Hey--Look Out, World, Here I Come

ELIANA, 14 MONTHS OLD, REFUSES TO WALK. SHE has never attempted to stand alone, much less take a step. The other four children in her play group, however, have all earned the right to be called toddler: they started staggering on two feet right around their first birthdays. Even Eliana's friend Rachel, not yet 10 months old, has taken a few precarious steps. But Eliana is seemingly oblivious to her playmates' advanced locomotion. A demon crawler, she is still perfectly content to navigate her Washington, D.C., house on hands and knees. Her parents, of course, are certain that they have the first healthy kid in human history who will never learn to walk.

They shouldn't worry. And not only because the annals of child development are replete with stories of "late walkers" or "delayed talkers." For generations, anxious new parents have sat up nights comparing their child's accomplishments with the all-important "milestones" mapped out by pediatricians and development researchers. Babies sit up at 6 months, for instance; they blurt out "ma-ma" and "da-da" by 9 months, and start walking at a year. But scientists now understand in more detail than ever before how motor and sensory skills develop. The new findings call into question the very idea of developmental milestones and suggest that it is not meaningful to use the labels "early" and "late" when it comes to a baby's accomplishments. That isn't to say broad guidelines are useless. Parents should be concerned if, say, their 10-month-old can't sit up by himself. But the best evidence now shows that each young brain forms the neuronal and muscular connections required for sitting and crawling, walking and talking, at its own pace. There is no prize for finishing first--and, in most cases, no need to panic just because your tot isn't keeping up with the junior Joneses.

Considering how cruelly uncoordinated humans are at birth, it's a wonder babies learn to walk at all. A newborn has virtually no control over his limbs. Except for the most primitive reflexes, like sucking and grasping, he cannot will his arms or legs in any direction. These reflexes originate in the brain stem, the only part of the brain that is fully functional at birth. But between the fourth and seventh months, as the cortex of the brain develops, these reflexes become inhibited. After a while, the primitive reflexes seem to disappear altogether, allowing an increasingly sophisticated progression of motor skills to take their place.

Before that can happen, the brain must learn to deliver precise commands to the muscles. And for that, neurons must be myelinated. A white, fatty substance that coats nerve cells like the plastic insulation on telephone wires, myelin keeps electrical signals traveling along a neuron rather than leaking out and dissipating. Myelin also prevents "cross talk," in which the electrical signal in one neuron interferes with that of a nearby one: thanks to myelin, a nerve impulse telling the right hand to reach up and scratch the nose doesn't accidentally move the left foot instead. At the same time that nerves are being myelinated, the rapidly maturing brain is forming and pruning synapses (junctions between neurons), creating well-organized networks out of a chaotic jumble of billions of nerve cells. It will be two years before all of a child's nerves are fully myelinated.

Newsweek subscription offers >

Sensory skills at birth are just as rudimentary. Newborns can usually distinguish between faces and other objects-and they can recognize their parents' voices even in the womb. A newborn can focus on objects no farther than 15 inches away, about the distance to his mother's face when he's bottle- or breast-feeding. He can track slow-moving objects, but loses them if they are more than 18 inches away. For the first few weeks, this is all the vision he needs--and about all his brain can handle. From there, vision improves gradually. By the seventh month, he has developed binocular vision, the ability to see in three dimensions. But it will take seven to nine years before he can score 20-20 on an eye chart.

As her neuromuscular and sensory systems mature, a baby at last gains some control over her wayward body by the age of 1 or 2 months. Placed on her stomach, she'll struggle to hoist her bowling ball of a head from the floor. Not long after that she will bring her chest off the ground. It makes intuitive sense that children develop control over their heads first. Without a steady visual field, they cannot develop hand-eye coordination and balance, both of which are prerequisites for crawling and walking. At 3 or so months, the baby will prop herself up on her forearms. She'll get control of her upper arms before her lower arms, her wrists before her fingers, her legs before her feet.

All of this is good practice for the next big step: rolling over, which a baby usually masters at anywhere from 2 to 6 months. To get it right, she must develop the ability to rotate her spine, something she's prepared herself to do with all the kicking and flopping over she's been doing since her earliest days. Eventually she shifts her weight so far to one side that momentum takes her completely over. The first few rolls are usually accidents. But by now she has enough control to repeat actions she likes. Much to her delight-and her parents' she can soon flip at will. Using muscles repeatedly gives them the strength and elasticity known as "muscle tone," which is as important as the development of the nervous system in producing intentional movement.

With muscles and coordination working in concert, babies can start working up to more demanding skills, like sitting up and crawling. This requires much more strength and balance than the relatively simple movements they've done up until now. Once they get the hang of it, typically by 7 or 8 months, it's usually not long before they're tipping forward on all fours for a little crawling practice. Most kids start out by rocking back and forth on their arms and legs. Eventually, they learn to put enough weight on one side that they free up one arm, then the other. Some never quite master the traditional crawl. They scoot along on their bottoms or drag their bellies across the floor using only their arms. "It's all part of baby problem solving," says Esther Thelen, professor of psychology at Indiana University. The point isn't to crawl for its own sake, "but to get someplace." About 15 percent of kids skip crawling altogether and move right on to walking. "Rather than go through gross motor development in some specific sequence," says Johns Hopkins neurodevelopmental researcher Bruce Shapiro, toddlers often "reach for their functional threshold" even if that means skipping milestones. In the case of early walkers, he says, "by the time the muscle tone comes in, they're ready to stand up."

Newsweek subscription offers >

THE PHYSICAL AND conceptual leap to walking can happen anywhere from 10 to 17 months. Why such a long span? It's a feat of balance and coordination that the brains and bodies of some children simply need longer to master. "It takes everything, including the big toe," says John Hopkins pediatrician Paul Lipkin. To prepare for the big step, most kids spend a few months "cruising" from one piece of furniture to the next, before shoving off from the coffee table and taking those first drunken steps.

What makes this elegant succession of skills all the more impressive is that otherwise helpless babies do so much of it completely on their own. "Blind children learn to walk," says Harold Klawana, a neurologist at Chicago's Rush Medical College. "They're not imitating anyone. The nervous system acquires that skill all by itself, as the body develops the anatomical and physiological sophistication to perform these tasks." Children are motivated by an irrepressible desire to reach beyond themselves. Sure, some get there a little quicker than others; but five years down the road you won't be able to tell which ones they were.

Hey--Look Out, World, Here I Come | News