What came after the dinosaurs? Well, land mammals went from small "vermin" to giant beasts in just 25 million years, according to a new study.
Writing in the journal Science, researchers say mammals rapidly filled a "large animal" void left by the demise of dinosaurs 65 million years ago.
They then went from creatures weighing between 3g - 15kg to a hugely diverse group including 17-tonne beasts.
The BBC's science reporter says the scientists believe further growth was capped by temperature and land availability.
Felisa Smith of the University of New Mexico in Albuquerque, USA, and colleagues looked at the fossil record of mammals to plot their course through the ages.
They concluded that the rise of the mammals was by no means inevitable, and owes most to the obliteration of the dinosaurs, be it by comet, asteroid or another event.
"Mammals actually evolved almost around the same time as dinosaurs, about 210 million years ago," she told BBC News.
"And for the first 140 million years, we were basically vermin scurrying around the feet of the dinosaurs and not really doing much of anything.
"A comet came and hit the Earth and killed off all dinosaurs... and mammals as a class probably had characteristics that helped them survive that impact."
She believes most were burrowers that lived through the ensuing environmental mayhem largely underground, feeding on whatever food they could find, be it plant or meat.
Mammals went "nuts"
"What came out of that impact at the end of the Cretaceous period was a bunch of really small mammals that were not particularly diverse, probably not much more than a kilo," said Professor Smith.
"But we had a giant Earth with nothing big on it anymore; and so I think that ecological opportunity allowed mammals to just go nuts."
"Going nuts" meant land mammals diverging in shape and size. Some mammals attained weights of 15-17 tonnes, including Indricotherium, a mammal related to horses, and Deinotherium, a member of the elephant family.
These weights were reached within about 25 million years of the demise of the dinosaurs, but the researchers say their weights then plateaued through to recent times.
This pattern was repeated across the continents.