How Lignin Helped Trees Grow Up
A new book details how lignin once protected algae from UV radiation, then later in evolutionary time became a structural support for trees.
The following is an excerpt from When the Earth Was Green: Plants, Animals, and Evolution’s Greatest Romance by Riley Black.
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When the Earth Was Green: Plants, Animals, and Evolution's Greatest Romance
307 million years ago Ohio
Reflected moonlight glows along the edges of a dark carapace, dozens of intricate exoskeletal pieces making up the back of what at first seems to be a moving part of the forest floor. Never before, and never again, will an arthropod as large as Arthropleura crawl over the Earth. The invertebrate is the largest land-dwelling arthropod of all time, a creature that can only exist in a forest of such extravagant greenery. The entire animal stretches more than eight feet from the front of its head to the trailing tip of its many-segmented body. Despite its intimidating size, however, it’s truly only the plants of this Carboniferous forest that have anything at all to fear from the animal. Arthropleura is an herbivore, its complex mouthparts best suited to munching through increasingly tough plant parts. The evolution of such a truly giant plant predator was spurred by the origin of forests capable of reaching far higher into the air than ever would have been possible for the shoreline liverworts of a hundred million years earlier. The key is lignin.
Plants didn’t evolve lignin in response to coming ashore. The biological compound had evolved in other circumstances and been co-opted to new uses. Evolution excels at refashioning what already exists into new biological possibilities. Hundreds of millions of years before this moment, in the single-celled world that makes up so much of life’s history, various photosynthesizing organisms evolved the precursors of lignin in the form of monomers—molecules that are like links that can be chained together into longer chains called polymers. Lignin monomers were already present in the red algae that wafted among the cyanobacteria, not to mention other cellular organisms that rely on photosynthesis like diatoms and dinoflagellates. It didn’t allow algae to grow twenty feet tall, but instead helped protect such cells from the UV radiation of the sunlight they so desperately needed to expose themselves to in order to photosynthesize while they spread through the sunlit shadows. Plants evolved their own sun- screen. And if early plants and their predecessors had continued to explore the life aquatic, the practically weightless environment may not have given lignin much of a nudge to become such a vital support structure. As plants began to establish themselves on solid ground, however, lignin began to change how plants grew.
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Linked together as a polymer, lignin is very strong. That’s a critical consideration in how high plants can grow above the ground. Without something to strengthen its cell walls, a tree without lignin would bend and flop over under its own weight like a stalk of week-old celery. Plants would have stayed low- growing, not even high enough to cast much shade on the ground. Earth would not have forest but endless shrubbery, at best. Lignin provided plant cells with the necessary strength to begin growing tall. But that’s hardly all. Lignin is also important in water transport, allowing plants to soak up water through their roots and move the liquid through the tree’s tissues, reaching higher into the air than would otherwise be possible. By pure evolutionary accident, a molecule that had evolved to help protect plants from the damaging effects of UV also included hidden potentials that trees needed to grow far above the surface of the soil, providing both strength and a way to move water from the ground into even the tallest parts of the tree.
This forest primeval has its own strange character. The trees of this Carboniferous forest aren’t anything like the forms that will thrive over the next 300 million years. Each towering trunk is immediately recognizable as a tree, but not quite like any that will flourish during the heyday of the dinosaurs or shade the proliferation of Cenozoic mammals. The shape of these plants identifies them as trees, but in future times their surviving relatives will return to growing low to the ground rather than reaching for the distant stars overhead.
Forests are composed of trees, and a tree is not a distinct group of living things but truly just one expression of what plants can become. In fact, the difference between a tree and a shrub often rests on how tall the mature plant can grow. Extend a shrub farther into the air and it gains a new title, just as a tree that branches low and has little trunk to speak of could easily be counted as a shrub. Shifting between such botanical expressions will happen again and again among different plant groups through time. Even though they’re more closely related to ferns and other ancient plant forms, the scale trees of this forest are trees because they are tree- shaped, just as the pines, palms, and peach trees that will evolve later are trees despite belonging to distantly related plant groups. If ever there was an Age of Plants, this would be it.
Adapted from WHEN THE EARTH WAS GREEN by Riley Black. Copyright © 2025 by Riley Black. Reprinted with permission by St. Martin’s Press, an imprint of the St. Martin’s Publishing Group.
Meet the Writer
About Riley Black
@LaelapsRiley Black is a science writer and the author of several books, including When the Earth Was Green, The Last Days of the Dinosaurs, Skeleton Keys and My Beloved Brontosaurus: On the Road with Old Bones, New Science, and Our Favorite Dinosaurs.