When it comes to evolution, bigger might really be better. A team of Stanford scientists analyzing the body sizes of marine mammals over nearly 550 million years have found that average body size has increased 150-fold.
The findings, published in the journal Science, provide evidence that animals tend to evolve toward larger bodies over time.
FOR THE RECORD
Animal evolution: In the Feb. 23 Section A, an article about the tendency of animals to get bigger over time said that scientists studied the evolution of marine mammals over a span of nearly 550 million years. The scientists examined many types of marine animals, not just mammals.
While the history of life is around 3.6 billion years old, most animal groups only appear in the fossil record around the Cambrian explosion, a dramatic diversification of animal life that occurred some 542 million years ago, and their descendants, great and small, fill the world today.
But Cope’s rule, named after paleontologist Edward Cope, argued that animal bodies would tend to get bigger over time. The idea came about in the late 1800s after paleontologists noticed this expanding trend in land animals such as horses.
The problem is, there doesn't seem to be a consistent pattern: For example, dinosaurs got bigger, but birds and insects didn't. This sense that animal sizes were generally increasing might actually be the result of random evolution that wasn't due to natural selection – a process called neutral drift.
To see if Cope's rule really held, the scientists analyzed marine animals as a whole, putting together a data set that included more than 17,208 different groups of marine animals. These groups represented five major phyla – arthropods (including crabs), brachiopods (creatures with hinged shells), echinoderms (such as starfish), mollusks (including sea snails) and chordates (which includes all mammals, fish and reptiles). The data covered the 542 million years since the Cambrian explosion and included 74% of the animal diversity in the fossil record, the authors wrote.
On average, body volume increased about 150-fold – but the extremes also tell an interesting story. The minimum body volume shrank a little bit (by less than a factor of 10), but the maximum body size spiked way up (by more than a factor of 100,000).
"The trends in minimum, mean, and maximum biovolume of marine animals are consistent with actively driven size increase and not consistent with simple neutral drift away from an initially small ancestor," the study authors wrote.
While some groups of species tended to get larger and others did not, the ones that grew actually became more diverse as time went on. That means that animal classes with a tendency to put on weight were more successful, and ultimately able to move into more niches in the environment. Among other advantages, larger animals are likely able to move faster as well as hunt for bigger prey.
Reptiles and mammals who returned to the sea have been the major heavyweights over the course of animal history, the authors noted – enormous reptiles prowled the seas during the Mesozoic era (which lasted from 252 million to 66 million years ago), and today’s oceans boast massive mammals (the blue whale, for one, is the largest animal on land or sea). There’s probably a good reason for that, the authors noted: Marine reptiles and mammals evolved from land animals, which breathe air instead of water.
"Air breathing is an exaptation [i.e., a characteristic that evolved but isn't considered an adaptation for its current use] that can explain the rapid and widespread attainment of large size in marine reptiles and mammals. Relative to water, air has 20 to 30 times the concentration of O2, is up to 100 times less viscous, has diffusion rates of O2 through membranes that are 300,000 times faster, and is about 1000 times less dense," the authors point out. "Thus, large animals are better able to meet their metabolic needs by breathing air than by breathing water."
While the study hints at a certain directionality when it comes to size in evolution, it’s still unclear exactly what factors may have played a role, the study authors wrote.
"A remaining question is the extent to which this differential diversification was enabled by intrinsic factors such as physiology, escalatory interactions between predators and prey, or changes in the physical and non-animal environment, such as oxygen availability or the amount and quality of primary production," the authors wrote. "Testing among these controls will be critical to understanding how the physical and biological environments combine to shape the evolution of global ecosystems."