Lignin: How a single mutation lit the spark of the industrial revolution

A Carboniferous forest depicting Meganeura, Image Source: Field Museum

What do giant insects, trees, and the industrial revolution have in common? All three are part of a 360 million-year-old domino effect that can be traced to a single mutation in the plants' genome, the biosynthesis of lignin.

Lignin is a class of complex organic polymers found in plants, and it's a key player in the formation of cell walls, especially in wood and bark, lending rigidity to the plant's stem and making it so that trees can grow large.

Before lignin was first synthesized, terrestrial plants couldn't grow much larger than a bush. If you found yourself walking through the continents of the Devonian 400 million years ago you would be immersed in a truly alien environment.

Instead of trees, you'd find small bushes made out of ferns, horsetails, and seed plants, with no flowers in sight. Earth would still have to wait 270 million years to see the first flower bloom. 

But what would possibly first catch your eye would be the 8-meter (26 ft) tall and 1-meter (3 ft) wide giant fungi.

The forests of the Devonian were more akin to the Mushroom Kingdom from the Super Mario games, with plants playing second fiddle to giant mushrooms.

This would all change by the beginning of the Carboniferous, 360 million years ago. A single mutation led plants to synthesize lignin. With this polymer plants were finally able to outgrow the fungi in whose shadow they stood for millions of years.

Thanks to lignin and to the absence of natural predators - although insects were already abound, vertebrates were still making their first steps onto land - trees were free to grow as large as they could, with some species growing as tall as a 10-story building with an average height of 30 meters (98 ft).

And since lignin was the new kid on the block fungi, which usually are responsible for degrading organic matter and turning it into nutrients, didn't actually know how to degrade lignin. This meant that if a tree were to fall its trunk would just lay on the ground.

So, for about 60 million years trees spread around the globe leaving their trunks on the forest ground to be buried by sediment. Time and the pressure of the Earth's crust turned this ancient wood into coal. The very same coal that was used to power the industrial revolution and that some countries still use today to produce electricity.

This was all possible due to the evolution of lignin and the fungi's inability to degrade it.

With so many trees around the oxygen levels in our atmosphere skyrocketed. Experts estimate that during the carboniferous atmospheric oxygen levels peaked around 35 percent, compared with 21 percent today.

This was good news for a particular group of animals: invertebrates. Today the largest invertebrate found on land is the coconut crab weighing about 4 kg (9 lbs), and with a leg span exceeding 1 meter (3 ft). But most other invertebrates that you may find in your backyard, be they insects or arthropods, usually aren't bigger than a few inches long.

Insects and other invertebrates can't grow any larger today because, well, because they don't have lungs and wouldn't be able to get enough oxygen.

Instead, insects have tiny tubes called tracheae that are distributed around their body. The air travels through these tubes by diffusion, a process that slows down the longer the tube is. This means that if these tubes are longer than 1 cm (0,4 in) the insect runs the risk of not being able to bring oxygen to its organs on time. Which is not ideal.

However, since oxygen levels in the carboniferous were almost double what they are today, this meant that insects could grow large. And I mean very large.

The stuff of nightmares

Have you ever had a dream where you're being chased by two-meter-long millipedes, dragonflies the size of your face, or scorpions larger than a medium-size dog?

If your answer is yes, you probably dosed off while watching a documentary on the insects of the carboniferous.

From Meganeura, an extinct order of insects similar to dragonflies but with a wingspan of over 60 cm (2 ft), to Arthropleura, a two-meter long millipede, and Pulmonoscorpius, a 70 cm (2,3 ft) scorpion, many species of large insects ruled the land from 360 to 300 million years ago.

And all this was possible due to a single mutation that made plants able to synthesize lignin.

Eventually, fungi did develop ways to degrade lignin using enzymes, such as peroxidase, which reduced the amount of wood left lying on the forest floor, as well as the number of trees. Lowering atmospheric oxygen levels closer to the ones we have today and leaving insects unable to grow larger than your garden variety grasshopper.

Arthropleura (left) and Meganeura (right), Image Source: DR

A Domino Effect 360 million years in the making

This single mutation led to a domino effect that dethroned the giant fungi that once ruled over the continents of the Devonian, and gave rise to the evolution of trees. This made the oxygen levels go up, and the insects grew along with them, finally leading to the fossilization of an immense amount of wood that would in time become the coal that humans used to power the steam engines that revolutionized the workforce in the 19th century.

We have lignin to thank for all this and for the extraordinary ecosystems that have evolved to be reliant on trees for nutrients, shelter, and shade.

Without it, our ancestors might've had a rough time finding a branch to swing on to.