Scientists from Monash University's School of Biological Sciences have discovered a mouse can actually grow to be as big as an elephant, after several million generations!

The news doesn't, of course, mean a mouse can actually evolve into an elephant (or an elephant-sized mouse) but it does suggest a time frame of sorts for the evolution of different sizes of animals on Earth.

The researchers estimate a mouse-sized animal may take 24 million generations to evolve to elephantine sizes; a rabbit-sized creature, on the other hand, may take only 10 million generations. Essentially, the scientists have measured the relative speed of large-scale evolution in mammals.

As many as 28 different groups of mammals were considered (including elephants, primates and whales) from various continents and the ocean basin, over the past 70 million years. The creatures' sizes were tracked in generations, rather than years. The results suggested intra-species changes happened faster at higher-level groups but the change didn't last long. As a result, there was a rapid increase in the sizes after the mass extinction of the dinosaurs.

The study also suggested that the amount of time a relatively large mammal needed to shrink in size depended on the target size - a large mammal could take up to 100,000 generations to become as small as a mouse; it could, however, take considerably less to reduce to a size smaller than its original one but larger than a mouse-like creature. One of the fastest growing groups of aquatic mammals - whales and dolphins - were actually land mammals and actually smaller in size.

"The huge difference in rates for getting smaller and getting bigger is really astounding - we certainly never expected it could happen so fast," said Dr. Alistair Evans of Monash University's School of Biological Sciences.

"Instead we concentrated on large-scale changes in body size. We can now show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change - a massive change, but also a very long time," he said.