As animals evolve they tend to get larger over time, researchers concluded in a sweeping study released Friday that tracked thousands of creatures over a half-billion-year period.
The mean body size of marine animals, for example, increased 150-fold over the past 542 million years, according to the study published in the US journal Science.
"That's the size difference between a sea urchin that is about two inches (five centimeters) long versus one that is nearly a foot (30 cm) long," said Noel Heim, a post-doctoral researcher at a lab at Stanford University's School of Earth, Energy & Environmental Sciences.
"This may not seem like a lot, but it represents a big jump."
Body-size increases have occurred since animals first appeared in the fossil record, some 550 million years ago, according to the study.
"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, a paleobiologist at the same Stanford school who led the research.
But it was not known if the average size of animals had changed over time and whether this reflected an evolutionary trend in body size, he added.
The phenomenon does not apply to all animal lineages, but it was particularly evident in those that were the largest and most diverse in the early evolutionary record, according to the researchers.
"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," Payne said.
The computer-modelled research reinforces the so-called Cope rule, named after the 19th-century paleontologist who determined that biological lineages tended to evolve to larger sizes over time.
Edward Cope observed that mammals like horses grew larger over generations.
Scientists had previously tried to verify Cope's theory in other groups of animals but the findings were mixed. Corals and dinosaurs seemed to confirm the theory, but birds and insects did not.
As a result, some scientists questioned whether or not the trend in terrestrial animals was a true evolutionary phenomenon.
Payne's team set about to conduct the most comprehensive-ever test of Cope's theory, compiling a database of more than 17,000 groups, or genera, of marine animals including arthropods and mollusks spanning the last 542 million years.
"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," Heim said.