Evolution of fungi

Fungi represent a distinct kingdom of life that is separate from both plants and animals. Scientists use DNA analysis to trace their evolutionary history. This genetic evidence shows that all fungi descended from a single common ancestor. This ancestor likely lived between 1.2 and 1.5 billion years ago.

While many early fungi likely lived in aquatic environments, new evidence suggests they may have been on land much earlier than once thought. For example, a fossil named Ourasphaira giraldae from the Canadian Arctic might have lived on land one billion years ago. This discovery suggests fungi colonized land long before plants arrived.

Early fungi likely lived in water and possessed flagella. A flagellum is a tiny, whip-like structure used for swimming. During the Paleozoic Era, which lasted from 542 to 251 million years ago, many fungi were aquatic. These organisms were similar to modern Chytrids because they used flagellum-bearing spores to move. Over time, most fungal lineages lost the flagellum as they adapted to new environments. Moving from water to land required fungi to develop new ecological strategies. They evolved into saprobes that eat dead organic matter and parasites that live on other organisms. They also developed mutualistic relationships, such as mycorrhiza, which are helpful partnerships with plants.

Finding fossils of fungi is a significant challenge for paleontologists. Unlike many animals, fungi do not biomineralize, meaning they do not create hard shells or bones. Most fungal structures are microscopic or consist of soft, fleshy tissues that decay quickly. This makes the fungal fossil record much more meager than the records for plants or animals. To find them, scientists often look for fungi preserved within a plant or animal host. They use thin-section preparations to study these samples under light or transmission electron microscopy. Researchers also use acid to dissolve the surrounding rock in compression fossils. This process allows them to see tiny surface details using scanning electron microscopy.

Fungal diversity grew significantly during the Devonian period, between 416 and 359.2 million years ago. The Rhynie chert provides a rich record of fungi from this era, including Zygomycota and Chytridiomycota. Around 400 million years ago, the Ascomycota and Basidiomycota lineages diverged from one another. By the Late Carboniferous period, roughly 318 to 299 million years ago, all modern classes of fungi were already present. Some organisms, like Prototaxites, may have been the tallest living things during the late Silurian period. These were likely fungi or lichens that grew very large.

Specific groups of fungi appeared at different points in Earth's history. Lichen-like fossils have been found in the Doushantuo Formation in China, dating back 635 to 551 million years ago. The oldest known fossil of a sporocarp, or a spore-bearing structure, is a species called Paleopyrenomycites from the Rhynie Chert. Much later, the first known mushroom-forming fungi appeared. A species named Archaeomarasmius legletti was found preserved in amber. This fossil dates to the mid-Cretaceous period, approximately 90 million years ago. This shows that while fungi are ancient, the familiar mushrooms we see today are relatively recent.

Fungi show incredible resilience during major biological crises. About 251 million years ago, following the Permian-Triassic extinction event, a "fungal spike" occurred. This was an extraordinary abundance of fungal spores in the sediment. During this time, fungi may have been the dominant life form on Earth. They also experienced a massive bloom approximately 66 million years ago. This occurred immediately after the Cretaceous-Tertiary extinction event. When many plant and animal species died out, the Earth became a massive source of organic matter. Scientists have described this period of fungal growth as being like a massive compost heap.

The ability of fungi to flourish during these times is linked to their role as decomposers. When large-scale deforestation or mass extinctions occur, they provide a sudden wealth of nutrients. This allows fungi to expand rapidly while other life forms struggle to survive. Molecular data supports this resilience, as phylogenetic analyses of over 5,000 agaricomycete species show no sign of a mass extinction at the Cretaceous-Tertiary boundary. While other kingdoms faced collapse, fungi utilized the changing environment to thrive. This connection between fungal evolution and global ecological shifts highlights their vital role in Earth's systems.