Some of them seemed to have partnered with plants that also left fossils behind in the rocks. Others appeared to have specialized in breaking down dead plant matter.

Until now, those fossils have been the oldest clear evidence of fungi. Many scientists considered them a snapshot of the early conquest of land. Fungi and plants came ashore together as ecological partners, it seemed. Together, they transformed barren lands into a soil-carpeted habitat.

Recently, though, some researchers have grown dissatisfied with this scenario.

By comparing the DNA of different species, scientists have drawn an evolutionary tree of fungi. If the Scottish fossils were among the earliest members of the fungal kingdom, you’d expect that living fungi would share a common ancestor not much before 407 million years.

But that’s not what the DNA trees tell us. The genes of living fungi indicate that their common ancestor lived over a billion years ago.

Could there be a 600-million-year gap in the fossil record? In recent years, scientists have searched for fungi in rocks older than those in Scotland, and they’ve found a few microscopic fossils that looked like they might be fungi. But they were too ambiguous to convince many experts.

The new fossils came to light during a geological expedition to the barren fringes of the Canadian Arctic. In 2014, Robert Rainbird, a research scientist at the Geological Survey of Canada, noticed black flecks on a piece of shale.

He knew that sometimes flecks like these turn out to be microscopic fossils. “I thought, ‘I should grab some of this stuff, because it looks juicy,’” he said.