Researchers investigate why a frozen Earth coincided with an evolutionary spurt

What happened during the "Snowball Earth" period is perplexing: Just as the planet endured about 100 million years of deep freeze, with a thick layer of ice covering most of Earth and with low levels of atmospheric oxygen, forms of multicellular life emerged.

Why? The prevailing scientific view is that such frigid temperatures would slow rather than speed evolution. But fossil records from 720 to 635 million years ago show an evolutionary spurt preceding the development of animals. Two University of Colorado Boulder scientists aim to help solve this puzzle.

If they succeed, they would not only help unravel an evolutionary mystery, but also extend the temperature record of Earth by 2 billion years.

Carl Simpson, a macroevolutionary paleobiologist at CU Boulder, has found evidence that cold seawater could have jump-started—rather than suppressed—evolution from single-celled to multicellular life forms. But to demonstrate that very cold temperatures could have sped up evolution, he needs an accurate temperature record from that period.

Temperature records using existing methods are accurate only to 500 million years ago. That could change, though: Lizzy Trower, a chemical sedimentologist, has developed a novel method of measuring global temperature from 500 million to 2.5 billion years ago.

Together, Trower and Simpson hope to test Simpson's hypothesis against temperature records from Trower's novel tool, and they recently won a $1 million grant to do so.

Both the fossil record and calculations based on a "DNA clock"—which calculates the age of current organisms based on the rate of mutations over eons—indicate that multicellular organisms emerged during Snowball Earth.

Simpson, who is an assistant professor of geological sciences and curator of invertebrate paleontology at the CU Museum of Natural History, has spent a lot of time since coming to CU Boulder trying to understand the connection between extreme, prolonged cold and evolution. He describes a breakthrough stemming from a "knuckleheaded" approach: "trying to imagine what the unicellular ancestor of an animal would have been experiencing" during Snowball Earth.

During this "cold, salty and dark" period, there would have been up to a kilometer of ice at the Equator, and liquid water below the ice would have been very cold, about -5°C (about 23°F).

"One thing that you learn about small organisms from a physics point of view is that they don't experience the world the same way that we do, as larger-bodied organisms," Simpson said. Unicellular organisms are affected by the viscosity, or thickness, of sea water.

The increase in viscosity—which increases as water temperature falls—could yield an evolutionary advantage to those single-celled organisms that clumped together, using their combined propulsion efforts to their mutual advantage. In his laboratory, Simpson and colleagues have found that a type of green algae responds as he hypothesized it would.

"And basically, that would trigger the origin of animals, potentially," he said.

How cold was it?

However, there is uncertainty about how cold it was and how much that cold varied during Snowball Earth. Current methods suggest that the average global temperature in this period was about 20°C, or 68°F, levels that wouldn't turn the planet into a snowball. That's where Trower comes in.

Trower, an associate professor of geological sciences, studies grains of sand made from calcium carbonate and called ooids. These sand grains can gather material and get larger as they roll around, "as opposed to any other type of sand grain, which generally just gets smaller the more it's transported around," she said.

Trower's idea was to explore whether the size of ooids could reveal things about the environments in which they formed. Ooids are affected by two kinds of processes: physical and chemical.

Physically, the sand grains are abraded as they roll around and collide with other grains. These abrasions and collisions make the grains shrink.

Chemically, the sand grains can grow with the precipitation of new minerals. Originally, Trower framed these reactions as reflecting the seawater in which they're forming. "So, for example, if it's more super-saturated with respect to these calcium carbonate minerals, then the rate of mineral precipitation is faster, and that might explain why you would get ooids that are larger."

But her calculations based on water viscosity didn't suggest that ooids would grow as large as they did during Snowball Earth. Giant ooids from this period have been found in some places worldwide. Trower is focusing on a form of calcium carbonate called ikaite, which forms only in very cold conditions and which was discovered in a Norwegian fjord.

The ooids built on these rare, cold-loving carbonate minerals can grow comparatively large, greater than 2 millimeters in diameter. Trower notes that ooids of this size and composition form only in certain temperatures; thus, the diameter of these ooids could be a proxy measurement of Earth's temperature for the last 2.5 billion years.

Answering a big question

With their funding, Trower, Simpson and colleagues will collect giant ooid samples from around the world, measure them and analyze the samples to determine the nature of minerals they were originally composed of.

"That, in turn, can tell us something about the chemistry and water temperature in which they formed," Trower said, noting that those results would be compared against the physical record.

The goal is to answer a big question: "Does the fossil record agree with the predictions we would make based on this theory from this new record of temperature?"

Undertaking such potentially ground-breaking research is both nerve-wracking and also quite exciting, Simpson and Trower said.

Anne Sheehan, professor and chair of the Department of Geological Sciences, praised the scientists. "The project benefits not only from the talent and creativity of Trower and Simpson but also from their willingness to step outside of their disciplines and take risks. This work exemplifies how cross-disciplinary collaboration can push the boundaries of Earth science and drive groundbreaking discoveries."

Nancy J. Stevens, professor and research institute director of the CU Natural History Museum, observed, "The origin of complex multicellular life is an exciting puzzle to solve, and it would be remiss not to point out how Trower and Simpson have selected a topic and approach that mirror the contemporary research landscape. Organisms able to join forces to unlock new solutions can navigate challenging environments, and ultimately evolve and thrive."

Trower and Simpson's work also has potential implications for the human quest to find life elsewhere in the universe, Trower said. If extremely harsh and cold environments can spur evolutionary change, "then that is a really different type of thing to look for in exoplanets (potentially life-sustaining planets in other solar systems), or think about when and where (life) would exist."

Provided by University of Colorado at Boulder