The Evolution of Early Plants

In modern terms, "plant" refers to organisms classified under the kingdom Plantae. However, other photosynthetic organisms, such as protists, green algae, and cyanobacteria, also played crucial roles in the evolutionary history of plants. Although this discussion focuses on the evolution of the Plantae kingdom, these other organisms offer valuable insights into the development of all photosynthetic life forms. All of these, including plants, green algae, and protists, are primary photosynthetic eukaryotes.

The search for fossil evidence of early plants begins with indirect indicators of photosynthesis in the geological record. The earliest evidence of photosynthesis dates back around 3 billion years, found in rocks and fossils of cyanobacteria, which are photosynthesizing prokaryotes. Cyanobacteria use water as a reducing agent, producing atmospheric oxygen as a by-product. This process drastically altered Earth's early reducing atmosphere into one that supported aerobic life forms. The oxygen produced by cyanobacteria reacted with dissolved iron in the oceans, forming sedimentary layers known as Banded Iron Formations (BIFs). These formations help pinpoint the origin of photosynthesis and provide constraints on when enough oxygen was available to form the ozone layer, which shields the planet from ultraviolet radiation. As oxygen levels rose, they created a highly oxidizing atmosphere, which pushed anaerobic organisms to extinction and opened up ecological niches for aerobic life.

Further evidence of cyanobacteria includes stromatolites, layered structures believed to be formed by microorganisms such as cyanobacteria trapping and binding sedimentary grains. While the direct evidence of cyanobacteria is less certain, their role as primary producers of atmospheric oxygen is well-established. Modern stromatolites, containing cyanobacteria, are found on the west coast of Australia. The chloroplasts in eukaryotic plants evolved from an endosymbiotic relationship between cyanobacteria and other prokaryotic organisms, eventually leading to photosynthetic eukaryotes in marine and freshwater environments. These early photosynthetic single-celled organisms gave rise to groups like Charophyta, freshwater green algae.

Major eras in Earth's evolution

Cambrian 

In the Cambrian Period, early plants were small, unicellular, or filamentous, with soft body tissues and simple branching structures. Identifying plant tissues in Cambrian strata is challenging due to their small size and the similarity of various groups with simple branching patterns. However, calcareous green algae from the genus Dasycladales have been found in Cambrian fossils. These algae are not ancestors of land plants but represent early green algae. By this time, no land plants with vascular tissues existed, although molecular evidence suggests that land plants might have originated earlier, around 480–440 million years ago, and fungi on land around 1 billion years ago. The fossil record, however, does not yet fully support these molecular clock estimates.

Ordovician 

The Ordovician Period marks a significant change in plant evolution with the appearance of spores in the fossil record. The first terrestrial plants, resembling liverworts, emerged in the Middle Ordovician, indicating the beginning of terrestrialization. These early plants lacked conducting tissues, limiting their size and tying them to moist environments. They reproduced with spores, which protected their future offspring and allowed for preservation in the fossil record. The ability to disperse spores enabled these plants to colonize new areas when moisture conditions were suitable.

Silurian 

The Silurian Period saw the first fossils of vascular plants, which have specialized conducting tissues. Early vascular plants, such as those in the genus Cooksonia, exhibited simple branching patterns with flattened sporangia. By the end of the Silurian, more complex vascular plants, including zosterophylls and primitive lycopods like Baragwanathia, had diversified.

Devonian 

During the Devonian Period, land colonization by life forms advanced significantly. Primitive plants, along with bacterial and algal mats, created the first recognizable soils and supported early arthropods. Early Devonian plants lacked true roots or leaves and had minimal vascular tissue. By the Late Devonian, a diverse array of plant types, including lycophytes, sphenophytes, ferns, and progymnosperms, formed the first forests. Notable examples include the tree-like Archaeopteris and giant Cladoxylopsid trees. The "Devonian Explosion" refers to the rapid diversification of plant groups and growth forms. This period also saw the co-evolution of plants and arthropods, leading to modern insect-plant interactions. The proliferation of plants likely impacted carbon dioxide levels and climate, contributing to a major extinction event.

Carboniferous 

Early Carboniferous plants were similar to those of the Late Devonian but saw the emergence of new groups. Dominant plants included Equisetales (horse-tails), Sphenophyllales, Lycopodiales (club mosses), Lepidodendrales (scale trees), Filicales (ferns), Medullosales, and Cordaitales. Late Carboniferous saw the rise of cycads, Callistophytales, and Voltziales. Carboniferous lycophytes, such as Lepidodendron and Sigillaria, were massive trees with trunks up to 30 meters tall. Fossil ferns, like Pecopteris and Sphenopteris, were similar to modern species, with some probably being epiphytic. The period also featured the giant form Calamites and climbing plants like Sphenophyllum. True coniferous trees appeared later in the Carboniferous, preferring drier conditions.

Permian 

The Permian Period began with a continuation of Carboniferous flora, but saw a major shift in vegetation around its middle. Swamp-adapted lycopod trees were replaced by conifers, which were better suited to changing climatic conditions. Lycopods and swamp forests persisted in the isolated South China continent, which had high oxygen levels. During the Permian, conifers, Ginkgos, and cycads diversified, with some gigantopterids possibly being early ancestors of flowering plants, though true flowers evolved later.

Triassic 

In the Triassic Period, plant communities included lycophytes, cycads, ginkgophytes, and glossopterids. Seed plants, or spermatophytes, became dominant, with conifers flourishing in the northern hemisphere and Dicroidium being a prominent southern hemisphere tree.

Jurassic

The Jurassic Period saw a transition from the arid conditions of the Triassic to a warmer, humid climate with lush jungles. Conifers continued to dominate, with families like Araucariaceae and Pinaceae thriving. Other notable groups included the Mesozoic conifer family Cheirolepidiaceae and shrubby Bennettitales. Cycads, Ginkgos, and tree ferns were common, with smaller ferns likely dominating the undergrowth. The Cretaceous Period saw the rise of flowering plants (angiosperms), which co-evolved with insects. Modern trees like figs and magnolias first appeared, while earlier Mesozoic gymnosperms, such as Bennettitales, became extinct.

Cenozoic

The Cenozoic era is marked by the rise of savannahs and the evolution of grasses around 35 million years ago. Human agriculture began about 10,000 years ago in the Fertile Crescent, with the cultivation of Neolithic crops leading to increased human populations. Around 8,000 years ago, common bread wheat (Triticum aestivum) emerged in southwest Asia through hybridization, and two rice species, Oryza sativa and Oryza glaberrima, were domesticated around 6,500 years ago.