A Living Time Capsule Shows the Human Mark on Evolution

Imagine you’re a looking for a place to shoot a monster movie. The plot involves animals kept in suspended animation for seven centuries springing back to life. Chances are you wouldn’t pick South Center Lake for your location. The charming 898-acre lake sits on the outskirts of the small town of Lindstrom, Minn., known as America’s Little Sweden. Gothic it’s not.

But in real life, South Center Lake has become the setting for a remarkable resurrection. Scientists have revived shrimp-like animals that have been buried at the bottom of the lake for an estimated 700 years. If this estimate holds up to further testing, they are the oldest animals ever resurrected.

“The time frame is pretty remarkable,” said David M. Post, an evolutionary ecologist at Yale University who was not involved in the research.

What impresses other researchers about the new study, published today in Ecology Letters, is the detailed look it offers at the changes in South Center Lake over the past several centuries. They found that the evolution experienced a major jolt about a century ago, as Europeans transformed the landscape.

“Going back in time has always been science fiction,” said Luisa Orsini of the University of Birmingham in England, who was also not involved in the research. “But with these biological archives, you can actually do it.”

The resurrected animals are known scientifically as Daphnia and informally as water fleas. About as big as a grain of rice, the shrimp live by the billions in lakes. Each fall, some species produce eggs sealed in tough cases. The cases fall to the bottom of lakes, and the next spring many produce new water fleas. But some cases get buried in sediment, their eggs unhatched.

In the mid-1990s, Lawrence J. Weider, an evolutionary ecologist then working in Germany, figured out how coax the eggs to hatch. His first success came with eggs buried for decades in a German lake. Some of the revived animals were in such good shape they could reproduce in his lab.

In 2009 Dr. Weider, now at the University of Oklahoma, and his colleagues set out to resurrect eggs from some lakes in Minnesota. The chemistry of those lakes has been carefully documented for decades, making it possible to see how changes in pollution levels affected the water fleas.

To gather the animals, Dr. Weider and his colleagues took a boat out on the lakes. “It’s a smaller version of a party barge, with a hole cut out of the deck,” he said.

Through the hole, the scientists lowered a tube and pushed it about three feet into the sediment — deep enough, Dr. Weider thought, to gather water flea eggs a few decades old.

The scientists then went back to Oklahoma, sifted the cases from the mud, and started resurrecting the animals. They also extracted Daphnia DNA, giving them more data to analyze.

Only then did Dr. Weider get an estimate for the age of the sediment in South Center Lake from another lab.

“I said, ‘Are you kidding me?’” said Dr. Weider.

The lab concluded that the bottom of the lake’s sediment core was about 1,600 years old. The oldest eggs that Dr. Weider and his colleagues had successfully hatched were about 700 years old.

To estimate the age of the sediment, the lab measured levels of a radioactive isotope called lead-210. The researchers are now confirming the dates by measuring another isotope, carbon-14. While the dating remains provisional for now, Dr. Post said he was confident that the oldest of the water fleas lived before Europeans colonized the United States.

“This paper opens the door to the pre-colonial,” he said.

Dr. Weider and his colleagues realized they could look at the history of South Central Lake from long before Europeans had arrived, back when it was surrounded by prairies.

The researchers began studying the chemistry of the sediments, with a particular focus on one element: phosphorus. “Phosophorus is a key element for life, but there’s not that much around,” said Punidan D. Jeyasingh of Oklahoma State University, a co-author of the study.

For centuries, phosphorus was scarce in the lake. But in the late 1800s, its concentration grew drastically — a result of the fertilizer running off from new farms in the area.

That environmental shift coincided with a drastic change in the genes of the water fleas. The oldest DNA the scientists obtained from the lake dates back to around the time the Vandals were ransacking Rome. The scientists found that one genetic strain of water fleas dominated at the time — and continued to until the late 1800s. As phosphorus flooded the lake, a previously rare strain emerged and took over.

The scientists studied how the resurrected water fleas had used phosphorus to grow. When Daphnia from the Middle Ages fed on the substance, they could hold onto much of it for days — a good strategy for surviving in a phosphorus-starved lake. “It was good at the time that they lived,” said Dagmar Frisch of the University of Oklohoma, a co-author on the study.

The more modern water fleas, on the other hand, used a different strategy. When they were put in phosphorus-rich water, they didn’t bother to hold onto it for long. With so much phosphorus on hand, they didn’t need to waste that energy.

The new research adds to a growing number of studies indicating that humans are influencing the evolution of wild species. Along with water pollution, people are also creating new urban environments, fishing and hunting, and affecting the climate.

Dr. Orsini predicted that in the future scientists would find other biological archives that would shed more light on evolutionary impacts. By measuring changes in the past, it may be possible to predict how different species will cope with challenges in the future.

“You can understand how a species can evolve, and so you can understand how it may survive drastic changes,” she said.