About 500 million years ago, most of Earth’s land surface was bare rock and dry soil. There were no trees, no grass, no flowers. Life existed almost entirely in the oceans. Long before dinosaurs roamed the planet, Earth looked nothing like the world we know today.
Then something remarkable happened: plants began to grow on land. It was one of the most important events in the history of the planet, because it changed Earth forever.
The explanation comes from a geoscientist, a professor of geography and doctoral researcher in Earth sciences at Binghamton University, who studies how the diversity of flora and fauna has shifted over time.
The ancestors of plants lived in water
The story of plants begins in water. The first plant-like organisms were simple, tiny green life forms, similar to algae. We still see algae today as seaweed on beaches or as a slimy green layer on lake rocks.
Algae have lived in Earth’s oceans and lakes for more than a billion years. They can make their own food, using sunlight, water and carbon dioxide to create sugars.
That process is called photosynthesis, and it releases oxygen, the gas we need to breathe, as a byproduct.
At first, Earth’s atmosphere held very little oxygen. Over millions of years, photosynthetic organisms such as algae and some bacteria slowly released oxygen into the air.
That shift, sometimes called the Great Oxidation Event, made it possible for larger and more complex life forms to evolve. Without oxygen-producing organisms, animals, including humans, would never have existed.
Scientists believe the first true plants evolved from green algae about 470 million years ago. Those early plants lived in shallow water near the coast, where conditions changed often: sometimes they were submerged, and other times exposed to the air. That habitat helped them gradually adapt to life on dry land.
Gaining a foothold on dry land
The move to land was not easy. Aquatic plants are supported by water and absorb nutrients easily, but land plants faced new challenges: how to avoid drying out, how to stay upright without floating, and how to draw water and nutrients from dry soil.
To survive, the first plants developed important new features. One key adaptation was a waxy layer called a cuticle, which helped hold water inside the plant. They also formed tougher cell walls that let them stand upright against gravity.
Simple root-like structures called rhizoids anchored the plants to the ground and absorbed water and minerals.
The first land plants were very small and simple. They resembled today’s mosses, liverworts and hornworts, which still grow in damp places such as forest floors and stream banks. They had no true roots or stems and stayed close to the ground.
Fossils of early land plants, such as Cooksonia, date back about 430 million years and show small branched stems only a few inches (a few centimeters) tall. The Y-shaped fossil of Cooksonia barrandei, at 432 million years old, is considered the oldest land plant in the world and can be seen at the National Museum in Prague, in the Czech Republic.
Though they were tiny, these plants had an enormous impact. As they spread across the ground, their roots helped break down rock and turn it into soil, a process called weathering. That created more fertile ground, able to support more life.
The plants also released more oxygen into the atmosphere and improved air quality. They created new habitats and food sources, allowing insects and other animals to move from water onto land.
Growing more complex over millions of years
Once plants took hold on dry land, evolution kept going. About 420 million years ago, plants developed vascular tissue: tiny tubes that carry water and nutrients throughout the plant. This adaptation let plants grow taller and stronger, because water could travel from the roots to the leaves. These vascular plants included early relatives of ferns and clubmosses.
With vascular tissue, plant life truly began to flourish. About 360 million years ago, vast forests covered much of Earth. Giant ferns and tree-like plants, some more than 100 feet (30 meters) tall, dominated the landscape. Over time, the dead plant matter from these forests was buried and compacted, eventually forming coal, still used as an energy source today.
Another major step was the development of seeds, about 380 million years ago, found in seed ferns. Fossilized leaves such as those of Furcula granulifer, from the Upper Triassic of Greenland, already showed a reticulate leaf venation.
Other seed plants, such as the first conifers, a group that includes modern pines, could reproduce without needing water for fertilization. Seeds protected the plant embryos and allowed them to survive harsh conditions such as drought or cold.
The most recent major plant evolution occurred about 140 million years ago, with the rise of flowering plants, also known as angiosperms. Flowers helped plants attract animals such as insects and birds, which spread pollen and seeds. Fruits developed to protect the seeds and make them easier to disperse.
One example is Nanjinganthus dendrostyla, a Jurassic angiosperm from China that, according to research by Fu and colleagues, confirms that angiosperms existed before the Cretaceous.
Today, flowering plants make up most of the vegetation we see, including trees, grasses, fruits and vegetables.
The first plants did not just survive: they transformed Earth. They changed the atmosphere, built the soil and created ecosystems that let animals thrive on land. Thanks to plant evolution, Earth became a green and living planet, full of diverse life.
Reporting: Anne Silva / Amazonia Mag