During the two years of its monthly appearance, this column has looked at many objects—cars, turbines, airplanes, windows, mobile phones, and nuclear reactors—made by humans. Today’s focus is on the human body, specifically the way it keeps itself cool.

Before the development of long-range projectile weaponry some tens of thousands of years ago, in Africa, our ancestors had only two ways to secure meat: by scavenging the leftovers of mightier beasts or by running down their own prey. Humans were able to occupy the second of those ecological niches thanks, in part, to two great advantages of bipedalism.

The first advantage is in how we breathe. A quadruped can take only a single breath per locomotive cycle because its thorax must absorb the impact on the front limbs. We, however, can choose other ratios, and that lets us use energy more flexibly. The second, and greater, advantage is in our extraordinary ability to regulate our body temperature, which allows us to do what lions cannot: to run long and hard in the noonday sun.

It all comes down to sweating. The two large animals we have mainly used for transport perspire profusely,…[Read more]