Years ago, geologist Neil Davies traveled to Bolivia to pick through heaps of fossilized fish. He wanted to know more about the ancient shoreline these fish swam along roughly 460 million years ago, and perhaps learn how they died. The fish, he found, appeared to have been choked by muddy sand that rivers washed rapidly into the sea, maybe during a storm.
Similar heaps of smothered fish appear elsewhere around the world in rocks of similar age. This was before plants had colonized continents, so riverbanks had no roots or stems that could trap muddy sediments on land.
Magnify this effect globally, and the impacts would have been substantial — not just on coastal life but on the landscape of the entire planet. Before plants, rivers would have stripped continents of silt and clay — key constituents of mud — and sent these sediments to the seafloor. This would have left continents full of barren rock, and seas with smothered fish.
Once plants arrived on land, things began to change. Mud clung to vegetation along riverbanks and stuck around rather than shuttling straight to the seafloor. Davies, now at the U.K.'s University of Cambridge, and his colleagues have found that the expansion of land plants between about 458 million and 359 million years ago coincides with a more than tenfold increase in mud on land — and a significant shift in the ways that rivers flowed. The arrival of first plants and then mud "fundamentally changed the way the world operates," he says.
Life evolved tools to cope with the new muckiness and new river shapes, resulting in a diversification of life and landscapes that persists to this day. Plants are responsible for much of this change, but mud contributed too, by adding a cohesiveness to the continents — unlike sand, wet mud sticks together.
Davies is now working to figure out whether early plants increased the creation of mud, trapped more of it in place, or played both roles. It's a story worth getting straight, says Woodward Fischer, a geobiologist at the California Institute of Technology in Pasadena. "Mud is one of the most common, abundant things you can think of," he says. "The recognition that for most of Earth's history it was not like that is a big deal." The research could also help inform modern-day decisions around river engineering projects like dam construction, Fischer says. Understanding the ways that vegetation manipulates river flow and sediment buildup could help prevent some of the failures that have contributed to flooding along the Mississippi River and other major waterways across the world. "Every little bit that we can do better there has huge impacts," he says.
Of mud and riverbanks
When geologists talk about mud, they're referring to tiny particles that stick together when wet. Those particles have often broken down from larger rocks over time due to the forces of wind, rain, ice, and snow. Fungi and microbes can break down rock and form mud, too.
Before plants arrived on land, mud was around — it was just mostly sent to the seafloor by rivers. Once plants showed up, they not only held sediments in place but their roots also physically broke down rock and released chemicals that further crumbled it. In these ways, plants accelerated what geologists refer to as the "continental mud factory."
Since the 1960s, geologists have noticed that rivers that flowed before plants arrived on land often look different in the geological record than those that formed once continents greened. The earliest rivers resembled those that tumble along the gravelly coast of Alaska today, says Taylor Perron, an earth scientist at the Massachusetts Institute of Technology in Cambridge who wrote about the factors that control landscape formation in the 2017 Annual Review of Earth and Planetary Sciences.
Those gravelly Alaskan rivers have many channels that braid across sand banks, continually slumping and forming more channels as they periodically overflow — like rivulets at the edge of a beach. Without anything anchoring these riverbanks in place, they continuously collapse to form new channels. But the arrival of plants kept that erosion at bay — and mud added to the riverbanks' cohesion — so rivers were less likely to slump into these braided forms. Instead, they developed a single channel that meandered through the landscape in a cohesive "S" shape, like parts of the Mississippi and Amazon rivers do today. In this sense, the arrival of plants "is one of the best natural experiments in landscapes that has ever happened on Earth," Perron says.
The shape of a river may seem trivial, but it has far-reaching effects on the life in and around it. Bends in a sinuous channel, for example, can alter the water's temperature or chemistry, making it different from sections that run in a straight line and creating new microenvironments that plants and animals need to adapt to, Davies says.
Even the earliest plants, which resembled mosses, could have begun to alter how sediments accumulate on riverbanks, says Kevin Boyce, a paleontologist at Stanford University who co-wrote about the evolution of plants in the 2017 Annual Review of Earth and Planetary Sciences. "Those weren't big trees," Boyce says, "but they still would have influenced the movements of water" by slowing its flow. As plants evolved to become tree-sized by about 386 million years ago, they gained the power to slow wind. Fine particles caught up in winds would drop to the ground when gusts died in the branches, leaving more sediment caught among trunks and stems.
Life in the muck
This posed new challenges to animals like early millipedes and wormlike creatures. "Mud is providing a totally different medium for things to live in," says Anthony Shillito, a geologist at the University of Oxford, U.K.
To get through mud, an animal such as a worm creates cracks to shuffle through by contracting its body, extending it, squeezing water out of the way and moving forward. This is mechanically different from traveling through sand, which requires an animal to excavate material out of the way, Shillito says. So early land worms and insects would have had to evolve body parts equipped to deal with muckier movements.
And those movements, in turn, could have helped shape the mud itself, says Lidya Tarhan, a paleobiologist at Yale University. "The act of digging and excavating those burrows and keeping them clear can move around sediments and change the distribution of sediments and also affect the chemistry," she says. For example, some invertebrates ingest sediments to extract nutrition, and chemical reactions in their guts can form fine particles that come out in their feces as mud.
Here are three examples of traces left in the fossil record by animal movements. It's often not possible to figure out exactly what critter left what trace, because different types of animals can produce similar-looking traces. But studying the different patterns can offer information about how animals may have traveled through ancient landscapes. As early animals began to colonize land, some wormed or pushed their way through sediments (A and C), whereas others scratched their way along (B). | (A.P. SHILLITO & N.S. DAVIES / GEOLOGICAL MAGAZINE 2020/Courtesy Knowable Magazine)
But the strongest influence early burrowing animals likely had on their muddy environments, Tarhan says, would have been loosening up mud and allowing it to disperse within rivers and across landscapes. With the rise of single-threaded rivers, mud would have had more opportunities to spread onto floodplains. Such plains don't develop as easily alongside braided rivers, whose banks easily collapse as waters rise, says Chris Paola, a sedimentologist at the University of Minnesota in Minneapolis.
Modern rivers that people have deforested show how the absence of vegetation can destabilize riverbanks and cause them to become less cohesive. Along California's Sacramento River, for example, areas that farmers cleared for cropland are far more susceptible to erosion than areas that remain forested. Conservationists have worked to stabilize the river by planting more than a million seedlings along its banks.
Understanding the interplay of plants and mud in river flow can inform efforts to restore eroding rivers back to a more stable state. "If you don't understand what's driving the river into one state or another, it's hard to do that well," says Paola, who coauthored an article about restoring river deltas in the 2011 Annual Review of Marine Science. And since so much of life revolves around rivers today, it's important to do that well.
But this has always been true. Life has always congregated around rivers, from the very first emergence of plants and animals onto land. That's why the early accumulations of mud alongside rivers — and how mud influenced their flow — is nothing to throw dirt on.
"Once you take it out of the equation and imagine the world without as much mud on the land," Davies says, "then it becomes a very different kind of planet."