Cradles of Evolution

Geosciences professor Phoebe Cohen studies what sponge fossils reveal about life today.

High atop the Mackenzie Mountains in northwestern Canada, a group of scientists including geosciences professor Phoebe Cohen are studying the remains of ancient reefs to improve our understanding of the development of complex life on Earth.

The jagged, shallow water reefs, known as the Little Dal Group, were formed nearly one billion years ago. They are home to the earliest purported remains of sponges, considered to be the simplest and possibly oldest known animals in the fossil record.

How the reefs and the surrounding ecosystem were formed is something scientists are still trying to determine. In 2021, Canadian geologist Elizabeth Turner published a paper in Nature investigating structures in the Little Dal reef system, which she and other scientists interpreted as being formed by sponges.

“This is a pretty big claim, because the reefs are approximately 200 million years older than the oldest evidence of sponges,” says Cohen, adding that “it would push back the oldest known animals by about 100 million years.”

With the support of a two-year, nearly $106,000 grant from the National Science Foundation, Cohen leads a team that includes Caroline Nadalin ’27 and six other researchers from the University of Washington, Dartmouth University and the University of Oxford. The group spent two weeks over the summer exploring the geology and paleontology of the Little Dal Group, collecting samples that will help reveal what kinds of organisms built these massive reef systems and help shine light on what ecosystems were like almost 1 billion years ago, when most life was microscopic.

Using new approaches and technology, the team aims to collect more extensive information from rock samples than what’s been gathered previously. University of Washington professor Akshay Mehra developed a serial grinding mechanism that polishes a section of a stone, captures an image, then shaves a millimeter from its surface to be repolished and photographed again. These images are then turned into three-dimensional models that can help identify the reef’s original makers.

To better understand how the reef was built over time, Cohen’s team used drones to map the various parts of the reef system, from the original portion called the core to the rubble, pieces of rock that have fallen away over time.

“Mapping the reef system helps us to better understand its original size and geographic extent, and how it grew in relationship to the other sediments that were being deposited around it,” Cohen says. “This data will be used by the team as a part of our efforts to reconstruct the reef system.”

Working in such a remote location—where a helicopter could fly in only two to three people and their equipment at a time, and where grizzly bears, caribou and moose make their home—was grueling. Nadalin says she felt the mental and physical toll of the backcountry terrain.

“From spending whole days on loose talus slopes to scrambling up steep drainages and climbing towering 800-million-year-old carbonate reefs, the hiking was some of the most intense I’ve ever faced—not to mention all the rocks in my pack,” she says, referring to the samples the team brought back. “However, it was also the most rewarding work I’ve ever done and led us to some of the most beautiful places I’ve ever seen.”

Cohen and her colleagues returned to their respective schools with almost 500 pounds of rock samples to be sorted, cataloged and examined for fossils. They also plan to look at the isotopes of carbon in the specimens, which can reveal information about what the environment and climate were like more than 800 million years ago.

Cohen, whose research areas include paleontology and geobiology, plans to discuss her work on the Little Dal Group in her spring semester course, “The Co-Evolution of Earth and Life.”

“It’s helpful for students to see how I talk about process and how we actually know the things that we know—and to see examples from my own research, especially when other students are involved,” she says.

Adds Nadalin, who plans to keep working in Cohen’s lab this year: “I can’t wait to continue the learning process.” 

Top photo: A helicopter-eye view of the Stone Knife River flowing through the Mackenzie Mountains of Canada’s Northwest Territories. Photograph by Phoebe Cohen

Below: Photos from the team’s fieldwork in the Canadian Rockies, summer 2024. Photographs by Phoebe Cohen