Why Papua New Guinea’s Highlanders Differ Physically From Those Living Near Sea Level

In the rugged, remote mountain highlands of Papua New Guinea, more than a mile and a half above the sea, cool mornings produce a dense fog that permeates the tropical forest. Villagers tend small gardens, producing sweet potatoes, pineapples or bananas, and roam forests on the hunt for cuscus, a type of possum, and wild pig. From one village to the next residents speak different languages, some of the almost 850 heard across the polyglot paradise. For thousands of years this lifestyle continued relatively unchanged—but living at elevated altitude for so many generations may have physically changed the highlanders.

New research published today in PLOS ONE suggests that highlanders who have dwelled in Papua New Guinea’s rugged mountains for some 20,000 years show some interesting adaptations to life at high altitude. Individuals living among Oceana’s highest peaks are shorter, have slimmer waistlines and enjoy larger lung capacity when compared to lowlanders living closer to sea level. They also boast higher concentrations of hemoglobin, the proteins in red blood cells that carry oxygen throughout the body. Studies of other high-altitude communities from Tibet, the Andes and Ethiopia have uncovered genetic drivers behind physical adaptations to life in thin air. Exploring genomes in Papua New Guinea, searching for clues to the sources of physical high-altitude adaptations, is the intriguing next step and it might reveal another way our least-known close ancestors, the Denisovans, continue to influence human genes to the present day.

Modern humans reached the island, a stepping stone between Asia and Australia, perhaps 50,000 years ago. Since that time they’ve evolved in relative isolation from the rest of the world, and sometimes one another.

A recent large study found that Papuan genetics, like Papuan languages, are diverse. Sharp genetic divisions appear between highlanders and lowlanders starting between 10,000 to 20,000 years ago. The highlanders, who number some three million, also developed agriculture independently some 9,000 years ago, as long as 6,000 years before later migrations of individuals brought such practices to Papua New Guinea’s lowlands. “Despite this extended time of life at altitude, Papua New Guinean highlanders have been understudied in regard to altitude adaptation in comparison with other high altitude populations like Andeans, Tibetans or Ethiopian highlanders,” says Mathilde Mathilde André, a PhD student at the University of Tartu, Estonia, and lead author of new research exploring the effects of altitude in the highlands.

As part of the Papuan Past Project, a joint effort between researchers from the universities of Tartu, Toulouse (France) and Papua New Guinea, scientists recruited healthy adults from high-altitude communities on Mont Wilhelm, some 7,545 to 8,860 feet above sea level, and excluded those who didn’t have a full local ancestry dating back at least two generations. They tested whether a representative group of 70 highlanders were different from 89 compatriots who dwelled near the sea in Daru at elevations of less than 330 feet. The team studied and compared 13 different phenotypes that are known to have changed among other humans dwelling at high altitude—including body shapes and proportions, and pulmonary and circulatory functions.

Participants breathed through a spirometer with a turbine to reveal their lung function. They had their heart rate and blood pressure taken, and yielded small blood samples to be analyzed for hemoglobin concentrations. They stood for measurements of height, bodyweight, waistline and minimum/maximum chest depths during breathing exercises.

All these measurements were put through statistical analysis, controlling for effects like sex and age, to determine where differences might lie between highland and lowland groups. The results showed six areas where the two diverged. In the mean, highlanders are shorter, by more than 1.5 inches, and their waistlines are also slimmer by about half an inch. Highlanders also have significantly greater minimal and maximal chest depth, and much larger forced vital capacity—the amount of air a person can exhale after taking the deepest breath possible. Hemoglobin concentration is also higher among the highlanders, though the authors caution that malaria has also been shown to impact this measurement and could be a confounding factor in the differences between the two groups.

In the world’s other high places, communities who’ve lived at altitude for generations show various adaptations to that environment. In Peru and Bolivia, across the altiplano of the Andes Mountains, humans have distinctive barrel-shaped chests, the better to inhale more air, and oxygen, with each breath. But in Tibet individuals appear to have coped differently with life at altitude. They don’t have barrel shaped chests or high hemoglobin concentrations which make blood thick and viscous. Instead, thinner, low hemoglobin blood runs through their veins. While their blood isn’t able to carry as much oxygen the heart and circulatory stem have an easier time moving it around the body, which may make these humans less prone to altitude sickness. Tibetans seem to compensate by breathing more often. It’s also possible that instead of evolving to acquire oxygen more efficiently, their bodies have perhaps evolved to make do with a bit less oxygen.

Genetic studies have helped scientists uncover the ways in which some of these traits began and grew in importance with time. Searching for such clues is the next step in Papua New Guinea. The team has genomes from the same individuals used in the study of physiological differences, and will next comb through their DNA to look for genetic differences between highlanders and lowlanders that may be linked to the physical adaptations.

“Such strong phenotypic differences between New Guinean highlanders and lowlanders suggests that altitude might have acted on the New Guinean genome, as it did in the Tibetan and Andean genomes,” says co-author Nicolas Brucato, a biological anthropologist at the University of Toulouse. They’ll also look further afield, to see if any genetic oddities they find are present in other high-altitude populations, from the Andes to the Himalaya and the Ethiopian Highlands

One partially intriguing genetic puzzle has to do with the genetic inheritance of the Denisovans, close human relatives who left behind a strong signature in many living Asian and Pacific Island people’s DNA. Scientists don’t have many fossils to reveal what Denisovans looked like, but they’ve been able to trace their genetic legacy with DNA from just a few teeth and bits of bone.

Several research labs have identified a key hemoglobin-regulating gene in most Tibetans, called EPAS 1, which had its origin with the Denisovans, ancestors to both Tibetans and Han Chinese. Today the gene is seen in very few Han, among whom it seems to have dwindled over the millennia, but about four out of every five Tibetans carry it.

“New Guinean populations are known to have the highest genetic inheritance from Denisovans, leading us to question if Denisovan genetic sequences might have also helped for the adaptation to altitude of human populations in New Guinea,” Brucato says.

Cynthia Beall, a physical anthropologist at Case Western Reserve University who specializes in human adaptation to high altitudes and wasn’t involved in the research, notes that these genetic investigations could be especially interesting in Papua New Guinea, given the diversity of humans on the island. “One thing we’ve learned studying altitude elsewhere is that sometimes, as in the case of Ethiopia, closely related ethnic groups respond differently to altitude. So it’s possible that they’ll find things like that.”

But when it comes to how humans deal with high altitude, causation can be tricky to untangle. When individuals who live at sea level trek to high altitudes their own bodies begin to respond immediately, and in some of the same ways that can be attributed to evolutionary selection, like producing higher levels of hemoglobin. And some studies suggest that hemoglobin begins adapting to altitude almost immediately, and that those changes may last for months.

“This research presents an interesting opportunity to try to separate out acclimatization responses,” Beall explains. “Many of the traits they suggest could be acclimatization or developmental adaptions. Lung volumes are famous for being examples of developmental adaptations, that individuals from most all populations could achieve in a lifetime of exposure to altitude, particularly in people who migrate before adolescence.”

And of course, altitude isn’t the only factor that might influence changes, both biological and genetic, among humans who spend countless generations living in high regions. Papua New Guinea’s tropical, wet highland environment differs notably not only from the nearby lowlands, but also from the far flung, high-altitude regions like the Tibetan Plateau and the Andes where most studies of humans at altitude have occurred.

Communities on the Tibetan and Andean plateaus exist at altitudes of 13,000 feet or more, far higher than the highland villages of Papua New Guinea. Beall notes that makes this study interesting, because it fits into a range of altitudes which researchers generally don’t study. “Typically what people do is look for the biggest contrasts that they can find,” she says. “This group is studying a very interesting range of altitudes.”

Diets also differ widely and could be a significant factor in some of the observable differences among humans who live at altitude in different places. In Papua New Guinea, a relatively rich and diverse highland diet might be one reason why weights aren’t different among Papua New Guinea’s highlanders and lowlanders, as they are in some other areas where highland diets may be less diverse. Research has even shown that individuals who are iron sufficient and getting enough vitamin C are more likely to adapt effectively to altitude.

Before genetic analysis revealed how environmental factors help regulate the molecular pathways for responses like hemoglobin production, Beall says, she and others wouldn’t have suspected such a role for diet in influencing adaptations to altitude. The ongoing genetic analysis of Papua New Guinea’s highlanders might provide equally interesting insights into the Papuan past, and more understanding of the human body’s incredible ability to adapt to its environment.