap sunlight for the plant to use in order to grow. Then they needed a strong skeletal support because the air could not hold them up like the water could.