PALO ALTO, Calif., Feb. 19 (UPI) -- Evolutionary biology is an especially complicated field. Understanding the trajectory of evolutionary lineages is hard enough, and predicting them is near impossible.
But a new study purports that one component of evolution -- one trend -- is consistent: Species keep getting bigger and bigger. According to scientists at Stanford University, marine animal lineages evolve to feature larger species over time.
It seems obvious. Apes and humans, and the other larger hunters and gatherers that dominate the modern landscape, are certainly much bigger than the microorganisms that emerged from the primordial ooze of ancient Earth and jump-started it all.
"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 University's School of Earth, Energy & Environmental Sciences, explained in a recent press release.
But proving that an evolutionary tendency toward size really exists over the course of biological history is a tall order.
"It's not something that you can know by just studying living organisms or extrapolating from what you see over short time scales," Payne said. "If you do that, you will absolutely be wrong about the rate, and possibly also the direction."
Payne and his colleagues aren't the first to suggest that average animal body size has been trending upward. What's known as Cope's rule -- named after Edward Cope, one of America's most prominent early paleontologists -- was conceived in the 1890s after paleontologists, including admirers of Cope's work, observed that terrestrial mammals like horse species seemed to grow in size over time.
But over the last 100-plus years, efforts to confirm Cope's rule among land mammals has produced inconsistent results. The rule holds true for dinosaurs, horses and corals but not for birds and insects.
Payne and his colleagues chose to train their scope on animals of the sea. In plotting the size of thousands of aquatic animal species stretching across 542 million years, the researchers found average body size increased 150-fold. They also found that larger lineages tended to produce greater diversification over time.
"That's also something we didn't know before," Payne said. "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."
In the past, scientists have argued that any upward trends in animal body size were the result of neutral drift, or chance. Bolstered by the research that showed neither birds nor insects were getting larger over time, neutral drift proponents contended that lineages could grow in size as the result of chance, but not as a result of selective or evolutionary advantages.
But Payne and his colleagues built a computer model to recreate the history of evolution, testing different theories about evolutionary trends. This allowed them to account for the possibility of neutral drift, and he says the numbers confirm Cope's rule.
"The degree of increase in both mean and maximum body size just isn't well explained by neutral drift," said Noel Heim, a postdoctoral researcher in Payne's lab. "It appears that you actually need some active evolutionary process that promotes larger sizes."
The new research was published this week in the journal Science.