Evolutionary history of plants

Lore & Background

The evolution of plants began with unicellular archaeplastids that evolved through endosymbiosis, leading to algal mats and multicellular green algae. The closest living relatives of land plants are charophytes, specifically Charales, though some evidence points to a unicellular terrestrial charophyte ancestor similar to Klebsormidiophyceae. Early land plants likely had a haplontic life cycle, with the diploid condition limited to the zygote, which immediately underwent meiosis. The late Devonian saw the rise of forests of tall trees such as Archaeopteris, which had secondary vascular tissue and wood, and the evolution of early seed ferns like Elkinsia. The Permo-Triassic extinction event largely spared most plant groups but altered community structures, setting the stage for flowering plants in the Triassic (~200 million years ago) and their diversification in the Cretaceous and Paleogene. Grasses, the latest major plant group, became important around 40 million years ago, and many groups evolved new metabolic mechanisms to survive low CO2 and warm, dry conditions over the last 10 million years.

Reader's Guide

The evolutionary history of plants is significant because it documents the transition from aquatic to terrestrial life, a pivotal event that transformed Earth's surface and atmosphere. Plants were not the first photosynthesisers on land; microbial organisms capable of photosynthesis existed as early as 1.2 billion years ago, but they were small and formed little more than algal scum. The emergence of land plants in the Ordovician introduced sporopollenin-walled spores, enabling survival in dry conditions. Adaptations such as cuticles, stomata, and vascular tissue allowed tracheophytes to resist desiccation and grow upright, leading to the first forests in the Devonian. These forests altered global weathering and carbon cycles. The evolution of seeds and later flowers and fruits enabled plants to dominate terrestrial ecosystems. The Permo-Triassic extinction reshuffled plant communities, and the rise of flowering plants and grasses in the Mesozoic and Cenozoic shaped modern biodiversity. The legacy of plant evolution includes the creation of soils, the regulation of atmospheric CO2, and the foundation of terrestrial food webs. Ongoing evolutionary innovations, such as new metabolic pathways in grasses, continue to adapt plants to changing climates.

Did You Know?

• The closest living relatives of land plants are the charophytes, specifically Charales, which are branched, filamentous algae dwelling in shallow fresh water.
• Grasses, the latest major group of plants to evolve, became important in the mid-Paleogene, from around 40 million years ago.