Between two and five million years ago, something akin to a giant guinea pig roamed South America. Weighing about a tonne, it would have loomed large over its modern relatives – diminutive rodents such as mice and rats.

Such extraordinary contrasts in body mass are part of the story emerging from an international study led by Monash University and published in the Proceedings of the National Academy of Sciences (PNAS). The research tells the story of mammalian body size over the past 70 million years.

Body size plays a critical role in survival, explains lead author Dr Alistair Evans, senior research fellow with the Monash School of Biological Sciences. Being large, for example, can help you regulate body temperature in a cold climate. Being small can help you survive when there is fierce competition for food.

"Believe it or not, the ancestors of elephants were once as small as mice," Dr Evans says. "So we were curious to find out how long it would take a 20-gram mouse to evolve into a two-million-gram elephant … and vice versa."