Babies must crawl before they walk, parents and pediatricians agree. Crawling has also been held up as a prerequisite to the normal progression of other aspects of neuromuscular and neurological development, such as hand-eye coordination and social maturation. But new research may knock the legs out from under this conventional wisdom.

According to anthropologist David Tracer of the University of Colorado at Boulder, babies of the Au hunter-gatherers of Pa­pua New Guinea do not go through a crawling stage. Instead their parents and other caregivers carry them until they can walk. Yet Au children do not appear to suffer any ill effects from skipping this phase. In a presentation given to the American Association of Physical Anthropologists in Chicago this past April, Tracer argued that, in fact, not crawling may be entirely normal and possibly even adaptive.

In his observations of 113 Au mother-child pairs, Tracer found that babies up to 12 months old were carried upright in a sling 86 percent of the time. On the rare occasions when the mothers put their infants on the ground, they propped them up in a sitting position, rather than placing them down on their stomachs. As a result of spending all of that time upright, Au kids never learn to crawl. (They do, however, go through a scoot phase in which they sit upright and propel themselves along on their bottoms. Tracer says the Au believe that this scooting, rather than crawling, is the universal human prewalking phase.)

The Au are not alone in discouraging their children from crawling. Tracer notes that babies in a number of other traditional societies—including ones in Paraguay, Mali and Indonesia—are raised this way. Furthermore, he observes, neither do our closest living relatives, chimpanzees and gorillas, put their youngsters on the ground very often. Thus, it may well be that our early hominid ancestors toted their babies around, too, rather than letting them crawl.

Citing a study of Bangladeshi children showing that crawling significantly increases the risk of contracting diarrhea, Tracer proposes that carrying infants limits their exposure to ground pathogens. It also protects them from predators. He therefore contends that the crawling stage is a recent invention—one that emerged only within the past century or two, after humans began living in elevated houses with flooring, which would have been much more hygienic than dirt.

Wenda Trevathan, an anthropologist at New Mexico State University, agrees that babies were probably rarely placed on the ground in the past, adding glowing embers as another potential hazard. Tracer’s work “highlights how narrowly we view normal infant development,” she remarks, “and calls into question the tendency to judge all human infants on the basis of Western infants.”

Note: This article was originally printed with the title, "Hitching a Ride."