The first plants on land

The story

A land without green

Try to imagine a landscape with mountains, plains, rivers and deserts, but without a single blade of grass, a bush or a tree. Bare rock, mud, sand. That is what the continents were like for almost the whole history of the Earth. Life lived in the seas, in lakes and perhaps in thin films of microbes on the shores. Then, about 475 million years ago, something began to change: the first plants came out of the water.

The traces: microscopic spores

The oldest evidence for these plants is not leaves or trunks, but microscopic grains, spores. Spores are very tiny cells that plants without seeds use to reproduce, as mosses and ferns do today. They have a very tough outer wall, which survives in rocks for hundreds of millions of years. The oldest spores generally accepted as belonging to land plants are about 470 to 475 million years old and come from Ordovician rocks, in particular from the Middle East and Argentina. They have a regular shape and are found in groups of two or four.

In 2003, in Ordovician rocks in Oman, a group of palaeobotanists found not only scattered spores but also tiny fragments of plants with the spores still inside. For the researchers they are proof that those spores were really produced by land plants, perhaps like liverworts, small green plants without roots that grow today on damp stones. Exactly which plants they were and how they were built remains largely a mystery: no whole fossil of them has been found.

Where they came from

Scientists are almost certain that land plants descend from freshwater green algae, relatives of those that still grow in ponds today. By studying DNA, they have found that the closest living relatives of plants are certain groups of green algae, such as the Zygnematophyceae. To come out of the water, these algae had to face new problems: not drying out in the air, putting up with the Sun's strong light without the filter of water, staying upright without the support of water, and finding mineral salts in the soil.

The first land plants solved these problems one step at a time. They developed a waterproof film, the cuticle, so as not to lose water. They invented pores, the stomata, to exchange gases with the air. And they made a partnership with fungi: their primitive roots, or simple filaments, joined with soil fungi, which brought them water and mineral salts in exchange for sugars. This cooperation still exists today and involves most plants.

The first whole plants

Fossils of whole plants, with stems and capsules that held the spores, appear later. Cooksonia, found in Ireland, Wales and other places, lived in the Silurian, about 430 million years ago. It was only a few centimetres tall, made of thin stems that forked in two, each with a small capsule on top. It had neither leaves nor true roots.

An exceptional deposit is the one at Rhynie, in Scotland, discovered in 1912 by the geologist William Mackie. It is a siliceous rock, called chert, about 407 million years old, formed by hot springs that buried small ecosystems: the stems of plants, fungi, algae and insects are preserved in detail, down to the cells. For the first time it showed what a land ecosystem was like at its beginnings.

Then, in a few tens of millions of years, plants made great progress. In the Devonian there appeared tissues strengthened with lignin, a substance that makes wood rigid, vessels that carry water, the first true leaves and deep roots. Around 385 million years ago, in places such as Gilboa, in New York State, the first forests formed, with trees many metres tall, such as Wattieza, like a tree fern; a little later, in nearby places such as Cairo, came Archaeopteris, which had leaves like those of a fern but a trunk of real wood.