lignum

Latin lignum meant wood, timber, firewood. The Swiss chemist Anselme Payen identified the two major components of wood in 1838: cellulose (the fibrous part) and a darker, harder substance he called lignine, from the Latin root. Lignin is a complex polymer that fills the spaces between cellulose fibers in plant cell walls, providing rigidity and compressive strength. Without lignin, plants would be limp.

Lignin's evolution, roughly 450 million years ago during the Silurian period, was one of the most consequential events in the history of life. Before lignin, all plants were small, soft, and confined to wet environments. Lignin allowed plants to build rigid cell walls, grow tall, and compete for sunlight. Trees were the result. Forests were the result. The terrestrial ecosystems we know exist because a polymer evolved.

For roughly 60 million years after lignin evolved, no organism on earth could decompose it. Dead trees accumulated without rotting, piling up in vast deposits that were eventually compressed into coal. The Carboniferous Period (359-299 million years ago) is named for this coal, and the coal exists because lignin had no predator. White-rot fungi eventually evolved the enzymes to break down lignin, ending the era of coal formation.

The paper industry regards lignin as waste. In the Kraft pulping process, which produces most of the world's paper, lignin is dissolved out of wood to isolate cellulose fibers. The extracted lignin is usually burned for energy. Researchers are now finding uses for it—bioplastics, carbon fiber precursors, adhesives—turning the molecule that built the first forests into materials for a post-petroleum world.