Sprouting from the base of tree trunks, roots are the arboreal equivalent of a digestive tract, exchanging water and nutrients with surrounding soils. Roots literally anchor a plant, and the more extensive they are, the bigger and stronger the stuff above ground can grow. In their modern forms, they helped trees dominate their habitats—and spread across the globe.
But roots didn’t always look as they do today, and researchers have long puzzled over how and when trees evolved their expansive underground plumbing.
“This pushes … of this kind of root system back in time,” says University of North Carolina, Chapel Hill’s Patricia Gensel, a paleobotanist specializing in plants of the Devonian, which spans the period from 419 million to 360 million years ago. “By the mid-Devonian, we have pretty sophisticated trees,” says Gensel, who was not involved in the study. “Before this, we never would’ve been able to say that.”
Splaying out some 18 feet from the base of their trunks and digging deep into the soil, the roots were sturdy, branched, and intricate, with delicate rootlets splaying from their tips. They looked, in other words, “strikingly modern, essentially what you’d see outside in my yard right now,” says lead author William Stein, a paleobotanist at Binghamton University. But separating the fossils from Stein’s neighborhood spruces is an evolutionary chasm of about 385 million years.
The fossilized roots, the researchers realized, belonged to Archaeopteris (not to be confused with the unrelated bird-like dinosaur Archaeopteryx), a genus that researchers think produced the first “modern tree.” Like today’s oaks and maples, Archaeopteris boasted flat, green leaves ideal for absorbing sunlight and girthy, lumber-worthy trunks that helped the plant grow out as well as up. The massive roots revealed at Cairo now added another contemporary characteristic to Archaeopteris, giving the trees a trifecta of resource-utilizing features that likely helped them take over the world’s forests toward the end of the Devonian, Stein says.
What prompted Archaeopteris to evolve its suite of nutrient-guzzling traits is still unclear. But whenever and however that shift occurred, it signaled a dramatic departure from the shrimpy plants carpeting the planet a few million years prior, Gensel says. “What’s at the Cairo site … is mindblowing in a sense.”
The team’s find also tells us a little about who was growing with whom in the mid-Devonian, says Cindy Looy, a paleobotanist at the University of California, Berkeley, who wasn’t involved in the study. “There are very few Devonian sites … where you can get an idea of what a forest looked like three-dimensionally,” she says. But Cairo, with its exquisite preservation, is a notable exception.
Collectively, these forests and others like them went on to reshape the entire planet. Woody trunks sopped carbon from the air, before dying and depositing the molecules underground to fertilize new life. Leaves shaded the soil, protecting its residents from the sun’s relentless rays. Roots wrestled into the dirt, altering its chemistry and shuttling carbonic acid toward the sea. Moored by trees, entire landscapes became bolstered against floods and inclement weather.
Drained of carbon dioxide, the atmosphere cooled dramatically, likely helping to plunge the globe into a prolonged period of glaciation. Several branches of the tree of life fizzled out, while other species moved onto land and diversified. “The arrival of these forests was the creation of the modern world,” Berry says.
https://www.smithsonianmag.com/science-nature/385-million-year-old-fossils-reveal-worlds-oldest-forest-had-modern-tree-roots-180973810