The World’s First Trees Were More Structurally Complex Than Trees Today

A 374-million-year-old fossil reveals that the earliest trees on Earth lacked the internal rings that we've come to see as the signature feature of trees.

Life that arose from Earth’s primordial soup hundreds of millions of years ago was usually simple compared to the highly evolved creatures that exist today.

But new research has found that the world’s first trees – the cladoxylopsids – were more complex than their successors, creating a new mystery about the origins of plants that once dominated the globe.

Writing in the journalProceedings of the National Academy of Sciences, a team of researchers identified never-before-seen growth patterns in a 374 million-year-old cladoxylopsid trunk discovered in China.

“It’s a new way of being a plant and particularly a new way of being a tree,” Cardiff University paleobotanist and study co-author Chris Berry told Seeker. “That just in and of itself is extremely interesting.”

Rather than growing in the familiar ring pattern that lets schoolchildren determine the age of most felled trees today, cladoxylopsid trunks were hollow. Around the empty core of the trees were 2-inch-thick strands of xylem, or the material that carries water from tree roots to their branches and leaves.

The strands didn’t behave like ordinary xylem, however. Instead, they bore rings like the cores of modern-day trees, as if the cladoxylopsid was a collection of saplings. What’s more, the tree-like strands connected to each other in an extremely sturdy manner that resembled a network of interconnected water pipes, the researchers wrote. While palm trees and other plants also don’t grow rings, their xylem don’t interconnect, Berry said.

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Cladoxylopsids grew weirdly, too. The strands developed vertically and thickened. But more didn’t appear once the tree was growing. Instead, the soft wooden tissues between the strands would split apart and heal as the tree’s diameter expanded.

“It builds a skeleton at that point, and it’s sort of fixed,” said Berry. “This skeleton is gradually ripping itself apart to accommodate the expansion. As it does, it repairs itself.”

Capable of growing more than 30 feet tall with diameters of more than a foot, their appearance reflected their primordial environment. “These trees didn’t have leaves in a big, green flat sort of sense,” he said. “They were just twigs, little appendages, sticky things.”

Berry didn’t understand why or how cladoxylopsids grew in their unique way. But they were a monoculture, which could hold clues to their evolution.

“You’d think the oldest tree would be the most primitive and simple,” he said. “I have no explanation for that. The only thing you could argue is that there was no competition. There were no other trees around to compete with.”

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In addition to researching the tree’s origins, Berry and his colleagues intend to examine how cladoxylopsids drew nutrients from the soil and how the plant used carbon dioxide.

Compared to today, atmospheric carbon dioxide concentrations were around 10 times higher in the period of 360 to 420 million years ago when cladoxylopsids flourished. Learning more about how cladoxylopsids consumed carbon could also hold clues about the role of the greenhouse gas in the atmosphere today, he added.

“It’s widely thought early trees and early plants played a significant role in shaping the system of the Earth we are used to having,” Berry said.

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