While some animals—like dragonflies and penguins—have gotten smaller over time, sea creatures seem to be getting bigger.

This news may not be particularly shocking—after all, living creatures started out as microscopic, and have evolved into things like the blue whale, currently the largest creature on Earth. Sure, it took a couple billion years to make that progress, but it happened.

The idea that animals are always getting bigger is known as Cope's rule. Named after paleontologist Edward Drinker Cope, the rule suggests that animal lineages increase in body size over time. The hypothesis is based off the scientist's observations after looking at various fossils. The rule has been tested and documented for over a century, but a recent report published by Science provides the most convincing evidence yet.

A team led by Stanford University's Noel Heim analyzed more than 17,000 fossils of various marine animals, some as old as 542 million years. The study found that marine animals were, on average, 150 times larger than their Cambrian era counterparts.

"That's the size difference between a sea urchin that is about 2 inches long versus one that is nearly a foot long," said Noel Heim, a postdoctoral researcher in Payne's lab. "This may not seem like a lot, but it represents a big jump."

What's interesting is that not all the animals are getting bigger; this trend is being driven by larger animals surviving and diversifying to create more species.

"That's also something we didn't know before. For reasons that we don't completely understand, the classes with large body size appear to be the ones that over time have become differentially more diverse," said Jonathan Payne, a paleobiologist at Stanford School of Earth, Energy & Environmental Sciences.

The data leads scientists to believe that larger animals are evolutionarily favored, as there are many advantages to being greater in size. Larger size makes for a more formidable opponent, that can swim faster, burrow deeper, and eat larger prey.

"As time marches forward, each species is assigned some probability of producing a new species, of remaining the same, or of going extinct, at which point it drops out of the race," Heim said.

The study will hopefully encourage other scientists to look for other trends in evolution. If size is a clear driving force, then perhaps evolution moves in other directional ways.