Our first plant like life shows up about 3,500 million years ago as cyanobacteria. The blue-green bacteria floats in the vast oceans covering the plant and begins to convert sunlight into food. As it does this it releases oxygen into the atmosphere. Much of the life that followed would not have come to be without that oxygen.
The cyanobacteria then forms colonies called stromatolites. These colonies form because the bacteria is sticky and as it gets bounced around the ocean surface it meets and sticks to other cyanobacteria. Each cell tries to be the one on top closest to the sunlight so the stromatolites are mushroomed shaped. These can still be found in warm, shallow ocean areas today.
1,500 million years ago cells begin to capture other cells. In time the captured cells differentiate into nucleus, organelles and chloroplasts. The chloroplasts are what convert sunlight into energy for these cells. All the green parts of your plant have cells containing chloroplasts.
There is but one continent now and most of the planet is ocean.
Critters evolve and grow, but plant life is limited to cyanobacteria until about 450 million years ago in the Ordovician period when liverwort makes its appearance.
Liverwort is a low growing plant with dark green ribbon like leaves. It floats to the edges of the oceans and tentatively reaches onto land. It doesn’t completely leave the water, it has no way to carry water to various parts of the plant so it remains on the ocean’s edges.
Mosses begin to appear right behind the liverwort. They too need to remain in the water as they have no vascular system. But both liverwort and mosses photosynthesize their food. The era starts out warm but descends into an ice age.
Around 430 million years ago in the Silurian period the ice melts, the world warms and the continents continue to break up and drift apart. Plants are now finding ways to support themselves upright outside of the water. They still live at the edges of the water but they begin to develop tubes to carry water from their bases to their tops. Very slowly the plants begin to venture onto land. Cooksonia is about 4″ tall, has no leaves and no roots but we believe it to be the first plant to venture out of the ocean onto land. The cooksonia would not survive the next era.
Club moss appears, it is about 10″ and it and other plants of this time reproduce by spores. Leaves are no more than scales, much like on tillandsia today. The few plants on earth are clustered at the edges of the ocean. Lichen is here and it is converting rocks to soil.
415 million years or so back in the Devonian period our first leafed plants appear. While they are true leaves, the veins are a random collection with no real form to them. There is a dramatic drop in CO2 in the atmosphere which we believe is due to the plant life in this time. Horsetail ( rush ) is common. Ferns appear and some of them bear seeds rather than spores. Plants get larger, stems get stronger and forests begin to emerge. Trees are often around 60′. Our first insects show up now. Most import real soil is appearing across the planet. It also draws down CO2. As the soil spreads so do our forests. But alas, global cooling comes and kills off most of our plant life.
Next comes the Carboniferous period around 350 million years back, global warming takes over and while the poles are still covered in ice, most of the planet is warm. Continents are sliding together into one again. Huge tropical forests cover the land. They grew so fast they used up huge amounts of CO2 and bumped the oxygen up to 35%. ( it’s only 21% today ) Club mosses form trees 160′ tall. The world is warm, humid and shady. Insects are huge too. Dragon flies have 26″ wingspans and some centipedes are 6′ long. For the first time the planet is no longer silent.
Spores begin to differentiate into different sexes. Seeds are retained in the plants for germination. Seed ferns vanish during this time. And again the planet begins to cool.
The Permian period starts about 290 million years ago. We have one continent, seas are shallow, swamps are giving way to desert and our first plant eating critters appear. As things cool diversity declines. Only plants that can handle drier, cooler climates survive.
Triassic period begins with little life. The planet is harsh, cold and most everything has gone extinct. But we still have some ferns, small trees remain, cycads, ginkoes, tree ferns, horse tails are all on the planet at this time. Seed bearing plants are doing better then the spore bearers, flowering plants appear. This is the time of the dinosaurs.
The planet warms, life expands and diversifies including insects and small mammals. The Jursassic period is upon us. More warming occurs, the polar caps melt and our big continent begins to break up. Tropical plants do well, as do temperate forests. Bald cypress make their appearance. Dinosaurs give way to mammals and the planet again cools. Ice caps form again and much of the life on the planet dies off.
The Creaceous period has continents sliding apart and climates begin to localize. Magnolia trees and other early flowering plants spread forth. In a very short time 90% of plants will be flowering. Leaves change from haphazard veins to a more regular vein pattern, a huge break through for plants. Plants and insects begin their symbiotic relationship in this time.
But again the planet cools, wiping out most life. A large meteor hits Mexico further stressing the planets life. Then world is again warming. Palms, laurels, vines and citrus plants are now with us and doing well. But the warmth is temporary as we slide yet again into a cold spell. Plains and prairies appear and with them grasses make their stage entrance.
At about 3 million years back human type creatures appear on the planet. Cooling continues followed by more global warming. 250,000 years back neaderthals show up, followed by homosapiens 180,000 years ago. Herbaceous flowering plants appear about 30,000 years ago. They are better adapted to handle the changing seasons. The last ice age retreats about 10,000 years ago. The first farms show up not long after at about 6500 years ago.
I did a much more indepth look at plant evolution on Herself’s Houston Garden in Sept. of 2008 if you’d like more indepth explaination