After trawling through a formidable amount of data, spanning almost 550 million years of evolutionary history, scientists have found that marine organisms have been evolving towards larger sizes since the Cambrian period. According to the comprehensive new study, by far the largest of its kind, the average size of ocean dwelling animals has increased by a factor of 150 over the past 542 million years.
As compared with the teeniest marine animals alive during the Cambrian, the minutest sea creatures alive today are smaller in terms of volume by a factor of 10; however, at the other end of the spectrum, the largest animals have increased in volume by more than a factor of 100,000.
But this trend towards bigger body sizes isn’t due to chance, nor is it simply due to the persistent selection for larger size. Instead, the researchers say, it’s due to the diversification of certain animal groups early on in their evolutionary history. This new study, conducted by Stanford scientists, has been published in Science.
Being big may have its drawbacks, such as requiring more food and space and resulting in longer generation times, but it also has clear advantages, like more efficient body heat conservation and fewer predators. Because of these perceived benefits, scientists have long contemplated whether animals tend to evolve towards larger sizes. Indeed, back in the 19th Century, paleontologists began to notice such a trend in certain land animals, such as horses, and coined the term “Cope’s rule,” after fossil expert Edward Drinker Cope, to describe such a pattern.
But while some animal groups, such as dinosaurs, exemplify Cope’s rule, others, such as birds, don’t fit the bill. This led scientists to speculate whether the inclination towards bigger body sizes is down to active selection of advantageous traits, or rather the result of random gene changes leading to so called “neutral drift” away from initially small sizes.
To find out more, scientists gathered 542 million years’ worth of body size and volume data on 17,208 marine genera, the rank above a species. And it didn’t take them long to notice a distinct trend throughout history. But it wasn’t that all marine animals have been getting bigger; rather, those that were already bigger tended to diversify more, leading to not only larger, but more varied life within these groups.
At this stage, it was still unclear whether this was driven by evolution or simply due to neutral drift, so the team came up with a computer model designed to play out different potential evolutionary scenarios with regards to body size. Each new species plugged into the simulation could either be smaller or bigger than its ancestor, or alternatively die out. They also added in different situations, such as allowing random fluctuations in body size which don’t affect the survival of the species as a whole. In another scenario, a bigger size was advantageous in terms of survival and was thus more likely to propagate.
From these simulations, it was clear that this trend could not simply be explained by chance, or neutral drift, and that active selection must have been at play. Now that the researchers know this is the case for at least one trait, they would like to look at others to see if they can identify any more trends throughout history.