A new study, published this week in the Proceedings for the National Academy of Sciences, estimates the time necessary for mammals to evolve from the size of mice to the size of elephants—about 24 million generations. Led by scientists at Australia’s Monash University, the researchers looked at how long it took large scale changes in body size to evolve, rather than small changes within species, and found that land mammals changed size more slowly than whales. While it took about 5 million generations for land mammals to increase in size 1000-fold, cetaceans increased by a similar factor in just 3 million generations. Researchers also saw that decreasing in size occurred about 30 times faster than increasing.

“This tells us how much slower so-called macroevolution is compared to microevolution,” lead author Alistair Evans of Monash University told Nature. “The kinds of short-term rates...

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