Published by the Students of Johns Hopkins since 1896
August 5, 2020

Ask a scientist: Why aren’t humans taller?

By Ann Wang | February 24, 2011

Why aren’t we taller? I usually ask myself this question several times a week, whether I’m hopping on my kitchen counters to get the salt, carting a stepstool around lab to reach the buffers on the top shelves, or when people standing next to me rest their elbows on my shoulders.

According to the CDC, the average height for both men and women in the US has increased about an inch in the last 50 years. But why is the average height for an adult American male just over five feet nine inches instead of, say, four feet? Or seven feet?

It makes intuitive sense that there would be a happy medium for human height.

For example, if Shaquille O’Neal and I were arm wrestling over a steak, there’s no question I’d be going hungry. On the other hand, I’d only need one steak to be full and I’m guessing Shaq would need something like six.

However, the answer to how we evolved to be as tall as we are has less to do with arm wrestling and more with how fast we can throw.

How you can throw a baseball goes up with the mass and length of your muscles — in other words, how tall you are. Our prehistorical ancestors probably didn’t engage in the great American past-time, but they did need to throw rocks and spears to hunt.

When you throw a rock, the force with which the rock hits its target (such as a juicy mammoth) is proportional to the square of how fast the rock is moving. More specifically, the trauma the rock inflicts is related to the rock’s kinetic energy — one half its mass times its velocity squared.

Factoring in your mass, which goes up with the cube of your height, your ability to kill prey increases roughly proportionate to your height to the fifth power.

So then why didn’t our ancestors get taller and taller?

You’ve seen little kids fall down, pick themselves up, and run off without a scratch. Can you imagine a giraffe doing the same thing?

The damage you do to yourself when you fall, like the damage you can do throwing a rock, also goes up with your mass and the square of the speed with which you hit the ground.

Additionally, the mass of food you need to consume goes up with your mass — which is proportionate to the cube of your height.

Since our ability to throw a rock or sphere increases with our height to the fifth power, a small increase in height meant a significant increase in hunting ability.

Combined with the negative consequences of increasing height, our ancestors found that they didn’t need to be all that tall to be effective hunters. Learning to dunk — that’s another story.

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