What does that say about medieval farming?

The key is variety. Medieval agriculture wasn't one crop on one field — it was many kinds of farming woven through a landscape that still held "old bits of woodland, you had heaths, you had ponds, and bits of secondary forest," Gordon said. These patchworks, or "mosaics," created more habitats, and more habitats meant more plant species.

"In that landscape, you had lots of natural features that remained from the Middle or Early Holocene," which began after the last Ice Age about 12,000 years ago, he added. "It was all interlaced with these agricultural mosaics, and that led to raised biodiversity."

Outside experts say the study backs a growing body of evidence that human shaping of land can add diversity rather than erase it. Many people assume a "stable and pristine world in which the entry of people has only one result: net loss of species and homogenization," Erle Ellis, an agroecologist at the University of Maryland, Baltimore County who wasn't involved in the research, said by email. "The palaeoecologists have always known the story is far more complicated."

Patrick Roberts, director of the Department of Coevolution of Land Use and Urbanisation at the Max Planck Institute of Geoanthropology, put it this way: rather than "the monoculture, plantations, or uniform changes to landscapes we see in the 20th and 21st centuries, emphasis was placed on maintaining 'mosaics.'"

How does this connect to today's biodiversity crisis?

The contrast with modern farming is stark. Research suggests global biodiversity declined by up to 11% during the 20th century, driven largely by land-use change, pollution and climate change. Monoculture — growing a single crop over a wide area — is a major offender in agriculture.

The study shows "how people can actively use landscapes while maintaining biodiversity and mixed ecosystem functions," Roberts said. In other words, the medieval lesson isn't that humans should leave the land. It's that mixed, varied land use can support more species than either abandonment or monoculture.

What's still unknown?

Plenty, by Gordon's own account. Species count is only one measure. "There are a lot of other types of diversity — not just the number of types, but what the different types present actually do," he said.

A landscape could hold a hundred plant types that all do the same job — what ecologists call low "functional diversity." That matters, because functional diversity underpins how ecosystems cope with climate change and how resilient they stay.

Functional diversity "is super important for how ecosystems adapt to changes in climate and how resilient they are, which is something we need for future-proofing our agricultural system," Gordon said.

The findings are preliminary in one sense: pollen records capture plant presence, not the full ecological picture, and much of Europe's abandonment history remains unstudied. Still, the 40% signal is hard to ignore — and it flips a common assumption on its head. Sometimes, the most biodiverse landscape is the one with people in it, tending a patchwork of fields, woods and ponds.