How long did it take evolution to turn a mouse-sized mammal into one the size of an elephant? A thousand generations? A million?
Actually, it's 24 million generations.
That's the "speed limit" of large-scale evolution, according to a team of biologists and paleontologists at Australia's Monash University. Their research, based on the fossil record and published in the Proceedings of the National Academy of Sciences, estimates the number of generations for 28 mammals to become larger and smaller over the past 70 million years.
But if it took 24 million generations for the elephant to get so big, scientists say it takes only about 100,000 generations for an animal to evolve into a significantly smaller size.
"The huge difference in rates for getting smaller and getting bigger is really astounding," study author Dr. Alistair Evans said in a written statement. "We certainly never expected it could happen so fast!"
With size lost taking so much less time than size gained, the scientists' findings emphasize just how rare bigness really is today - and how rare it is likely to remain.
Evolution of another huge animal like the elephant is unlikely to happen again as long as humans remain Earth’s dominant species, researchers say. The only reason we saw evolutionary growth spurts in elephants and other mammals was because dinosaurs ceased to be the dominant animals on our planet.
“Very large land mammals need a huge area to be able to source enough food,” Dr. Evans told Wired. "And there just isn’t enough remaining land. It’s likely that animals will not get enough food or live long enough to grow as large as they have, even compared to 100 years ago...Big animals represent the accumulation of evolutionary change, and change takes time.”