If you've ever wondered whether mammalian evolution has a speed
limit, here's a number for you: 24 million.
That's how many generations a new study estimates it would take
to go from mouse- to elephant-sized while operating on land at the
maximum velocity of change. The figure underscores just how special
a trait sheer bigness can be.
"Big animals represent the accumulation of evolutionary change,
and change takes time," said evolutionary biologist Alistair Evans
of Australia's Monash University.
Evans and co-authors revisit a fossil record dataset of mammal
body size during the last 70 million years, in a study published on
31 January in Proceedings of the National Academy of
Sciences. The data was originally used to describe the
evolutionary growth spurts experienced by mammals soon
after dinosaurs ceased to be Earth's dominant animals.
For the previous 140 million years, mammals had been rat-sized
or smaller. With dinosaurs significantly reduced, mammals had a
chance to fill newly vacant ecological niches, particularly that of
the large-bodied plant-eater.
In this context, size isn't simply a visible sign of change, but
a proxy for modifications to diet, metabolism and body structure.
To become big is to change, radically and fundamentally.
"How fast can all of these interconnected changes be made? This
to me is the main question that drives why maximum evolutionary
rates are fascinating," said Evans.
In the new study, Evans' team measures the time taken, in total
years and likely number of generations, for 28 mammal lineages to
become larger and smaller over the fossil record.
Odd-toed ungulates, including horses and rhinoceroses, had the
highest maximum rates of growth. (The largest land
mammal ever, the now-extinct Paraceratherium, was
part of this group.) Rodents placed in the middle of the pack,
while carnivores changed quite slowly, and primates even more
At the fastest observed terrestrial rates, going from rabbit- to
elephant-sized takes roughly 10 million generations, while the
aforementioned mouse- to elephant-sized jump takes 24 million
generations. In the oceans, however, body size could change twice
as fast, perhaps because water's support of body weight lessened
The researchers also found that mammals shrink more rapidly than
they grow, with size lost 100 times faster than it's gained. An
implicit conservation message: Treasure bigness, because it's
difficult to achieve, and won't likely happen again so long
remain Earth's dominant species.
"Very large land mammals need a huge area to be able to source
enough food," said Evans, 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," he said.