How fast can evolution increase an animal’s size? An international team of scientists led by Alistair Evans of Monash University in Victoria, Australia, recently set out to answer that very question. Their study puts evolution in perspective and is the first to measure how fast large-scale evolution can occur in mammals. Their exploration of increases and decreases over the eons led to several impressive estimates. It takes “only” 24 million generations for a mouse-sized mammal to become as big as an elephant. Ocean mammals can increase in size twice as fast as terrestrial mammals—most likely because animals with large surface areas can spread their weight out for extra support in the water. However, this process in reverse—a decrease in size—occurs 10 times faster for both sea and land creatures. Shrinking so much faster probably has to do with the relative benefits of becoming more compact. Animals faced with limited resources survive better if they are smaller and need fewer nutrients to subsist, and a shift toward this more advantageous structure would theoretically take a shorter amount of time (think natural selection). But what about humans? An overwhelming amount of evidence suggests that we have been getting taller over the centuries—we are, on average, about 4 inches taller than we were 150 years ago. But unlike the long process of animal-size change, this short-term change that we’re experiencing probably isn’t due to natural selection. Childhood nutrition has seen a great improvement during the past two centuries, a development that has had more immediate impact on stature. So if you want your descendants to grow up strong and tall, no need to wait 24 million generations—just feed them well!

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Find the original article in the Proceedings of the National Academy of Sciences.

Read more about human height change from a 1998 posting at

Published On: April 10, 2012

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