The evolution of Plants

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Anupama Nair


The evolution of plants in itself resulted in a wide range of complexity i.e., from the earliest algal mats, through multicellular marine and freshwater green algae, like terrestrial bryophytes, lycopods and ferns, to the complex gymnosperms and angiosperms which in layman terms are flowering plants we see today. You can still find some of the earliest groups thriving, as exemplified by red and green algae in marine environments. The more recent 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 multi-cellular photosynthetic eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, multicellular photo-synthesizing organisms existed on land in the late Precambrian, Period that was around 850 million years ago.

Evidence of the emergence of embryophyte land plants first occurred in the mid-Ordovician Period which was nearly 470 million years ago, and by the middle of the Devonian Period nearly 390 million years ago, and has many of the features now found in land plants today including roots and leaves. By late Devonian period some plants such as Archaeopteris had secondary vascular tissue that produced wood and had formed forests of tall trees. Evolutionary innovation continued throughout the rest of the Phanerozoic eon and still continues today. Most plant groups were relatively unharmed by the ‘Permo-Triassic extinction event’. This may have set the scene for the appearance of the flowering plants in the Triassic nearly 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 nearly 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 places near the equator called tropics around 10 million years ago.

Land plants evolved from a group of green algae, perhaps as early as 850 million years ago, but algae-like plants might have evolved as early as 1 billion years ago. The closest living relatives of land plants are called the charophytes, specifically Charales and we can assume that the habit of the Charales has changed not much since the divergence of lineages. We can safely say that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools. However, some recent evidence suggests that land plants might have originated from unicellular terrestrial charophytes resembling the  extant Klebsormidiophyceae. The algae would have had a haplontic life cycle. It would only very briefly have had paired chromosomes  i.e., when the egg and sperm first fused to form a zygote that would have immediately separated by meiosis to produce cells with half the number of unpaired chromosomes

Plants were not the first living organisms that photosynthesizes on land. Weathering rates suggest that organisms capable of photosynthesis were already living on the land 1,200 million years ago, and microbial fossils have been found in freshwater lake deposits from 1,000 million years ago. What are fossils? “Fossils are the preserved remains, or traces of remains, of ancient organisms. Fossils are not the remains of the organism itself! They are rocks. A fossil can preserve an entire organism or just part of one. Bones, shells, feathers, and leaves can all become fossils”. 

However, the carbon isotope record suggested that they were too less to impact the atmospheric composition until around 850 million years ago. Evidence of the earliest land plants occurred much later at about 470 million years ago, in lower middle Ordovician rocks from Saudi Arabia, and Gondwana in the form of spores with decay-resistant walls. These spores, called as crypto spores, were produced either singly or in pairs or groups of four, and their microstructure resembles that of modern liverwort spores,

A ‘snowball earth’, from around 720-635 million years ago in the Cryogenian period, is believed to have been caused by early photosynthetic organisms, which reduced the concentration of carbon dioxide and increased the amount of oxygen in the atmosphere.

The evolutionary history of plants is recorded in fossils were preserved in lowland or marine sediments. Some fossils preserve the external form of plant parts, while others show cellular features and still many others consist of micro-fossils such as pollen and spores. In rare instances, fossils may even display the ultrastructural or chemical features of the plants they represent.

In fact, plants and trees today are a necessity as they purify the air.


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