The study, published Thursday in the journal Science, finds that over time, the average size of marine creatures has increased by a factor of 150 times since the Cambrian period 542 million years ago, BBC News reports.
In the sometimes tempestuous explosion of divergent life forms near the start of this time window, size definitely favored bulkier creatures.
Today’s itty-bitty-est sea animal is less than 10 times smaller than its Cambrian counterpart when measured by volume, the BBC reports. Both give the word tiny a new definition, being sub-millimeter-in-size crustaceans. At the opposite end of the scale is the blue whale, which is more than 100,000 times the size of the largest Cambrian creature — a trilobite measuring less than a half meter long.
Size matters
The idea that natural selection possibly leads to animal lineages gaining weight over time isn’t new. The American paleontologist Edward Drinker Cope proposed the idea, now known as Cope’s rule, BBC News reports. Cope had plenty of experience with this — he and other 19th-Century paleontologists noticed that the prehistoric ancestors of modern mammals were often smaller. Horses, for instance, can be traced to propalaeotherium, which lived between 49-43 million years ago and hyracotherium, which lived 55-45 million years ago. Both of these primitive horses were only about the size of a small dog.
Not all critters grew larger over time, The Washington Post Notes. Birds are notable for being smaller than the dinosaurs they evolved from — and this was crucial for the evolution of flight. Scientists think birds shrank quickly, and this allowed them to exploit new resources and habitats.
It’s with marine creatures, however, that Cope’s rule really resonates.
Stanford University researcher Noel Heim wanted to see if the theory held true for creatures in the ocean’s depths, so he analyzed data regarding body size from every available source.
“We’ve had at least 50 high school students working on it over the last five years,” Heim told the BBC.
Not every kind of animal grew larger over time. Arthropods, such as crustaceans, got smaller. However, other creatures from less primitive species that grew larger had many more evolutionary offspring, The Washington Post reports. Species that are more modern descended from common ancestors that grew larger in size.
Heim wanted to make sure this was true, so he and his team ran computer simulations to see which species would survive, flourish, and reproduce when given different parameters. Some simulations allowed for random size variation, while others gave bigger animals an advantage over their smaller counterparts. What the researchers found is that the simulation that gave size as an advantage matched the fossil record the best, and this indicates that the size change wasn’t random.
“In the short term, there’s certainly evidence [the animals] will continue to get bigger,” Heim said, per The Washington Post, but for marine animals, if they become larger, it would become trickier for the creatures fo feed and find homes for their gargantuan bodies. So it’s an evolutionary advantage that has its limits.
So will sea creatures grow as large as super-tankers? No, not really, IFL Science points out. The minimum size hasn’t changed much, and when you switch from evolution to ecology, most species are small. This is really noticeable in the oceans because marine food webs are highly structured. It’s more along the lines of big critters eat little critters. It takes a multitude of small fish to meet the energy demands of a big fish, and this means that the only way that these food webs can be successful is if small organism outnumber the big guys by a substantial margin.
What Helm and his colleagues also show is that much of the overall increase in body size across marine animals can be explained by the evolution of major new groups, and these groups employed lots of anatomical and physiological innovation along the way. Within any existing group there is less of a drive towards larger sizes, and many of the animals that we consider ocean giants are already more or less as big as they could be, largely due to physical and physiological limits, IFL Science reports.
In another study, Sizing Ocean Giants, which was published earlier this year in the journal PeerJ, marine biologist Craig McClain and his colleagues document factors that often limit size in many of the more conspicuous large marine creatures. The study accounted for risks of tentacle tangling in jellyfish, metabolic constraints on giant clams, the physiological limitations faced by large bony fish when pumping water over their gills, and the reliance of blue whales on dense concentrations of their crustacean prey, IFL Science reports.
When it comes to most groups of marine animals, it’s unlikely that massively larger creatures will evolve any time soon. Cope may rule unchallenged, but a visitor to our future oceans is less likely to find whale-sized fish and supertanker whales than with new giants whose storied history we won’t know.
Send in the humans
If there’s one thing we’re good at doing, it’s removing large creatures wherever we live. McClain and colleagues note that in the case of manta rays (though this is also applicable to most exploited marine animals) “In the face of fishing pressure and other anthropogenic (human-caused) threats, it is likely that individuals in many populations may not be near their maximum possible ages or sizes.” In some species of fish, like plaice and cod, fisheries seem to have driven selection for smaller body sizes, and IFL Science notes that our burgeoning understanding of extinction risk in the oceans means that we shouldn’t take for granted the ongoing existence of the ocean’s giants.
We haven’t been around long enough to know if human-driven selection will continue to compete with Cope’s rule in the long run. As this new research demonstrates, previous mass extinctions have led to dramatic increases in body sizes among survivors. So perhaps once again, Cope’s rule will continue to rule following the current human-driven extinction event.
If we don’t eat everything out of existence, that is.
