Researchers in Washington state said a sewage cleanup in Lake Washington caused a species of fish, the threespine stickleback, to evolve in reverse.

The researchers from the Fred Hutchinson Cancer Research Center and University of Washington said the cleanup, which began in the 1960s, left the small fish without their usual hiding places from trout, the Seattle Post-Intelligencer reported Friday.

The scientists said the need for protection led to the stickleback's reverse evolution into an ancestral version of the species with bony armor and prickly spines. They said they compared samples of the fish dating from 1957 until the present and found that while most fish previously had small amounts of plating, nearly half are now fully-plated.

As the water transparency increased, that made it easier for predatory cutthroat trout to see the sticklebacks better and they could capture them more easily, said Katie Peichel, a scientist with Fred Hutchinson's Division of Human Biology. The sticklebacks with the complete set of plates could escape so they were favored.

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