Plant Evolution: Fashionably late

The Evolution Of Plants: Weird and Surprisingly Late

And what if they moved?
Would you see yourself in them then?

Plants are the most important organisms on earth. Not only are they the biggest, they are also most important for the overall functioning of our planet. So one would think they were some of the earliest organisms? But actually, plants came into the scene a lot later than one would think, in particular, much later than the first animals. And even then, the early ones were very different from modern plants. Flowering plants, the ones we know and love, actually only evolved after the dinosaurs. Plant evolution is such an understudied and underappreciated area of science. Let’s dive in.

What are plants? How did they evolve?

The earliest plant-like things were the green cyanobacteria we discussed previously that oxygenated the Earth’s atmosphere. They photosynthesized, so they are sort of like plants, and evolved roughly 3 billion years ago. However, they don’t really seem to count: they were single-celled, they floated in the ocean and they don’t have trunks, stems or roots. When we say plants, we generally mean multi-cellular things that live on land.

The oldest land plants: Mosses, liverworts and lichen (400 million years ago)

This is a drawing by Ernst Haeckel of the structures of different types of Lichen. Lichen were one of the earliest types of plants on Earth roughly 1 billion years ago. A lichen is actually a cyanobacterium living on a fungi. The fungi provide the structure and shape and the cyanobacteria gets energy from photosynthesis and feeds the fungi. They don’t have roots, stems, trunks, flowers, or any of the more complicated structures we associated with modern plants. But they were where plant life on Earth started.

The first things that people really thought of as ‘plants’ were probably liverworts, moss, and lichen. These came around about 1 billion years ago. These plants are all “non-vascular” plants – that means they don’t have structures to transport water around them, or anything like stems, or trunks. Instead, moss, lichen, and liverworts have a lot less structure, and are basically green all over, able to absorb water and light any point of their body. There were no trees at this stage of the early earth and no grass. However, amusingly, there were giant tree-sized mushrooms instead, which could get up to 8 meters tall and that have now all gone extinct.

The First Complex Plants: Ferns (360 million years ago):

The first vascular plants – plants that had complex structures for transporting water around them were ferns. These are Dicksonia Antarctica, the Australian Antarctic fern, one of the more common species in Australian forests. Ferns are really forms of plants, because they don’t have flowers, they instead reproduce with spores. Read more about them here (Image: Shutterstock).

The next big step with plants was evolving vascular tissue. The limitation of the early plants like lichen is that they were not very good at distributing water or sugars around their body. This meant that they could not grow particularly large and or collect large amounts of light – they were limited to about 30 centimeters tall. Vascular plants developed two new types of tissue: phloem and xylem. The xylem and phloem are the plant equivalent of blood vessels, empty tubes of dead cells that transport water and sugars to where they need to go (like up the trunk of a tree or along its branches).

The first seeds and trees (200 million years ago):

The oldest seeded plant is the Gingko – also known as the Maidenhair tree. They are some of the hardiest and resilient trees – a few of them survived within a kilometer of atomic bomb being dropped in Hiroshima Japan. They evolved about 200 million years ago and the living ones are identical to their fossiled ancestors. They were also some of the first trees cultivated by early humans. Read more about these fascinating trees here (Image: Cayambe, Wikimedia).

The next step in plant evolution is seeds. Up until this point, all plants reproduced using spores, instead of seeds. Spores are very similar to seeds, they are small cells released by a plant (or fungi) to let the animal reproduce. The difference between a spore and a seed is that spores don’t have any nutrients in them to help the plant germinate. Seeds generally have some fats and protein in them, so that if the seed lands far away from the original plant, in a place without any water or good soil, the baby plant might have a bit of chance of surviving after germination, because it carried some nutrients with it. This makes seeds a bit more difficult to make, so a plant generally releases fewer of them (one fungus can give off millions of spores). However, it means the plant has more of a chance to reproduce, particularly further away from water in harsher environments.

Seeds seem to have first evolved. about 350 million years ago. Once this happened, there was an explosion of different types of plants as they completely covered the earth. The first woody trees evolved (not grasses, they occurred much later). But finally, forests that would look vaguely familiar to us arrived.

Why do plants matter?

This is a diagram of a tiny portion of the complex chemistry that plants undergo to produce certain types of chemicals called ‘secondary metabolites’. All animals have a fair bit of this, but plants are so much more chemically complex. They produce structures to absorb light, toxins to fight off parasites, sugars and proteins to build and repair their cells, and all sorts of other chemicals (Image: Genome.jp).

Most people look down on plants. They see them as a less sophisticated life form of life.

We’ve already talked about the importance of photosynthesis (see our post). But there are many other senses in which plants are vastly more adaptable organisms than animals.

Imagine you could not move. Instead of going inside a house or a cave when it got cold, or putting on a jumper, you had to stay completely still and endure the elements. Same thing when it rained or got windy. And the only thing you could eat is the dirt beneath your feet and breath the air. If an animal came and had a nibble on you, the only thing you could do was repair yourself, and evolve a stronger body to prevent it from happening again.

This is the life of a plant. The complete inability for them to change their environment means they need to be vastly more chemically adaptable. Plants have thousands of different chemical pathways that they can use depending on many different factors – temperature, humidity, how much water or other nutrients they have available, and many more. A tree isn’t doing “nothing”, because it’s not moving (also – they do move a lot, just slowly). It is waging a constant chemical war against bugs and microbes attacking and defending, while managing its resources of water and nutrients, pumping them around its body. Some trees (deciduous ones) even randomly decide to drop all their leaves as they head into winter, just to conserve their chemical resources. Others can even survive having all their leaves and most of their branches chopped off – let’s see an animal try and do that.

A tree hugger is a phrase that is almost always used as an insult nowadays. And that is absurd. If you can’t hug trees for their contributions to keeping the planet going, there isn’t much point in hugging anything else.

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