Biologists have long debated whether bigger is better.
A Stanford team now has proof that evolution favors growth: the researchers found that many lineages of sea creatures have evolved into behemoths.
In one of the most comprehensive studies of the evolution of body size, researchers Jonathan Payne and Noah Heim found that the fossil record shows that a century-old — and much-argued — maxim, known as Cope’s Rule, is true.
Over the past 542 million years, the mean size of marine animals has increased 150-fold, they report last week in the journal Science.
That fat scallop on your dinner plate? Its predecessors were about one-quarter inch long.
“There has been this open question of whether animals get bigger, over time — but there’s been a lack of data,” said Heim, a postdoctoral researcher in paleontologist Payne’s lab at Stanford’s School of Earth, Energy and Environmental Systems. “We found that things are getting bigger. And bigger,” he said.
Cope’s Rule, named for a pioneering 19th century American paleontologist, argues that animals often start out small and get bigger over evolutionary time. For proof, one need look no further than horses. They started out no bigger than a house pet. Now they pull heavy wagons filled with beer barrels.
But this linear model for evolution fell out of favor starting in the 1970s, when a few major exceptions were exposed. The late eminent Harvard paleontologist Stephen Jay Gould dismissed it in his book “Full House,” saying it was not an invariable law of nature.
The Stanford team subjected Cope’s Rule to rigorous scrutiny. “Our study is the most comprehensive test of Cope’s Rule ever conducted,” said Heim.
With the help of dozens of Bay Area high school students and Stanford undergraduates, they measured more than 17,000 groups, or genera, of marine fossils. That’s three-quarters of marine animals in the fossil record — and almost two-thirds of all animals that ever lived.
Using calipers, students meticulously measured photographs and detailed illustrations of fossils in the 50-volume Treatise on Invertebrate Paleontology, revered as the bible of every invertebrate animal genus with a fossil record known to science. Then the data were analyzed using advanced computer models.
A biological trend emerged, revealing this: Cope was correct. Organisms get bigger.
And, even more profoundly: Evolution can be predicted.
In the warm, shallow and salty oceans that sloshed around the Western U.S., the typical sea urchin was about 2 inches long, said Heim. Now the creatures can reach nearly a foot.
The increase isn’t because each animal lineage grew steadily larger, they found. Instead, bigger species tended to trump small species, giving rise to more diversity and thus a broader array of giants.
There is this final twist: Humans overrule Cope’s Rule. Because hunters and fishers target large animals, they tip natural selection in favor of smaller animals, said Heim.
The million-year-old trajectory finally proven, “people might be playing a role in changing it.”