How trees changed the world

For many of us, encountering a tree is part of everyday life. They line our cities, pepper the countryside and play a vital role in the planet’s health, but what exactly are trees and how did they spring up on land? Our fossil plant expert Dr Paul Kenrick is here to tell us more about the evolutionary story of trees.

What is a tree?

Even though trees are everywhere, defining one is actually quite difficult. “It’s quite a deceptively simple question, but one that I suppose botanists have been struggling with for more than 100 years. It’s a grey area. There’s no real definition, but there’s a sort of working idea,” explains Paul.

“Generally, as opposed to a shrub or herb, a tree would be a plant that’s perennial – something that’s been living for many years. They also normally have a distinctive single trunk and a sort of crown of leaves at the top. However, not all trees are woody – tree ferns and palms don’t have any wood at all. Also, there are lots of cases where a shrub looks like a tree and a tree looks like a shrub.”

When did trees first appear?

We don’t know exactly when trees first appeared, but clues in fossil record help scientists to estimate when they emerged. The world’s oldest known fossil forest is located near Minehead on the Devon coast in the UK. It dates back around 390 million years. The site comes from the Middle Devonian – a time when plants were beginning to thrive on land.

The trees, known as Calamophyton, were very different from those we see today. They had thin trunks composed mostly of non-woody tissues and likely resembled palm trees more than typical modern woodland species.

During the Devonian Period, this region wasn’t part of England as we know it now. Instead, it was connected to land that would later become Germany and Belgium, where similar fossil sites have also been found.

Another famous fossil forest lies at a site called Gilboa in New York State, USA, where trees dating back around 380 million years have been discovered.

How did trees first develop?

During the Devonian Period, Earth underwent a major transformation as life began to spread from the oceans onto land. The earliest animals to make this transition were mainly arthropods.

Although it’s still unclear exactly why or how the first plants colonised land, we do know that these early plants, which were known as eophytes, moved onto land between 450 and 500 million years ago. Eventually they gave rise to the first trees.

“The very first plants were tiny. They didn’t have leaves or roots. But once these organs evolved, they were able to combine to form a sort of trunk,” says Paul. “Roots grew around the leaves creating a root mantle. The leaves and roots bound together to form a fibrous and lightweight trunk that kept being added to like in modern tree ferns.”

Why did trees develop?

There are a few reasons why plants developed into trees.

“There’s various reasons why you’d want to be a tree if you’re a plant,” says Paul. “One idea about the evolution of trees is that it’s a race for the light – about getting your head and shoulders above the competition so that you can get up there, produce a canopy and trap that light.”

“Plants are static – they don’t move around like animals. They need to compete for resources where they grow and the thing they’re really looking for is sunlight, which is where they get their energy from.”

Trees are also perennials. This means they continue living for years, unlike annuals, which complete their entire life cycle – from germination to seed production – within a single year and then die off.

“Another life strategy is longevity, and the tree growth form lends itself to that. Trees put a lot of resources into their offspring year-on-year. If they’re in a stable environment, they can just sit there for decades, hundreds of years even and keep producing seeds.”

Why are trees important?

Trees play a crucial role in maintaining the planet’s health because of their ability to store carbon dioxide. Forests act as carbon sinks, which means they absorb greenhouse gasses, such as carbon dioxide.

Carbon dioxide enters the atmosphere from a variety of sources. Human activities such as burning fossil fuels, industrial processes and deforestation release large amounts of carbon dioxide, while natural events – including volcanic eruptions and wildfires – also contribute.

Through photosynthesis, trees absorb carbon dioxide from the atmosphere and lock it into their trunks, branches, roots and surrounding soil. This process helps regulate Earth’s climate by reducing the amount of greenhouse gases in the air.

During photosynthesis, trees convert the carbon dioxide to sugars and in doing so produce oxygen as a biproduct, which is vital for life on Earth. “Trees draw carbon dioxide out of the atmosphere and lock it away. The net result is an increase in levels of oxygen,” remarks Paul.

Trees were also responsible for producing coal. Paul explains, “coal has been very important economically. In this country, for example, it fuelled the Industrial Revolution. Coal in the UK is made from plants that once grew in tropical swamp forests.”

In the Carboniferous Period, around 360 to 300 million years ago, there were forests made up of trees and herbaceous plants like ferns. When they died, they piled on top of each other. Over time, thick layers of dead material built up and formed peat. As sediments like sand and mud buried the peat, increasing heat and pressure over millions of years squeezed out water and gases, leaving behind carbon-rich coal.

Trees also support a huge amount of life. For example, birds use trees as a place to build their nests, and squirrels eat the seeds. Numerous insects rely on trees for food and shelter too. In urban areas, trees help to create wildlife corridors and they also play an important role in our wellbeing.

How can you help to protect trees?

Huge numbers of trees are being cut down. In 2024, roughly 18 football pitches’ worth of forest were being lost per minute. However, there are plenty of ways that you can help.

• Reduce, reuse and recycle paper products and choose items made from recycled or sustainably sourced materials. Look for certification labels, such as FSC, that show wood and paper products come from well-managed forests.

• Support sustainable farming practices by buying food and drink with a Rainforest Alliance seal.

• Plant trees in your garden or community to help local ecosystems. It’s also just as important to care for existing trees so they can thrive.

• Support local efforts to preserve parks, woodlands and urban trees.

• Use less energy, travel more sustainably and reduce waste to help limit climate change, which is a major threat to forests worldwide.

What trees can you see in our galleries?

In the Museum, we have fantastic fossilised trees that you can come and see. In one of the bays in Hinze Hall, you can see a display of fossil tree trunks, all from different periods. The display includes one of the oldest known trees from Gilboa in New York State, a tree from the Carboniferous Period as well as a tree from England dating back to the Jurassic Period, when dinosaurs roamed the land.

There’s also the giant sequoia trunk on display on the upper balcony of Hintze Hall. This modern-day species from California, USA, is one of the largest trees on Earth today.

Outside, in our Evolution Garden, there are several petrified trees, including the Craigleith tree specimen from the Carboniferous Period, which was found in Scotland. This extraordinary specimen is about 330 million years old.

The specimens in the collections we care for are helping scientists as they continue to discover more about the incredible evolutionary history of trees.