Marine Animals Just Getting Bigger and Bigger

Marine animals just keep getting bigger and bigger, and it's not because they are eating their Wheaties. New research finds that over time evolution favors ever-increasing body size - and not by chance.

This idea seems obvious given that humans, apes and other large creatures are colossal in size compared to the miniscule microorganisms that once dominated early Earth.

"We've known for some time now that the largest organisms alive today are larger than the largest organisms that were alive when life originated or even when animals first evolved," Jonathan Payne, a paleobiologist at Stanford's School of Earth, Energy & Environmental Sciences, explained in a press release.

But proving that evolutionary biology trends toward animal lineages that are larger in size is easier said than done.

"It's not something that you can know by just studying living organisms or extrapolating from what you see over short time scales," Payne noted. "If you do that, you will absolutely be wrong about the rate, and possibly also the direction."

So Payne and his colleagues chose to focus on life beneath the oceans, plotting the size of thousands of aquatic animal species since the Cambrian period until today.

It turns out in the past 542 million years the average size of a marine animal has increased 150-fold. This is the equivalent of a two-inch sea urchin jumping to nearly a foot long.

They also found that larger lineages tend to produce greater diversification over time. This pattern of increasing body size cannot be explained by random "drift," but suggests that bigger animals generally fare better at sea.

These researchers aren't the first to come up with this idea. In the late 19th century, paleontologist Edward Cope noticed increased body sizes of terrestrial mammals such as horses over time, and so that is when the seed was first planted. However, efforts to confirm Cope's rule among land mammals have ended up with inconsistent results. For example, the trend holds true for dinosaurs, horses and corals but not for birds and insects.

To account for any random drift, Payne's team used a new computer model to recreate the history of evolution for the more than 17,000 groups, or genera, of marine animals they studied.

"Our study is the most comprehensive test of Cope's rule ever conducted," said Noel Heim, a postdoctoral researcher in Payne's lab. "Nearly 75 percent of all of marine genera in the fossil record and nearly 60 percent of all the animal genera that ever lived are included in our dataset."

So if Cope's rule is right, then in the next several million years who knows how big things will get.

The results were published in the journal Science.

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