Compared with other animals, human babies take much longer to learn to walk. Does this have something to do with our big brains?

—Mahmoud Dhaouadi, via e-mail

John Bock, an anthropologist at California State University, Fullerton, provides a reply:

A HORSE can walk within an hour after birth. A newborn baboon baby can cling to its mother’s hair while she jumps through the trees. Even among our closest evolutionary relatives—chimpanzees and bonobos—babies are far more agile than their human counterparts. That’s because humans are born with brains that are largely immature, leaving babies with little control over their movements. This uniquely human attribute is the result of a lengthy evolutionary battle between big brains and narrow pelvises.

One of the first traits that differentiated humans from our ancestors was upright gait. There are several hypotheses about the emergence of this trait, but it seems to have offered a way to move more efficiently in open environments such as the savanna. Although our earliest human ancestors were very apelike in terms of their brains, their upright gait had changed their pelvis to look much like our modern one. This reshaped pelvis came with a narrower birth canal, making childbirth more difficult.

Meanwhile the new roaming grounds afforded advantages in acquiring resources and negotiating social relationships to those with flexible, problem-solving behavior. Over time, natural selection increased brain size in these early humans. But at some point, the selection for bigger and bigger brains collided head-on, so to speak, with the narrow pelvis. If babies’ heads got any bigger, they would get stuck in the birth canal and kill both mother and child. Although natural selection worked to maximize what could be done—for instance, babies’ heads compress as they twist their way around the bones in the pelvis—there simply is not enough room for a big, mature brain to pass through.

As it turned out, the evolutionary answer was to let the brain keep growing outside the womb before it matures. So in contrast to other mammals, humans have a good bit of development to do after birth. The result is a relatively undeveloped infant who needs lots of care and can do much less for itself than other newborn primates.

Physiologically, why is the sound of fingernails on a blackboard so unnerving? Is this effect particular to human beings, or are other creatures similarly affected?

—Rowan Snyder, via e-mail

Neuroscientist Josh McDermott of New York University explains:

PROBABLY A COUPLE of factors combine to make such sounds unpleasant. The first, perhaps unsurprisingly, is the presence of high frequencies. The range between two

and four kilohertz—approximately that covered by the highest octave of a standard

piano—seems to contribute the most to the nastiness of the sound. It is unclear why people tend to fi nd these frequencies unpleasant, but we know that noise-induced hearing loss most commonly occurs in roughly this region, so it is conceivable that the aversive reaction partly reflects the ear’s vulnerability.

The spectrum of screeching sounds is also much noisier than that of an instrument; that is, there is a strong random component to the sound. The noisiness probably results from the fingernails repeatedly catching on part of the chalkboard surface before sliding forward.

This catching and sliding also causes rapid fluctuations in intensity, giving the sound a “rough” character.

Roughness is known to be unpleasant—car manufacturers, who aim to produce minimally unpleasant engine noise, for instance, find that smooth sounds with minimal variation in intensity are preferred by listeners over those that are rough. It’s a bit harder to say why sound roughness is considered unpleasant—as far as we know it is not harmful to the ears.

Some scientists have proposed that screeching sounds are acoustically similar to screams, a sound to which we might plausibly have evolved an aversion. If this hypothesis were true, one might expect to find similar reactions in nonhuman primates, which also produce screams. Thus far only one species of monkey has been tested, and it did not

display the same aversive response to screeches that humans have. It may therefore be more appropriate to simply regard screeching sounds as a “perfect storm”—combining two properties that we know to be unpleasant, resulting in a single sound that is awful to listen to.

Note: This article was originally printed with the title, "Ask The Brains."