Modern oceanic creatures are 150 times larger than their Cambrian-era counterparts, which roamed the seas about 542 million years ago.

This massive size gain lines up with an evolutionary trend promoting species diversification among the largest marine animals, according to recent study from Stanford University paleontologists

“We’ve known for some time now that the largest organisms today are larger than the largest organisms that were alive when life originated or even when animals first evolved,” said Stanford paleobiologist Jonathon Payne in a press release.

The purpose of the study was to test the validity of a scientific theory called “Cope’s Rule,” dubbed in honor of the 19^th century paleontologist Edward Cope. The rule is simple: animals’ evolutionary lineages tend to produce larger species as time passes. Larger creatures can simple avoid predators more easily, eat bigger prey and move more swiftly through the water.

To test Cope’s Rule, researchers examined the body sizes of 17,208 genera, or groups of species, and entered this data into a custom computer program meant to simulate evolution.

“As time marches forward, each species is assigned some probability of producing a new species, of remaining the same, or of going extinct, at which point it drops out of the race,” explains Payne’s postdoctoral researcher Noel Heim.

The team suspected that neutral drift, the “non-selective” natural fluctuation of body sizes within animal lineages, may have accounted for some change. However, the data trends told a different story.

“The degree of increase in both mean and maximum body size just aren’t well explained by neutral drift,” notes Heim. “It appears that you actually need some active evolutionary process that promotes larger sizes.”

While not all genera increased in size over time, bigger marine mammals showed striking rates of diversification “for reasons that we don’t completely understand,” reports Payne.

The study was originally published in the scientific journal Science.