The anatomy of middle childhood can be subtle. Adult teeth start growing in, allowing children to diversify their diet beyond the mashed potatoes and parentally dissected Salisbury steak stage. The growth of the skeleton, by contrast, slows from the vertiginous pace of early childhood, and though there is a mild growth spurt at age 6 or 7, as well as a bit of chubbying up during the so-called adiposity rebound of middle childhood, much of the remaining skeletal growth awaits the superspurt of puberty.

“Adulthood is defined by being skeletally as well as sexually mature,” said Jennifer Thompson of the University of Nevada, Las Vegas. “A girl may have her first period at 11 or 12, but her pelvis doesn’t finish growing until about the age of 18.”

The 18-year time frame of human juvenility far exceeds that seen in any other great ape, Dr. Thompson said. Chimpanzees, for example, are fully formed by age 12. With her colleague Andrew J. Nelson of the University of Western Ontario, Dr. Thompson analyzed fossil specimens from Neanderthals, Homo erectus and other early hominids, and concluded that their growth pattern was more like that of a chimpanzee than a modern human: By age 12 or 14, they had reached adult size.

Life for Neanderthals was nasty and short, Dr. Thompson said, and Neanderthal children had to get big fast, which is why they hurtled through adolescence at the equivalent of today’s chapter-book age. Our extreme form of dilated childhood didn’t appear until the advent of modern Homo sapiens roughly 150,000 years ago, Dr. Thompson said, when adults began living long enough to ease pressure on the young to hurry up and breed.

And what an essential luxury item middle childhood has proved to be. “It’s consistent across societies,” Benjamin Campbell, an anthropologist at the University of Wisconsin in Milwaukee said. “In middle childhood, kids start making sense.”

Parental expectations rise accordingly. “Kids can do something now,” said Dr. Campbell, who edited the special issue. “They can do tasks. They have economic value.”

Boys are given goats to herd and messages to deliver. They hunt and fish. Girls weave, haul water, grind corn, chop firewood, serve as part-time mothers to their younger siblings; a serious share of baby care in the world is performed by girls not yet in their teens.