Small winners in the mammalian race to evolve


It takes at least 10 times as many generations for a mouse to reach elephantine proportions as for the reverse transition, reveals a vast study of mammalian evolution over the past 70 million years.

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.”

Written By: Julian Cribb
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  1. Hogwash.
    Indricotherium died out because it was too big to fit on Noah’s Ark so got left behind to drown.

  2. There was a National Geographic article (based on the work of Monash University)  in August 2012, picking out some key points – but it does not seem to be available on line.

    It suggested that it took 1.6 million generations for a sheep-size land animal to increase to the size of an elephant, but only 0.1 million generations for an elephant-size land animal to decrease to the size of a sheep.

    It also suggested, 5 million generations for a rabbit size animal or 24 million generations for a mouse size animal, to increase to the size of an elephant.  It takes about 10 times longer to scale up than to scale down.

    The problems seem to be re-engineering hearts, eyes, metabolism, and diet, with limbs needing to carry more weight.

    Whales can grow twice as fast as the water bears their weight. – 3 million generations for a dolphin-size aquatic to increase to the size of a Blue Whale.

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