What is the latest theory of why humans lost their body hair?

—J. Yablon, Adelaide, Australia

Mark Pagel, head of the evolutionary biology group at the University of Reading in England, replies:

Scientists have suggested three main explanations for why humans lack fur. All revolve around the idea that it may have been advantageous for our evolving lineage to become less and less hairy during the six million years since we diverged from the common ancestor we shared with our closest living relative, the chimpanzee.

The “aquatic ape” hypothesis suggests that six million to eight million years ago, our apelike ancestors had a semiaquatic lifestyle, foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to replace it, as other aquatic mammals have, with relatively high levels of body fat. Imaginative as this explanation is—and helpful in excusing our girth—paleontological evidence for an aquatic phase of human existence has proved elusive.

A second theory is that a loss of fur allowed for better control of our body temperature when we adapted to life on the hot savanna. Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated. This idea seems sensible, but even though hairlessness might have made it easier to stay cool during the day, our ancestors also would have lost heat at night when they needed to retain it.

Recently my colleague Walter Bodmer of the University of Oxford and I suggested that natural selection favored nakedness because it reduced the prevalence of external parasites. A furry coat provides an attractive safe haven for ectoparasites such as ticks, lice and biting flies. These creatures not only bring irritation and annoyance but also carry an assortment of diseases, some of which can be fatal. Humans, being capable of building fires, constructing shelters and producing clothes, would have been able to lose their fur, and thereby most of their parasites, without suffering from the cold.

How can an opera singer be heard over the much louder orchestra?

—A. Dean, Elgin, Ill.

John R. Smith, a physicist at the University of New South Wales in Sydney, Australia, explains:

Opera singers are able to maximize their sound output in frequencies where the orchestra is less powerful and to which the ear is more sensitive.

In both speech and singing, we produce sustained vowel sounds by using vibrations of our vocal folds—small flaps of mucous membrane in our voice box—that periodically interrupt the airflow from the lungs. The folds vibrate at a fundamental frequency, which determines the pitch: typically between 100 to 220 hertz (Hz), or vibrations per second, for normal speech and 50 to 1,500 Hz for singing. Speech and singing also contain a series of harmonics, which are basically multiples of that frequency. Singers, especially sopranos, can learn to tune the resonances of their vocal tract to match the fundamental frequency, providing a dramatic increase in acoustic power.

An orchestra is typically loudest around 500 Hz, with the sound level dropping off quickly at higher frequencies; the ear is most sensitive around 3,000 to 4,000 Hz. Many opera singers learn to increase the power in the harmonics at frequencies above 2,000 Hz, which helps to distinguish their voices.

Finally, opera singers often use much more vibrato—a slow, cyclic variation, or “wobble,” in pitch—than orchestral musicians do. This effect aids the signal processing within our auditory system in distinguishing the voice of a singer as something quite different from the accompaniment of the orchestra.

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