Cope’s rule states that evolution favors larger bodies, suggesting that the average size of animals on the planet should always trend upward. The hypothesis has its proponents, but there has been scant evidence to support it.

Now, an expansive study of marine animals suggests that Cope’s rule may hold true, at least for life underwater.

Researchers at Stanford University spent four years combing through fossil records dating to the Cambrian Period 542 million years ago, ultimately compiling a data on body sizes for more than 17,000 genera of marine animals.

The scientists found that the minimum body size for marine animals has decreased by only 10 percent over that time, while maximum body size has increased by a factor of 100,000. Over all, mean body size has increased 150 percent, the researchers reported.