Marine Animals Have Been Getting Bigger Over Millions of Years of Evolution

Scientists have uncovered a new trend in evolution.  They've found that animals tend to get larger over time.

"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," said Jonathan Payne, one of the researchers, in a news release.

While researchers have known that body size goes up over time in evolution, what remained unclear was whether the average size of animals also changed and grew. They wanted to see if this refleted a trend in body evolution.

In order to find out, the researchers studied the body size of animals over time. It turns out that over the past 542 million years, the mean body size of marine animals has increased 150-fold; that's essentially the difference between a sea urchin that's about two inches long versus one that's nearly a foot long.

In fact, this increase in body size has occurred since animals first appeared in the fossil record about 550 million years ago. This isn't due to all animal lineages suddenly growing bigger, though; it's due to the diversification of groups of organisms that were already larger than other groups early in the history of animal evolution.

"That's also something we didn't know before," said Payne. "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."

The findings reveal that over time, body size has increased among animals. This reveals a trend in evolution, and even shows why it's occurred.

"The discovery that body size often does evolve in a directional way makes it at least worth asking whether we're going to find directionality in other traits if we measure them carefully and systematically," said Payne.

The findings are published in the journal Science.

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