Scientists have identified what may be the "missing link" connecting fungi to plants, animals and bacteria.
In the journal Nature, the researchers outline the structures and properties of a primitive type of fungus that they name cryptomycota, or "hidden fungi".
They suggest it raises new questions about how fungi evolved.
Samples used in the research were isolated by Dr Meredith Jones in a pond near her laboratory at Exeter University, UK.
"They are the oldest currently known of the fungal group and they represent the missing link between fungi and the rest of the kingdom of life," she told BBC News.
"DNA studies show that it was around this point that fungi diverged, so this will tell us a lot about how they evolved."
The discovery has already thrown up one surprising finding. The group lacks what until now scientists had thought was a defining feature of fungi: a rigid cell wall that is used to feed.
According to lead researcher Dr Thomas Richards from the Natural History Museum in London, text books on fungi may well need to be re-written.
"This new group uses a different strategy. We need to rethink how we classify fungi and we may need a better definition of what a fungus is," he said.
The indications are that cryptomycota is an incredibly diverse group, according to Dr Jones.
"From the mushrooms we find at the shops, the yeast that we use to bake bread and brew beer, to the mould that's growing in the bathroom - you can see how diverse the fungi we already know about are," she explained.
"This new group is just as diverse, and so its discovery doubles our currently known understanding of the fungal kingdom."
Researchers had known about this group of fungi from DNA-based techniques.
But the Nature-reported study is the first time that a team has learned what cryptomycota actually look like by using advanced microscopic techniques.
Deciphering the group's shape and structure should help scientists understand better what this particular group of fungi do in the environment.