A Fungus the Size of a Tree Grew in a World Before Trees Existed

An eight-meter-tall organism named Prototaxites dominated the landscape some 420 million years ago — and it was a fungus, not a tree, growing in a world that had no trees at all. That is one of the central revelations in Nature's Magicians: How Leaves Conjure Up Our World, a new book by evolutionary biologist Jonathan Silvertown, excerpted this month by Scribner, an imprint of Simon & Schuster.

Silvertown opens with a striking fact: before the industrial revolution, leaves pulled carbon dioxide out of an atmosphere containing just 0.02 percent of the gas — half today's level — and turned it into solid wood. The machinery for that trick sits inside chloroplasts (tiny green bodies, hundreds per plant cell, that hold the pigment chlorophyll and run photosynthesis). And chloroplasts, he argues, are captives.

How did leaves capture their carbon-fixing engines?

Chloroplasts carry their own DNA, and that DNA reveals kinship with cyanobacteria — photosynthetic microbes that still live free in soil and water. According to the genetic narrative, an ancestral alga engulfed a cyanobacterium about 1.5 billion years ago, and instead of digesting it, kept it working. Silvertown points to living parallels: Cymbomonas tetramitiformis, an alga that swallows bacteria at one stage of its life, and sea slugs that sequester algae's chloroplasts undigested for months. The closest genetic match for the chloroplast ancestor is Gloeomargarita lithophora, a cyanobacterium that lives glued to rocks in fresh and brackish water.

There's a twist. The genes for oxygen-producing photosynthesis turn out to be older than cyanobacteria themselves — the first cyanobacteria were apparently not photosynthetic. Silvertown's analogy: he owns no family hat, yet wears a vintage one. The answer is horizontal gene transfer, the acquisition of genes by means other than parental inheritance, the same process that spreads antibiotic resistance through hospitals. Some of these ancient genes, stretching back more than 3 billion years, have been found in viruses that infect bacteria, which may have been the vector.

The scale is hard to absorb. Plants today make up 82 percent of Earth's living biomass, and chloroplasts plus cyanobacteria fix 100 billion tons of atmospheric carbon every year.

What delayed leaves for 40 million years?

Land plants descended from a freshwater green alga in the streptophyte group and arrived on land roughly 500 million years ago — long after cyanobacteria, which had been terrestrial residents for billions of years. The Rhynie Chert, a 407-million-year-old rock formation near the Scottish village of Rhynie outside Aberdeen, preserves seven plant species with knee-high cylindrical stems and no leaves in perfect three-dimensional cellular detail.

Yet leaves don't appear in the fossil record until the start of the Devonian period — at least 40 million years after land plants originated — and then evolved independently in at least six groups by the Devonian's close, 358 million years ago. Silvertown rules out a genetic barrier, noting that leaf-development genes were present in the earliest, leafless plants.

His explanation is environmental. In a hot, high-CO2 climate, flat leaves risked overheating while cylindrical stems stayed cooler and captured sufficient carbon through sparse stomata (the tiny pores that admit CO2 and release water vapor). When atmospheric CO2 fell — likely because plants were sequestering it — the climate cooled, and fossils show leaf size and stomatal density rose together. Evaporative cooling through stomata can drop a modern leaf's surface temperature 1°C below the surrounding air, unlocking larger leaves.

The cascade that followed, Silvertown writes, had 'planet-changing consequences' — a mechanism worth knowing as we watch today's CO2 curve move in the opposite direction.