Marine animals are becoming larger from evolution, according to a new study done by scientists, published the journal Science. Scientists have known that the largest animals on Earth are much bigger than the largest marine animals that were alive when complex life originated when animals first evolved. Johnathan Payne, a Stanford University paleobiologist, explained there was a factor that was unclear, whether the average size of marine animals had been increasing over time and what reflects the change in the evolution of body size. The study concluded that marine animals, over the past 542 million years, have increased in size by 150-fold.
According to scientists, they applied Cope’s rule, the hypothesis that animals continuously increase in size over time, to show the size increases in other animals. Cope’s rule has a few drawbacks, though. When marine animals reach a very large size, they are more prone to extinction. When using the rule to draw conclusions from the study, they realized that the rule applies perfectly to other animal groups, like dinosaurs and other land animals, but not to insects and birds. As a result, some scientists have wondered for a while if the growth pattern in land and marine animals is a true evolutionary phenomenon, or only a random, non-selective evolutionary characteristic, known as neutral drift. Payne explained this is the difference between a sea urchin that is only two inches long and a sea urchin that is one foot long. It represents a large jump in evolution, he said.
Payne’s research found that the increase in relative body size in marine animals first occurred around 550 million years ago, according to fossil records. Moreover, it is not due to all animal lines continuously getting bigger, but due to the diversification of different groups of marine animals that already became larger than others, due to evolution.
To test how marine animals became larger from evolution, Payne and his team of researchers used computerized algorithms to simulate body size evolution. Starting with smaller species from each phylum, the model showed that as time passes, a species is given a probability of creating a new species that either remains constant, or goes extinct, thus ending its lineage. When a computerized species was produced, the model allocated a new animal a body size that will be either larger or smaller than its ancestors. One scenario showed, due to neutral drift, that body size fluctuates randomly without affecting the future existence of the species. Another scenario assumed natural selection, theorized by Charles Darwin, in which having a larger body size means a better chance at survival as time passes and more generations are produced.
He explained the degree of the increase in body size and the maximum size allowed are not explained by neutral drift. Apparently, an animal needs some factor of active evolutionary process that produces larger sizes, Payne said. The researchers believe their computer model will aid the studies of other marine animals’ body sizes. For example, the want know whether organisms in tropical regions are larger or smaller than those inhabiting colder regions. Nonetheless, scientists have proven that animals, especially marine animals, have survived longer and grown larger due to the evolutionary process.
By Alex Lemieux
Photo by Graham Cook – Flickr License