Long before dinosaurs roamed the land, the Earth looked very different from the planet we know today. About 500 million years ago, most of the Earth’s surface was bare rock and dry soil. There were no trees, no grass and no flowers. Life existed almost exclusively in the oceans.
Then something incredible happened: Plants began to grow on land.
This moment was one of the most important events in Earth’s history because it changed the planet forever. As a geoscientist, I am interested in changes in the diversity of flora and fauna – that is, plants and animals – over time.
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Predecessors of plants lived in water
The history of plants begins in the water. The earliest plant-like organisms were simple, tiny green life forms such as algae. You can still see algae today as seaweed along beaches or as green slime on rocks in ponds.
Algae have lived in the Earth’s oceans and lakes for over 1 billion years. The can cook their own fooduses sunlight, water and carbon dioxide to make sugar. This process is called photosynthesis; it releases oxygen – the gas we need to breathe – as a byproduct.
To begin with, Earth’s atmosphere had very little oxygen. Over millions of years, photosynthesizing organisms such as algae and some bacteria have slowly released oxygen into the air. This change, sometimes called Great oxygenation arrangementmade it possible for larger and more complex life to evolve. Without oxygen-producing organisms, animals, including humans, could never have existed.
Scientists believe the first true plants evolved from green algae around 470 million years ago. These early plants lived in shallow water near shorelines, where conditions changed frequently. Sometimes they were underwater, and sometimes they were exposed to air. This habitat helped them slowly adapt to life on land.
Moving on land was not easy. Water plants supported by water and can easily absorb nutrients, but land plants faced new challenges. How would they avoid drying out? How could they stand upright without floating? How would they get water and nutrients from dry land?
To survive, early plants developed important new functions. An important adaptation was a waxy coating, called cuticlewhich helped to keep water inside the plant. Plants also developed stronger cell walls that allowed them to stand upright against gravity. Simple root-like structures, called rhizoids, helped anchor plants to the ground and absorb water and minerals from the earth.
The earliest land plants were very small and simple. They resembled modern mosses, liverworts and hornwort, which still grows today in damp places such as forest floor and stream edges. These plants did not have true roots or stems, and they stayed close to the ground. Fossils of early land plants, such as Cooksoniadating back to about 430 million years ago and showing small branched stems only an inch or two tall.
Although these plants were tiny, they had a huge impact on the earth. When plants spread across land, their roots helped break down rocks into soil, a process called weathering. This created richer soil that could support more life.
Plants also released more oxygen into the atmosphere, improving air quality and helping animals breathe. Plants created new habitats and food sources, allowing insects and other animals to move from water to land.
Increasing complexity over millions of years
Once plants were established on land, evolution continued. Around 420 million years ago, plants evolved vascular tissue: tiny tubes that transport water and nutrients throughout the plant. This adaptation allowed plants to grow taller and stronger because water could be moved upwards from the roots to the leaves. These vascular plants included early relatives of ferns and club mosses.
With vascular tissue, plant life really began to flourish. About 360 million years ago, large forests covered large parts of the earth. Giant ferns and tree-like plants, some over 30 meters tall, dominated the landscape. Over time, dead plant material from these forests was buried and compacted, eventually forming coalwhich people still use as a source of energy today.
Another major step in plant evolution was development of seedsaround 380 million years ago, found in seed ferns. Other seedlings, such as early conifers – a group that includes modern pines – could reproduce without needing water for fertilization. Seeds protected plant embryos and allowed plants to survive harsh conditions such as drought or cold.
The last major plant evolution occurred around 140 million years ago, when flowering plants, what scientists call angiospermsappeared. Flowers help plants attract animals such as insects and birds, which spread pollen and seeds. Fruit developed to protect seeds and help them travel. Today, flowering plants make up most of the plants we see, including trees, grasses, fruits and vegetables.
The first plants not only survived; they transformed the earth. They changed the atmosphere, built soil and created ecosystems that allowed animals to thrive on land. Thanks to the evolution of plants, the earth became a green, living planet full of diverse life.
This edited article has been republished from The conversation under a Creative Commons license. Read original article.
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