Plant Evolution

The entire event of plant evolution highlights the gradual adaptation of simple aquatic plants and how they developed features to adapt themselves to terrestrial environments. It is divided into the following key events:

Early Algae: The Precursors to Land Plants (~700 – 500 Mya)

The ancestors of modern-day plants probably evolved from green algae, the first non-vascular plants (similar to the stonewort) that could perform photosynthesis. They harbor chlorophyll a and b in their cells that could capture sunlight, a characteristic of modern-day plants. Their common ancestor is known as charophytes. These multicellular organisms could store their food as starch and could perform cell division.

Green algae, specifically charophytes, are thus considered the closest relatives to land plants. They lived in shallow habitats where they were exposed to frequent droughts and tides, which probably led to their adaptation to land.

Transition to Land (~500 Mya)

Early plants, such as bryophytes (mosses and liverworts), possessed hair-like structures called rhizoids instead of true roots and vascular tissues. Thus, they could not conduct water and minerals from the soil. Instead, they relied on moist environments to survive. 

However, when they moved to land, they faced some challenges. They lacked the adaptations necessary to survive there. 

• They needed a proper water-absorbing system to conduct water from the soil. It led to the evolution of roots.
• To overcome gravity, they needed to become more sturdy and thus gave rise to the stem. 
• In order to escape desiccation, they needed a waxy covering around the stem to trap moisture.
• They needed carbon dioxide to perform photosynthesis.  
• They had to protect the embryo from drying out.

Evolution of Vascular Plants (~450 – 400 Mya)

Before plants could completely evolve in the sea, animals were predominant in the ocean. Plants were dominant in the upper layer of water that could receive sunlight. Thus, plants never became dominant marine organisms. Eventually, plants started to colonize the land, which was almost 700 Mya. For them to colonize on land, they had to develop a number of adaptations:

• They developed a plumbing system in the form of vascular tissues (xylem and phloem) to conduct water and minerals from the soil to all its parts. They could also transport the food from the photosynthetic cells to the rest of its parts.
• To overcome gravity, they gave rise to the stem.
• To escape desiccation, they developed a waxy covering around the aerial parts called the cuticle.
• In order to intake carbon dioxide for photosynthesis, they developed tiny pores on the leaves and stems called stomata through which they could perform gas exchange.
• For the protection of their embryo, they evolved seeds.

They continued to evolve with time and developed adaptations to survive water scarcity and changes in the environment’s temperature.

Dominance of Seedless Vascular Plants (~400 – 250 Mya)

The first group of plants that have diversified are the Gymnosperms, which include plants like ferns, horsetails, and club mosses. They covered much of the Earth with dense forests. Seeds of gymnosperms protected the growing embryo and provided a food reserve so that they could reproduce without the need for water. It allowed gymnosperms to diversify. 

Diversification of Flowering Plants (~200 – 66 Mya)

After the extensive diversification of seedless vascular plants during the Paleozoic era, flowering plants (angiosperms) started to diversify in the Jurassic Period of the Mesozoic era. Flowers attracted pollinators like insects and birds, which led to more efficient reproduction. The development of fruits to protect the growing embryo and the efficiency of seed dispersal have caused a rapid rise in the angiosperm population.

Today, angiosperms are the most diverse group of plants, with over 300,000 species, including grasses, herbs, and higher plants.

Plants have evolved relatively slowly compared to humans and animals. Their evolution began during the Paleozoic era (~540 Mya) and continues today.

Pre-Cambrian Era (4000 – 541 Mya) 

• Cyanobacteria (blue-green algae) developed the ability to photosynthesize
• Green algae (Charophytes), the ancestors of modern-day plants, appeared in aquatic environments, having chlorophyll and the ability to store energy as starch.

Cyanobacteria and charophytes set the stage for plant evolution but are not considered true plants.

Paleozoic Era (541 – 252 Mya)

This era witnessed the colonization of land plants.

• Cambrian Period (541 – 485 Mya): Green algae diversified in aquatic environments, initiating terrestrial adaptation 
• Ordovician Period (485 – 443 Mya): The first non-vascular land plants, similar to modern mosses and liverworts, appeared. These early ancestors lived near the water source and had chlorophyll, which captures sunlight and stores energy, a characteristic of modern plants. They also developed a waxy cuticle to prevent desiccation.
• Silurian Period (443 – 419 Mya): During this period, the first vascular plants, such as Cooksonia, evolved, though their vasculature lacked differentiation into leaves, stems, and roots.
• Devonian Period (419 – 359 Mya): Ferns, club mosses, roots, leaves, and the first seed plants evolved. Progymnosperms, precursors to seed plants, also appeared, along with large trees.
• Carboniferous Period (359 – 299 Mya): Seedless vascular plants like ferns, horsetails, and lycophytes flourished. Seed-bearing plants (gymnosperms) became more prominent. 
• Permian Period (299 – 252 Mya): As the climate dried down, gymnosperms like conifers, cycads, and ginkgos flourished, and seedless vascular plants started to decline in number.  

Mesozoic Era (252 – 66 Mya)

It is the ‘Age of Gymnosperms,’ which witnessed the diversification of flowering plants. 

• Triassic Period (252 – 201 Mya): Gymnosperms like conifers and cycads dominated terrestrial ecosystems. 
• Jurassic Period (201 – 145 Mya): The first flowering plants (angiosperms) appeared and coexisted with their pollinators, while gymnosperms remained dominant.
• Cretaceous Period (145 – 66 Mya): Angiosperms diversified, producing flowers and fruits with seeds enclosed in protective coverings. By the end of this period, they became the dominant plant group.

Cenozoic Era (66 Mya – Present)

Angiosperms dominated this phase.

• Tertiary Period (66 – 2.6 Mya): Grasslands like savannas and prairies expanded as grasses and herbaceous plants thrived in cooler, drier climates. Angiosperms adapted to diverse ecosystems, dominating tropical and temperate regions, while conifers remained prominent in colder climates. 
• Quaternary Period (2.6 Mya – present): Cooling climates caused forests to recede in some areas, allowing grasslands and tundras to expand. Humans then appeared, domesticating plants like wheat, rice, and maize and thereby transforming ecosystems.

Plants need to adapt and diversify with respect to the changing environment. They generate oxygen, produce food (primary producers), and offer habitats and shelter for numerous species. It contributes to the development of biodiversity by driving speciation, which results in a wide variety of plant forms, ecological niches, and traits. Adaptations like photosynthesis and seed development sustain life on Earth.