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Study Measures Mammalian Evolutionary Growth

While everyone who believes in evolution is aware that all creatures, past and present, derived from a common ancestor,  not many consider the amount of time necessary. For instance, how long would it take for a “shrew-like” mammal, similar to a mouse, to evolve into an elephant?

This particular path in evolution was the subject in a recent study, evaluating the amount of generations necessary for this change to occur. The answer: a measly 24 million generations.

These researchers, led by evolutionary biologist Alistair Evans in Clayton, Australia, examined how maximum body mass increased among 28 orders of mammals on multiple continents during the past 70 million years. By comparing the sizes of the largest members of a mammal group at various points in evolutionary time, they were able to calculate the speed at which these mammals expanded.

 The evolutionary rate indicated by their calculations was actually slower than previously estimated. Former estimations had been based on smaller “microevolutionary changes,” and thus ranged from about 200,00 to 2 million generations.

“This tells us how much slower so-called macroevolution is compared to microevolution,” says Evans, explaining that small size changes can occur quickly, but larger-scale alterations require more time. To put this into perspective, “if we wiped out everything above the size of a rabbit, it would take at least 5 million generations to get to elephant-sized animals”, a 1,000-fold increase. That translates to about 20 million years.

One place in which this new calculation didn’t hold true was under the ocean. Whales went through evolutionary changes at a much quicker rate than their land counterparts. Due to the “constants of life in the sea,” the proportionally same increase in size would only take 3 million generations.

Evolution in the opposite direction also betrayed these calculations. For an elephant to evolve in to a mouse, per say, it would take one thirtieth of the time for generations to evolve the other way around.

Christine Janis, a palaeontologist at Brown University in Providence, Rhode Island, suggests that the “overbuilt” appearance of dwarfed mammals may be a clue as to why it is easier to become small. Size increases require the accumulation of skeletal traits to support a massive frame — such as stout limbs — but shrinking mammals bypassed these evolutionary obstacles because the body plan of a large mammal still functions at smaller sizes.

(via lyrasyndrome)