During Earth’s Cambrian period, around 542 million years ago, a foot-and-a-half-long trilobite was the biggest animal living in the ocean. That’s 100,000 times smaller than the blue whale—the largest creature in the world today, in the water or on land.
Now, a new study published in Science has found that, on average, most marine animals have considerably increased in size over the epochs—apparently as a result of natural selection rather than chance.
As BBC News reports, researchers have discovered that “the average size of a marine animal has gone up by a factor of 150” over the last 500 million years. The findings were gained by gathering information on more than 170,000 different marine species. By combing through volumes upon volumes of scientific records over a five-year period, the team was able to amass body-size data from “over 60% of all the animal genera ever to have lived.”
All of that info was used to test out a 19th century theory hypothesizing that evolution eventually leads to bigger animals. The proposal is known as Cope’s rule, and it has been previously applied to land animals like the horse, which has grown to its current proportions from the dog size of its ancestors living some 50 million years ago. But on land, at least, this isn’t a consistent pattern.. As BBC News explains, “Most groups of dinosaurs got bigger until they died out - but the birds that evolved from them grew smaller and lighter with the necessity of flight.”
But Cope’s rule had never before been applied to such a vast array of marine life. When it was, researchers found that the trend towards greater size was widely present. But, as lead researcher Noel Heim, from Stanford University, points out, “the consistent trend does not mean that every single genus of animals evolved to grow larger.”
Instead, the branches of the family tree that were populated by larger animals divided many more times - diversifying and expanding so that the ocean gradually built up a greater variety of bigger and bigger beasts.
To be sure that their findings were actually due to natural selection and not mere chance, Heim and his team applied the data to a computer program set up to simulate the evolution of an animal’s family tree. The software permitted species to die out, remain the same or change size, and, in some cases, allowed for animals’ sizes to randomly “drift” without influencing the species’ success.
In the end, they discovered that chance just doesn’t explain the change in animal size. "It appears that you actually need some active evolutionary process that promotes larger sizes," Heim told BBC News.
Researchers aren’t completely sure yet why, exactly, bigger is evolutionarily beneficial, but there are some theories. Larger sea creatures may be better at swimming quickly and eating larger prey. The changing oxygen levels of the ocean over time may also be involved.
And it isn’t just large mammals like the blue whale that have gained girth over millions of centuries. Heim says fish and invertebrates have also gotten bigger. This might mean that our far-in-the-future relatives could be facing quite the horror show should the colossal squid, the sea’s largest known invertebrate, adhere to the growing trend.