Astronomers are finding lots of exoplanets that are orbiting stars like the sun, significantly raising the odds that we will find a similar world. But if we do, the chance that the surface of that planet will look like ours is very small, thanks to an unlikely culprit: plants.
We all know how Earth's landscape came about, right? Oceans and land masses formed, mountains rose, and precipitation washed over its surface; rivers weathered bare rock to create soil and plants took root. Well, new research indicates that the last stage of this scenario is not right. Vascular plants—those with structures such as xylem and phloem that can conduct water—are what created the rivers and muds that built the soils that led to forests and farmland.
The evidence that vascular plants were a primary force that shaped Earth's surface is laid out in a special issue of Nature Geoscience, posted online today. (Scientific American is part of Nature Publishing Group.) In one article, Timothy Lenton, an Earth systems scientist at the University of Exeter in England, presents data from the biogeochemical record showing that the evolution of vascular plants around 450 million years ago is what really began to soak up carbon dioxide from the atmosphere, more so than organisms in the oceans. As a result, global temperatures dropped, initiating a cycle of widespread glaciation and melting that, over millions more years, would significantly grind Earth’s surface.
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Perhaps even more surprisingly, vascular plants formed the kinds of rivers we see around us today, according to another article by Martin Gibling of Dalhousie University in Nova Scotia and Neil Davies of the University of Ghent in Belgium, who analyzed sediment deposition going back hundreds of millions of years. Before the era of plants, water ran over Earth's landmasses in broad sheets, with no defined courses. Only when enough vegetation grew to break down rock into minerals and mud, and then hold that mud in place, did river banks form and begin to channel the water. The channeling led to periodic flooding that deposited sediment over broad areas, building up rich soil. The soil allowed trees to take root. Their woody debris fell into the rivers, creating logjams that rapidly created new channels and caused even more flooding, setting up a feedback loop that eventually supported forests and fertile plains.
"Sedimentary rocks, before plants, contained almost no mud," explains Gibling, a professor of Earth science at Dalhousie. "But after plants developed, the mud content increased dramatically. Muddy landscapes expanded greatly. A new kind of eco-space was created that wasn't there before."
Which brings us to the cosmic consequences. "Plants are not passive passengers on the planet's surface system," Gibling says. "They create the surface system. Organisms tool the environment: the atmosphere, the landscapes, the oceans all develop incredible complexity once plant life grows." So as Nature Geoscience's editors state in an editorial for their special edition, "Even if there are a number of planets that could support tectonics, running water and the chemical cycles that are essential for life as we know it, it seems unlikely that any of them would look like Earth." Because even if plants do sprout, they will evolve differently, crafting a different surface on the orb they call home.
