Scientists have discovered the world’s oldest forest in an abandoned quarry near Cairo, New York. The 385-million-year-old rocks contain the fossilized woody roots of dozens of ancient trees. The find marks a turning point in Earth’s history. When trees evolved these roots, they helped pull carbon dioxide (CO₂) from the air and lock it away, radically shifting the planet’s climate and leading to the atmosphere we know today.

“The Cairo site is very special,” says team member Christopher Berry, a paleobotanist at Cardiff University in the United Kingdom. The quarry floor, about half the size of a U.S. football field, represents a horizontal slice through the soil just below the surface of the ancient forest. “You are walking through the roots of ancient trees,” Berry says. “Standing on the quarry surface we can reconstruct the living forest around us in our imagination.”

Berry and colleagues first discovered the site in 2009 and are still analyzing the fossils it contains. Some of the fossilized roots there are 15 centimeters in diameter and form 11-meter-wide horizontal radial patterns spreading out from where the vertical tree trunks once stood. They seem to belong to Archaeopteris, a type of tree with large woody roots and woody branches with leaves that is related in some way to modern trees, the team reports today in Current Biology. Previously, the oldest Archaeopteris fossils were no more than 365 million years old, Berry says, and exactly when the tree evolved its modern-looking features has been unclear.

The Cairo site suggests Archaeopteris did so 20 million years earlier, says Patricia Gensel, a paleobotanist at the University of North Carolina in Chapel Hill who was not involved with the work. “The size of those root systems—it’s really changing the picture,” she says, adding that, even 20 years ago, researchers assumed trees with such large and complex root systems did not evolve so early in geological time.

Trees like those at Cairo had a big effect on the ancient climate, says Kevin Boyce, a geoscientist at Stanford University in Palo Alto, California. Deep roots penetrate and break up the rocks within and below the soil. Geologists call this processing “weathering,” and it triggers chemical reactions that pull CO₂ from the atmosphere and turn it into carbonate ions in groundwater. This ultimately runs off into the sea and is locked away as limestone.

Partly because of weathering and its knock-on effects, atmospheric CO₂ levels dropped to modern levels soon after the appearance of woody forests. A few tens of millions of years earlier they had been 10 to 15 times higher than today. Some research suggests the removal of so much atmospheric CO₂ led directly to a sustained rise in oxygen levels, with the atmosphere containing about 35% oxygen by 300 million years ago. This, in turn, may have led to the evolution of gigantic insects at that time, some with wing spans of 70 centimeters, which may have lived in the ancient forests.

The trees that grew a few tens of millions of years after the Cairo forest have also had an indirect impact on the modern climate. Berry has previously written about how the fossilized remains of these forests formed the coal that fueled the Industrial Revolution in Europe and North America.

This is not the first time Berry and his colleagues have explored a primitive forest. In the 19th century, researchers discovered a fossil forest in Gilboa, New York, about 40 kilometers from the Cairo site, containing 382-million-year-old specimens. Since 2010, Berry and his colleagues have been examining a quarry at Gilboa that also preserves ancient tree roots. But the Gilboa roots belong to more primitive trees that may be related to ferns and horsetails. They didn’t produce deep, woody roots with much potential for weathering.

This means the trees that grew at the Cairo site were the innovators, Berry says. “Woody trees with leaves that can produce shade—and a big rooting system—is something fundamentally modern that wasn’t there before.”