The evolution of plants part 1: The first conquerors of land





Editor and Artist
We live on a green planet. Today we may take them for granted, but plants are the most important living things on Earth. Their colonization of land made it possible for all animal life to survive, from the smallest ants to the largest dinosaurs. What were the first of these tough, green pioneers? And how did they find their way onto land in the first place?

For many millions of years, everything existed under the sea, including the first-ever photosynthetic organisms, known as cyanobacteria. They were about as simple as all other organisms at the time, single cells. Unlike other living things, their cells contained chlorophyll, the green pigment used to absorb light energy. These little organisms, formerly known as blue-green algae, were turning sunlight into food and expelling oxygen as a waste product. The water would soon be filled with breathable oxygen which would later bubble up into the air and start to accumulate in the atmosphere.

These early cyanobacteria were also the backbone of Earth’s many microbial ecosystems, the likes of which we still see today in the form of stromatolite beds. Stromatolites may look like no more than layered mounds of rock but they are actually microbial mats built by cyanobacteria. These organisms have been alive for 3.5 billion years and have colonized a number of underwater environments. Most of the time cyanobacteria remain unnoticed, only sometimes forming mats, like at Shark Bay in Western Australia.

Cyanobacteria were the dominant form of photosynthetic life for billions of years, remaining underwater in their colonies. More than 1.6 billion years ago, a second group of photosynthetic organisms emerged: the algae. The algae are eukaryotes, meaning they have specialized structures within their cells called organelles. One particular organelle type, the chloroplast, is shared by all algae and land plants. This is where photosynthesis takes place in the cell. The first ancestors of land plants evoled from green algae nearly 500 million years ago.

For any organism, life on land is a constant battle with gravity. You need to have a proper internal structure to fight the pull of these forces and to protect your soft internal organs. In the case of plants, they need an internal support system that would hold them above ground.

Non-vascular plants, which include mosses and liverworts, were the first-ever land plants. They do not have much of an internal support system, but have slightly simpler material for the same tasks. They cannot survive without contact with a body of water, and will soon dry out and die if there is no dampness around them. These restrictions meant that they could not spread across the planet when plants began to conquer the land.

The first plants to really fight against gravity are called vascular plants, and evolved special tissue types to move water and nutrients to all parts of the plant. Vascular plants have xylem and phloem tissues to transport water and nutrients respectively, along the interior of the plant stem. These vascular tissues are the plant’s strengthening inner skeleton. This enables them to grow tall against gravity, a necessary task for all land-living organisms.

The earliest plants to really battle against gravity arose in the Silurian Period, around 433 million years ago. These little plants had individual spore-producing organs that are all connected by a branched stem. Underground, their root-like rhizoids absorb water and work as an anchor to support the main body of the plant.

Fossils of such plants were first found in Shropshire in England. Called Cooksonia, these plants were an important part of Earth’s flora until at least the Early Devonian. Currently only their sporophytes or asexual parts are known. These plants had spore cases that would have exploded to release the spores all over the area like ferns and other primitive plants we see today.

While they may seem rather alien and almost simplistic today, the Cooksonia jungles were home to some of the first major land animals. The Shropshire fauna would be recognizable even today, among the vast range of insects, spiders and others that crawl about the undergrowth. Being smaller than their underwater cousins, they managed to fight the effects of gravity by developing exoskeletons of chitin to protect their internal organs.

One of the best places to see these fossil communities is the Rhynie Chert of Scotland. This incredible rock formation shows an ecosystem like no other, with a proper animal community living in the shadows of some of the earliest vascular plants. The formation itself was formed by volcanic hotsprings similar to Yellowstone. Rhynie is a type of formation called a lagerstätte, the kind that preserves an incredible number of organisms in impressive detail. These depositional environments are created when sediment completely swamps the dead organism underwater, in this case in an environment of hot springs. All the plants here had been buried quickly when silica-rich water from the volcanic springs covered them wherever they lay.

The Rhynie plants lacked true leaves, but their shapes were more diverse than ever. Rhynia is a typical plant of this kind. It was much more branched than Cooksonia and some of the other early vascular plants, and still had a markedly similar body. It generally grew on sandy surfaces rather than directly around the hot springs.

Among the biggest plants here was Asteroxylon, which grew in waterlogged areas of its habitat. This relative giant of its time had stems 40 cm (16 inches) long, and 12 mm (1/2 inch) in diameter. It was not a close relative of Rhynia. Instead, it was closer to the modern day clubmosses. Unlike today’s clubmosses, Asteroxylon did not have true leaves. It still held its spore cases aloft, supported by even thicker stems than its cousins. The stem is covered in scale-like projections, the kind present in most ancestral clubmosses. It stood around a meter or so high, and for a while it seems to have won the race to the sun.

Rhynie was, indeed, filled with a number of primitive clubmosses like Asteroxylon, each unique in its own way. Niche partitioning meant that competition was minimized between species. These little forests were still bound to water, though. While they were starting to come into their own they were still not the biggest organisms here. That place belongs instead, to a rather mysterious organism that has split scientists down the middle since 1843. Its name is Prototaxites. Its fossils look somewhat like massive tree trunks but in fact it may have been the fruiting body of an immense fungus. The tallest of them are known to reach eight meters in height, towering above their plant neighbors like columnar alien monoliths.

The age of giant mushrooms was not to last though. As the Late Devonian progressed, the clubmosses started growing much larger and became even better equipped to fight against gravity. Gigantic fungi had no place in this constantly evolving world and instead the plants took their place and evolved something extraordinary – wood. It is an adaptation that allowed plants to grow to enormous sizes in a variety of environments, changing the fate of the land plants for the next 400 million years.

This is part one of a five-part series on the evolution of plants.

1: The first conquerors of land

2: Birth of the forests

3: The Age of Coal

4: A tale of flowers and seeds

5: The grassland empire