Development of the Plant World on Earth

Life first appeared in water: about 3.5 billion years ago single-celled organisms arose; some of them acquired chlorophyll and resembled single-celled algae, and then multicellular algae arose. For a long time algae dominated the seas. As seas became shallow and the climate changed, some algae adapted to damp shores — these were the first land plants: psilophytes, Rhynia and Cooksonia. They had no roots or leaves, only a branched stem with sporangia at the tips. The textbook dates the move onto land to 570–510 million years ago; current fossil evidence places the appearance of land plants at about 470–420 million years ago (Ordovician–Silurian). Then mosses, club mosses, horsetails and ferns developed; in the Carboniferous tree horsetails and tree ferns 25–30 m tall formed forests whose remains turned into coal. Gymnosperms arose from seed ferns (the textbook says 200 million years ago; current estimates are much earlier, about 300 million years ago). Angiosperms appeared about 140 million years ago and from the Cretaceous spread very fast to dominate the Earth. Much earlier, a drying climate had led to the decline of tree-fern and horsetail forests and the spread of gymnosperms.