Evolutionary History of Plants:
– Land plants evolved from freshwater green algae.
– Closest living relatives are charophytes.
– Early plants adapted to terrestrial stresses with fungal help.
– Weathering rates suggest early photosynthesisers on land.
– Evidence of earliest land plants in lower middle Ordovician rocks.
– Middle Ordovician marks emergence of embryophyte land plants.
– By late Devonian, plants had roots, leaves, and wood.
– Late Devonian saw evolution of seeds in plants.
– Permo-Triassic extinction event changed plant communities.
– Flowering plants emerged in the Triassic and diversified later.
Evolution of Plant Metabolism:
– Grasses evolved mechanisms to survive tropical conditions.
– Plants adapted to low CO2 levels and warm, dry climates.
– Grasses became important in the mid-Paleogene.
– Evolution of new metabolic mechanisms continued.
– Grasses evolved to thrive in changing environmental conditions.
Plant Reproductive Adaptations:
– Plants developed resistance to desiccation on land.
– Bryophytes and tracheophytes adapted to deal with desiccation.
– Tracheophytes developed stomata to regulate gas exchange.
– Vascular tissue aided in water movement and upright growth.
– Plants evolved larger sizes due to reproductive adaptations.
Impact of Photosynthetic Organisms:
– Snowball Earth event partially caused by early photosynthetic organisms.
– Glaciation event led to decreased carbon dioxide levels.
– Molecular clock studies indicate origin of multicellular streptophytes.
– Cool Cryogenian period influenced streptophyte evolution.
– Warm Ediacaran period marked separation of streptophytes.
Evolution of Life Cycles:
– All multicellular plants have a life cycle comprising two generations: gametophyte and sporophyte.
– Plant evolution shows a shift from homomorphy to heteromorphy.
– Bryophytes have the gametophyte as the dominant phase, while vascular plants have the sporophyte as dominant.
– The diploid phase allows masking of deleterious mutations through genetic complementation.
– Two theories explain the appearance of a diplobiontic lifecycle: interpolation theory and transformation theory.
The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats of unicellular archaeplastids evolved through endosymbiosis, through multicellular marine and freshwater green algae, to spore-bearing terrestrial bryophytes, lycopods and ferns, and eventually to the complex seed-bearing gymnosperms and angiosperms (flowering plants) of today. While many of the earliest groups continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones; for example, the ascendance of flowering plants over gymnosperms in terrestrial environments.
There is evidence that cyanobacteria and multicellular thalloid eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, multicellular photosynthesizing organisms existed on land in the late Precambrian, around 850 million years ago.
Evidence of the emergence of embryophyte land plants first occurs in the middle Ordovician (~470 million years ago), and by the middle of the Devonian (~390 million years ago), many of the features recognised in land plants today were present, including roots and leaves. By the late Devonian (~370 million years ago) some free-sporing plants such as Archaeopteris had secondary vascular tissue that produced wood and had formed forests of tall trees. Also by the late Devonian, Elkinsia, an early seed fern, had evolved seeds. Evolutionary innovation continued throughout the rest of the Phanerozoic eon and still continues today. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the appearance of the flowering plants in the Triassic (~200 million years ago), and their later diversification in the Cretaceous and Paleogene. The latest major group of plants to evolve were the grasses, which became important in the mid-Paleogene, from around 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years.