Scientists hve resurrected animals that are known scientifically as Daphnia and informally as water fleas. About as big as a grain of rice, these shrimp-like organisms live by the billions in lakes. Each fall, some species produce hard-shelled eggs which fall to the bottom of lakes, and the next spring many produce new water fleas. But some of these eggs get buried in sediment, 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.