A hypothesis by a 19th century paleontologist that body size in the lineage of any animal -- and that includes us humans -- increases over time through evolution has been confirmed in a new study.

First proposed by American paleontologist Edward Drinker Cope, "Cope's Rule" holds that increasing body size in species is actively selected in evolution.

That's been confirmed in a Stanford University study reported in the journal Science, suggesting evolution does follow certain predictable rules  -- one of them being a tendency for organisms to increase in size as they evolve.

"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," says Stanford paleobiologist and study co-author Jonathan Payne.

There have been larger animals in the past, of course, like dinosaurs that evolved to be 85 feet in length, but the general principle holds that as any animal lineage evolves it tends to create ever larger specimens, as long as the environment, food sources and other factors are for the most part stable.

It's been true for humans; in Britain the average height of a 21-year-old man grew from 5 feet 5 inches in the 1870s to 5 feet 10 inches by the 1970s, research there has confirmed.

For the newest study, Payne and his colleagues put together a dataset of adult body sizes of individual species of more than 17,000 groups of marine animals.

"Our study is the most comprehensive test of Cope's Rule ever conducted," says lead author Noel Heim. "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."

Analysis of the data set showed the average marine animal living today is 150 times as large, in terms of mass, as the average ocean-dwelling creatures of 500 million years ago.

That includes the largest animals on the planet today, the blue whale, which can grow to 100 feet long.

So why the push to evolve to be always bigger? Heim has some ideas.

"It's easier to eat other animals if you're large. It's also easier to avoid being eaten," he says. "In water, larger animals can be more active because of the increased mass relative to their surface area. They feel less 'drag' than small animals. Larger animals also have a higher metabolic rate, which also contributes to a more active lifestyle."

If an evolutionary trend to bigger sizes is predictable, the researchers ask, what other rules might evolution be following?

"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," Payne says.