Getting Big Takes Time…Millions and Millions of Generations, Say Biologists

By Veronique Greenwood | January 31, 2012 3:21 pm

Creatures as large as elephants are unusual; it takes a long time to evolve such size.

How long does it take for a mammal as small as a mouse to evolve into something as large as an elephant? A really, really long time, a recent study has found: about 24 million generations, at minimum.

To get that number, researchers looked at the evolution of body mass over the last 70 million years, after the dinosaurs went extinct and surviving animals expanded into the ecological niches they left behind. That estimate is far longer than earlier estimates, which, extrapolating from bursts of super-fast evolution in mice, range from just 200,000 to 2 million generations. Such speedy evolution, in actuality, is probably not sustainable over the long term—hence the lengthy new estimate.

Getting smaller, on the other hand, is a much shorter process, happening up to 30 times faster. Evolving to a smaller size might be easier because smaller animals reach reproductive maturity more quickly, the researchers suggest. Or it could reflect the greater availability of ecological niches for tiny organisms: a scrap of grassland can feed a fieldmouse, but an elephant needs acres. Certain physical constraints—the pull of gravity, for instance—might also make great size difficult to attain without unusual, physiologically expensive adaptations. Tellingly, the rate of growth changes when you look at marine mammals, which are supported by the water around them. Body size can grow twice as fast in the water.

Do we have any future behemoths scuttling around our feet today? Such extreme growth requires a lot of resources, the researchers say, so as long as humans are dominating the planet’s resources, it’s unlikely that anything will evolve to such glorious heights as elephants have. That’s another side effect of taking up most of the space: stunting future creatures’ growth.

Image courtesy of wwarby / flickr

  • Sunny D

    Elephants are AWESOMMEEEE!!!

  • James Harmer

    Why would any future behemoths be scuttling around our feet. Why wouldn’t they be us?
    Look at human size over the last few thousand years, and the trend is towards bigger people…

  • JD

    Elephants and large dinosaurs were created this way, large. They may make some small changes in evolution but nothing dramatic. God creates how he wants except many humans can not fathom this because they don’t believe in God.

  • Gary B

    At first glance, those numbers seem problematic to me. 24 million generations in 70 million years implies average generations of less than three years. Even discounting the point that elephants and mastodons (for example) have been around for a while themselves. For this to work the vast majority of these generations would have to be multiple generations per year, among tiny mouse-size creatures. Must we assume that 23.5 million of those generations happened in the first 30 million years?

    OTOH, one characteristic that shows up in many trends in living systems is the inverse power law. So if we apply that to the length of a generation vs. time, then this might just work. IIRC it already has been shown to apply to the size vs. population distribution of mammals. So, maybe after all.

  • Techs

    Humans in America are getting shorter while most of the rest of the world is still growing taller. Diet seems to be a major part of the reason not immigration. Of course if you measure sideways then Americans are still growing.

  • Zachary

    I was under the impression that while evolution is a constant process, certain drastic changes can be increased in bottle neck scenarios. How steady is the increase in size over time?


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