After reviewing the evolutionary timeline of fungi, an international team of scientists has determined that their origin dates back to between 900 million and 1.4 billion years ago, a much earlier timeframe than previously believed. This means that fungi had already been living on Earth hundreds of millions of years before plants took root on our planet. The findings, published in open access in the journal Nature Ecology & Evolution, came thanks to the use of a new methodology and sophisticated evolutionary models combining a variety of dating techniques.

The study is the product of international collaboration between multidisciplinary researchers from various countries and institutions, including the evolutionary biologist Eduard Ocaña, Ramon y Cajal researcher at the Universitat Oberta de Catalunya (UOC).

"As a group, fungi are much older than previously imagined. It's highly likely that they were already around over a billion years ago, making them one of the oldest major groups of eukaryotes," he said. As a result, fungi (a kingdom that encompasses mushrooms, moulds and single-cell species such as yeasts) must be older than animals (which are thought to have appeared around 600 million years ago) and multicellular land plants (around 500 million years ago).


A new timeline for dating the origins of life

Unlike plants or animals, which have left a rich and recognizable fossil record, fungi's delicate and fibrous bodies are rarely preserved. Due to the limited number of fossils, their evolutionary history has so far been a puzzle full of missing pieces. To overcome this challenge, the researchers combined three different and complementary sources of information: the few fossils available, the genomic sequences of over a hundred species of fungi, and the effect of horizontal gene transfers, a key and innovative process that proved crucial in their endeavour.

These horizontal gene transfers are a rare but very important biological phenomenon whereby a gene crosses from one species to another. "When a gene jumps from one organism to another, that tells us that the two existed at the same time. This enables us to establish relative timelines, because any relative of the donor lineage must necessarily be older than any descendant of the lineage that received the gene," said Ocaña. By using these chronological markers from horizontal gene transfer events, together with other techniques and new computational tools that reduce calculation times, the experts were able to obtain new, more accurate and reliable evolutionary timelines for over 100 species of fungi.


Fungi: terrestrial ecosystem pioneers

The discovery is not just about dates. It has profound implications when it comes to our understanding of pre-Cambrian terrestrial ecosystems, as we have little information on them in terms of fossil records, especially as regards the proportions of different groups of eukaryotes. According to Ocaña, "our findings show that fungi were already present on land environments at least 800 million years ago and had ecological interactions with the ancestors of multicellular land plants, although we're currently unsure about the degree of complexity of these interactions. These ancestors probably shared similarities with the green algae groups that are evolutionarily closest to multicellular land plants, some of whose members have some degree of adaptability to non-aquatic environments."

Today's fungi form symbiotic relationships with most plants, providing nutrients in exchange for carbohydrates. These relationships, known as mycorrhizae, may date back to very ancient times: millions of years ago, early fungi may have supported algae and early plants as they adapted to living on Earth in exchange for new sources of energy. "If we accept that fungi were instrumental in helping plants colonize the Earth, our theory is that this partnership may have started much earlier than previously thought, in environments similar to biological soil crusts or the microbial mats that we still have today," said Ocaña, who works with the UOC eHealth Centre and the UOC-TECH Centre.


Rewriting the "empty Earth" narrative

According to the usual narrative about the origin of plants, our planet was bare and hostile until they started to take root around 500 million years ago. This new research challenges this perception: multicellular land plants appeared millions of years after their single-cell ancestors emerged and after fungi started to engage in ecological interactions on land. By breaking rocks, decomposing minerals and recycling nutrients, those early fungi helped generate the first soils, making the environment more hospitable.

Fungi were therefore involved in establishing the earliest terrestrial ecosystems, a finding that would not have been possible without the international collaboration of scientists from a very wide range of backgrounds, including evolutionary biologists, palaeontologists, fungal experts and creators of new methodological tools. "The idea originated from an innovative tool developed by Dr Gergely J. Szöllősi's Hungarian group, of which I was a member when I was doing my postdoctoral research. These findings wouldn't have been possible without this collaboration or the contributions made by researchers from Hungary, England, Japan and Catalonia."


New questions for the future

This discovery also paves the way for new lines of research. The authors are now considering applying the same methodology to other major groups of eukaryotes to obtain a more accurate chronology of the entire course of evolutionary history. "Fungi were a great subject of study, because the scarcity of fossil records meant that our approach provided significant added value. The next challenge is to extend these techniques to all eukaryotes to develop a much finer molecular clock for all complex life," said Ocaña.


Eduard Ocaña's work as a Junior Leader postdoctoral researcher, funded by "la Caixa" Foundation, has been carried out as part of the UOC's digital transition and sustainability, and digital health and planetary well-being research missions, and contributed to the UN Sustainable Development Goals (SDGs), especially 15, Life on Land.


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