The Quiet Architects of Decay: The Deep History and Importance of Polypore Fungi

All photographs are taken and owned by Noah Hilton unless otherwise stated.

When you hear the word mushroom, a polypore is probably not the first image that comes to mind. Before diving into their story, it helps to clarify some basic terminology so the distinctions between fungal structures and life stages are clear.

Polypores are a group of fungi that produce bracket- or shelf-like fruiting bodies, characterized by pores or tubes on their hymenium, the underside of the structure [1]. These fruiting bodies are the mushroom, and they function primarily to produce and release spores. Before a mushroom ever appears, however, the fungus exists as mycelium, growing within its substrate. Mycelium is a network of branching, tubular filaments that make up the main body of the fungus [2].

A useful analogy is this: mycelium is to a cherry tree what a mushroom is to a cherry. The tree grows continuously but only produces fruit when it is time to reproduce. In the same way, fungal mycelium can persist for years, sometimes centuries, before forming a mushroom to disperse spores.

Most polypore species are saprobic, meaning they decompose dead organic material [3]. Some polypores are parasitic, breaking down living tissue, while others form mycorrhizal relationships with plants. This article focuses specifically on saprobic polypores as we trace their remarkably long evolutionary origin story.

The first chapter of that story begins nearly 500 million years ago with the emergence of lignin, a complex organic polymer. Lignin evolved alongside the rise of vascular plants roughly 450 million years ago and served two major functions. It strengthened plant vascular tissues and created a powerful physical and chemical defense against decay and predation [4]. In simple terms, plants rapidly grew larger and more structurally complex while simultaneously becoming far more resistant to decomposition. The evolution of lignin ultimately allowed vascular plants to dominate Earth’s landscapes.

The second chapter brings fungi back into the picture. As mentioned earlier, saprobic fungi decompose dead organic matter, but lignin proved to be such a formidable barrier that an estimated 150 million years passed between the appearance of lignin and the evolution of fungi capable of breaking it down [5]. These fungi, known as white-rot fungi, were the first organisms able to degrade lignin using specialized enzymes.

By around 300 million years ago, white-rot fungi could finally access the vast energy locked within woody plant material. However, these early fungi still lacked a critical feature. They did not yet produce mushrooms. Instead of fruiting bodies, they reproduced using free-swimming, single-flagellated zoospores that required moist environments or direct contact with animals to spread [6].

Side note: dinosaurs did not appear until approximately 245 million years ago. This means the story of polypores was already more than 200 million years in the making before dinosaurs ever walked the Earth.

Although white-rot fungi could break down lignin by 300 million years ago, it took at least another 155 million years for fungi to evolve true fruiting bodies. According to research published in The American Journal of Botany, the earliest confirmed mushroom-forming fungi, known as homobasidiomycetes, date to the mid-Cretaceous period between 145.5 and 66 million years ago [7].

The earliest known mushroom was a bracket fungus, what we now call a polypore.

At last, the full origin story of the saprobic polypore comes into focus.

Why does any of this matter? On one level, the answer is simple. Origin stories are fascinating, mushrooms are fascinating, and mushroom origin stories are especially fascinating. Polypores are also deeply intertwined with both ecological history and human survival.

In 1991, a 5,000-year-old Neolithic human corpse, commonly known as the Iceman, was discovered preserved in an alpine glacier. Among his belongings were three fungal objects. Two were pieces of polypore mushrooms, and one was tinder made from another species [8]. The tinder, derived from Fomes fomentarius, the true tinder polypore, was likely used for fire-starting. The remaining polypore fragments were believed to serve medicinal purposes.

One of these species, Piptoporus betulinus, the birch polypore, is now known for its antibacterial, antiparasitic, and anticancer properties [9]. From penicillin that fights bacterial infections, to lovastatin that lowers cholesterol, and even cyclosporine that enables human organ transplants, fungi are responsible for some of the most important medicines in human existence [10]. As evident from the Iceman, humans have depended on nature for their basic needs, including medicines, since the earliest civilizations. Natural products today are responsible for over one-third of the approved drugs on the market [11].

The plants and fungi these medicines are derived from, as well as the environment they grow in, face an unprecedented threat by the same creatures that have used them to save their own lives. 10 million hectacres of forest, an area nearly the size of Germany, is lost every year by deforestation [12]. There are over a 1000 species of polypores that have been discovered, but there is an incredible amount of diversity still unknown even in well-researched landscapes. Even with the intention of reforestation, the current rate of old growth forest loss will inflict unimaginable damage and loss of species. It seems like a generally good idea for the planet, and selfishly for human existence, to stop the mass destruction of the very ecosystems that is giving us over a third our medicine. Although the thought of standing up against deforestation on this scale can seem daunting, there are steps that you can take in your own life to help. Planting a tree, using less paper, buying sustainable wood products, and doing your own research are just a few ways you can make a difference.

Here are a few resources that might help kickstart your support: 

• Rainforest Alliance

• Green Peace

• Green Tumble

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References & Acknowledgements:

1. Plantbiology.natsci.msu.edu. (n.d.)., https://plantbiology.natsci.msu.edu/mushrooms/polypores/

2.  Encyclopædia Britannica, inc. (n.d.). Mycelium. Encyclopædia Britannica., https://www.britannica.com/science/mycelium

3. Runnel, K., Miettinen, O., & Lõhmus, A. (2021, January 18). Polypore fungi as a flagship group to indicate changes in biodiversity – a test case from Estonia - Ima fungus. BioMed Central., https://imafungus.biomedcentral.com/articles/10.1186/s43008-020-00050-y

4. Renault, H., Werck-Reichhart, D., & Weng, J.-K. (2018, November 13). Harnessing lignin evolution for biotechnological applications. Current Opinion in Biotechnology., https://www.sciencedirect.com/science/article/pii/S0958166918300636#:~:text=It%20is%20thought%20that%20the,dominance%20of%20the%20terrestrial%20ecosystems

5. Behind the scenes: How fungi make nutrients available to the world. Energy.gov. (n.d.)., https://www.energy.gov/science/articles/behind-scenes-how-fungi-make-nutrients-available-world#:~:text=Although%20fungi%20appeared%20millions%20of,type%20to%20break%20down%20lignin

6. Laundon, D., Chrismas, N., Bird, K., Thomas, S., Mock, T., & Cunliffe, M. (2022, March 1). A cellular and Molecular Atlas reveals the basis of chytrid development. eLife., https://elifesciences.org/articles/73933

7. Hibbett, D. S., Donoghue, M. J., & Tomlinson, P. B. (n.d.). (rep.). Is Phellinites digiustoi the Oldest Homobasidiomeycete? (Vol. 48, Ser. 7). Botanical Society of America.

8. The Iceman's Fungi - researchgate. (n.d.)., https://www.researchgate.net/publication/222280519_The_Iceman's_fungi

9. Pleszczyńska, M., Wiater, A., Siwulski, M., Lemieszek, M. K., Kunaszewska, J., Kaczor, J., Rzeski, W., Janusz, G., & Szczodrak, J. (2016, September). Cultivation and utility of piptoporus betulinus fruiting bodies as a source of anticancer agents. World journal of microbiology & biotechnology., https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4963449/

10. McHale, E. (2022, June 30). Secret fungi in Everyday Life., https://www.kew.org/read-and-watch/everyday-fungi-food-medicine#:~:text=Penicillin%2C%20which%20fights%20bacterial%20infection,Aspergillus%20terreus%20(lowers%20cholesterol)

11. Cao, S., & Kingston, D. G. I. (2009, August 1). Biodiversity Conservation and Drug Discovery: Can they be combined? the Suriname and Madagascar experiences. Pharmaceutical biology., https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746688/

12. Ritchie, H., & Roser, M. (2021, February 9). Deforestation and forest loss. Our World in Data., https://ourworldindata.org/deforestation#:~:text=Globally%20we%20deforest%20around%20ten%20million%20hectares%20of%20forest%20every%20year