But in that time, the steel structures have also become a symbol of playground peril for many anxious parents and public officials who think they should be removed from parks and schoolyards to avoid the occasional bruises and broken bones.

A team of Dartmouth anthropologists takes a different view, marking 100 years since the jungle gym and monkey bars were patented by arguing that the iconic playground equipment and other forms of risky play exercise a biological need passed on from apes and early humans that may be critical to childhood development.

They write in the journal Evolution, Medicine, and Public Health that a trend toward “surplus safety” on playgrounds may come at the expense of children being allowed to independently test and expand their physical and cognitive abilities in a context in which injury is possible but avoidable.

“One of the ironies of modern parenting is that our children have never been physically safer and yet we have never been more worried about them. We need to consider the potential longer-term benefits of allowing them to engage in play where there is some level of risk so they can overcome challenges on their own and learn from it when it doesn’t work out,” says Zane Thayer, a co-author of the paper and associate professor of anthropology at Dartmouth.

“Generally, researchers have found that risky play helps children build resilience and confidence, skills that resonate throughout life,” she says. “We focus on jungle gyms and monkey bars as an easy way for children to engage in risky and thrill-seeking play.”

The researchers describe how the physiology — and fossilized injuries — of early humans show juveniles likely engaged in extensive swinging, climbing, jumping, and other risky play. The 3.3-million-year-old remains of a female Australopithecus afarensis child known as Selam exhibit shoulders, fingers, and feet adapted to climbing in trees and hanging from limbs, like modern apes. The 3.2-million-year-old skeleton of Lucy, an adult female of the same species, shows healed fractures thought to result from falls as high as 40 feet.

“Fossil evidence suggests that the children of early humans spent as much time in trees as adults did,” says Luke Fannin, first author of the paper and a PhD candidate in the Ecology, Evolution, Environment and Society program in the Guarini School of Graduate and Advanced Studies.

“If you’re spending all that time in trees as a juvenile, you need confidence, because falling from a tree can be devastating and possibly fatal for a large ape or hominin,” he says. “We see in modern nonhuman primates that juveniles test the limits of what they can and can’t do, what the risks are, and how to respond. That leads to the climbing skills we see in adults.”

The Dartmouth researchers cite a 2014 study reporting that infant and juvenile chimpanzees spend 15% and 27% more time, respectively, climbing and swinging than adults, which enhances their dexterity, skill, and awareness of their own mass. Though lacking the dexterity of other primates, modern humans are still competent climbers, Fannin says. People in hunter-gatherer cultures have been known to climb as high as 150 feet into trees to collect food.

“The past and the present point to children gaining physical and experiential skills by exploring their boundaries through play,” Fannin says. “Our physiology as children is still conducive to climbing, running, and jumping, as well as more easily recovering from injuries and short-distance falls.”

“It’s rare to see anthropology intersect so much with our daily lives,” Fannin says. “People don’t think about our ancestors very much, but play is a way that the past is reflected in the present.”

Nathaniel Dominy, the Charles Hansen Professor of Anthropology and study co-author, says that Sebastian “Ted” Hinton, the Chicago lawyer who patented the jungle gym and monkey bars in 1923 and 1924, also saw that reflection.

In one of his patents, Hinton wrote that children have a “monkey instinct” to climb as a form of play and exercise. Hinton lived during a fervor for the outdoors in the early 20th century that led to the establishment of the National Park Service, the plotting of the Appalachian Trail, and the creation of Scouting.

But Hinton saw climbing as a vestige of our simian lineage before that link was formally established, Dominy says. The remains of the Taung Child, a 2.8-million-year-old Australopithecus africanus that provided the first physical link between modern humans and ape-like ancestors, weren’t reported until 1925.

“Hinton was at the forefront of this cultural moment that embraced nature as essential to fitness, but it focused on bipedalism. Hinton described climbing as a product and necessity for childhood growth and development before we had the evidence for it,” Dominy says.

“One hundred years later, jungle gyms and monkey bars are still very much part of the conversation around childhood play. But the voice of anthropologists is nowhere in this debate, and that’s what we wanted to change,” Dominy says. “Our work shows how evolutionary theory has the potential to inform research and practice in the public health domain.”

Studies of hospital admissions show that jungle gyms and monkey bars result in more childhood fractures and hospital visits than any other playground equipment, the researchers report. But the risk of children being injured on a playground is relatively low.

The Dartmouth team cites a 2003 study that calculated the risk of playground injury at no more than 0.59 in 100,000, which is far less than injuries sustained through organized sports or even gym class. Another study found that 95% of children with playground injuries were treated and released between 2001-2013.

“Free play lets kids modulate activities to match their physical abilities and personal confidence,” Fannin says. “The rules and guidelines of free play develop on much longer timescales than supervised and organized sports where adults set the rules and expectations. Kids getting injured in organized sports has a lot to do with the social context in which they occur.”

But jungle gyms and monkey bars remain targets of efforts to make playgrounds safer, the researchers report. New York City removed them from most of their 862 public playgrounds in the 1980s and 1990s. While seven states have adopted the U.S. Consumer Product Safety Commission’s safety guidelines for monkey bars into law, enforcement is difficult, the Dartmouth team found. Municipalities find it easier to just remove the structures.

“We share the concerns of parents, school administrators, and policymakers in wanting to make sure our kids are safe. However, we also must consider the long-term benefits of engaging in this type of play,” Thayer says. “Risky play in which children challenge themselves is a normal part of our development, as it was for our ancestors.”