Fungus fossils show the complexity of life a billion years ago

Researchers have found billion year old fossils of a fungus that represents the oldest and most complex kind of life yet found. It's also our closest relative so far discovered from that time.

The fossil record has preserved little of the life this era when scientists think life was entirely microscopic and mostly consisted of bacteria.

The fungal microfossils were found in a remote part of the Northwest Territories.

Elizabeth Turner, a professor of sedimentary geology at Laurentian University was part of the team that discovered the fossils. She told Quirks & Quarks host Bob McDonald that finding the fossils was a major surprise. 

The existence of fungus in this time was not unexpected. Scientists studying the history of life had predicted, based on rates of genetic evolution, when certain types of life should have appeared. "But actually finding it was especially joyful because fungus doesn't fossilize very well," said Turner

Turner said prior to this discovery paleobiologists tended to think the Precambrian era — everything that existed before much more complex animal fossils burst onto the scene 540 million-years-ago — as "just being a whole bunch of bacteria."

This finding suggests a richer picture. Fungi are extremely important in ecosystems today because of the role they play in breaking down and recycling nutrients from dead plants and animals. They may well have played a similar role a billion years ago. 

"The reality is, actually, at a billion years ago, we had more complex forms of life in a fairly complicated ecosystem that included primary producers, consumers, and even predators, as well as the things that break down organic matter and recycle it — the fungi," said Turner.

[These fungus microfossils] would resemble what we would think of in the modern world as a fungal spore and fungal filaments.- Elizabeth Turner, Laurentian University

Similar to modern fungi

These tiny microfossils, described in the journal Nature, are less than one tenth of a millimetre in size and share many characteristics with their modern-day counterparts.

"They consist of a sphere connected to some filaments. The filaments branch off of one another at right angles multiple times. And the filaments contain divisions inside of them indicating individual cells separated from one another along the length of the filament," said Turner.  

"[These fungal microfossils] would resemble what we would think of in the modern world as a fungal spore and fungal filaments."

On top of looking at its physical appearance and its internal organization, Turner's European colleagues also analyzed the microfossil composition.

"Using a sort of a high-end type of spectroscopy, we were able to identify the material as a form of chitin," said Turner. 

Chitin is a major component of the exoskeleton of insects and crustaceans, as well as the cell walls of fungi.

The fungus' role in the ancient ecosystem

The fossils were discovered in shale sedimentary rocks in the Grassy Bay Formation in the Northwest Territories. A billion years ago this area is thought to have been an estuary — the mouth of a river where it met the ocean.

"I would say a conservative interpretation is that it lived in the marine environment — possibly on the sea floor, but it is remotely possible that it came from land, in which case it was facilitating very early microbial ecosystems on land," said Turner.

What this means for our tree of life

A major implication of this discovery is the light it sheds on when complex organisms — like animals and fungi — diverged from each other in the tree of life.

In the earliest branches of the tree, there are only three groups, two of which are bacteria. The other branch consists of more complex organisms called eukaryotes. At the tip of that branch is where fungi diverged from animals.

"That means that the branching off of those two lineages had to have happened still earlier than a billion years ago," said Turner.