Tuesday, January 31, 2012

Evolution of mouse-sized to elephant-sized animals

Life on Earth started off small. Nobody knows exactly how it came into being, but we do know bacteria were one of the first on the scene. Over millions and billions of years, life increased in size. Until about 65 million years ago, animals were much larger than we are used to nowadays. Around that time, something wiped out the dinosaurs, and much of Earth's life was lost. Evolution does not sit still, and our planet continued to 'produce' large animals. At the Monash University, studies were performed to uncover how long the evolutionary process needs to get a mouse-sized animal to evolve in an elephant-sized animal.

It takes about 24 million generations before a small mammal like a mouse evolves in something of the size of an elephant. That means a particular animal needs to procreate 24 million times before the evolutionary process makes them reach that size. Of course, there also needs to be a certain pressure in the environment to grow bigger: animals that grow larger in size should be more successful, and therefore have a better chance to procreate. The scientists tracked generations instead of years, because it allowed them to compare animals with different life spans: a generation is a universal measurement, while the concept of time has a different meaning for each animal.

However, when looking at the time evolution needs to make an animal shrink in size, the scientists found that this process is much faster. Only about 100,000 generations are needed to go from large to small. It seems when evolutionary pressure exists to become smaller, the consequences are much stronger than when there is a pressure to grow larger. The scientists give no explanation. It may seem unnatural, because larger animals are better able to defend themselves against predators, and therefore increase their chance on survival. However, a bigger body requires more energy to maintain. Apparently, getting big is not all that easy.

From their study, the scientists concluded it is much easier to achieve growth if you live in the water. That is perhaps not all that surprising, because weight and size have a much smaller impact than on land. Nature's biggest animal living today is also found in the oceans: the blue whale. It is also the largest animal ever known to have existed. This stunningly large animal typically reaches a length of about 30 meters.

It is one of the few studies that focus on large changes in evolution. Normally, scientists track rather small differences, which may lead to the formation of a new animal species. The study performed by Monash University provides an interesting insight in mammal sizes and evolutionary time. An interesting question would be whether currently living animals have the tendency to become bigger or smaller.

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