Some time in the past, a family sat on the top of the world and gazed at the stars. They lived on the Tibetan Plateau, 4200m (14,100ft) above sea level, in a site now known as Chusang. They called it home.
Although far from the comfort of more lowly climes, this location had its perks. Fuelled by the tectonic forces that raise and support the plateau, a hot spring at the surface provided a welcoming buffer against the chilled air. At night, the family lit fires in a hollow built into the slope, a lonely flicker against the peak of darkness.
Their fire has long gone out, but the family still left a lasting impression on the world. As they walked and played, 19 hand and footprints were pressed into the clay-like mud that seeped from the spring and, as they dried, were preserved into the present.
Judging by the size of the prints — and the hands and feet that made them — the family group contained six individuals, two of which were children. But who were they? And what brought them to such high altitudes? A foraging trip perhaps? Hunting? Or were they simply curious, always searching for lands untouched?
Their marks leave no answers to such questions. All that is known, as shown in a study from January 2017, is that the Chusang prints were made between 12,700 and 7,400 years ago, making it one of the oldest archaeological sites known on the Tibetan Plateau.
But what makes the Chusang family special is their isolation. Living at the centre of the plateau, they simply couldn’t migrate up and down the mountain with the seasons as other Tibetan people did during this period. They were here year-round, enduring the heavy snowfall, biting winds, and encroaching glaciers of winter.
Their survival is extraordinary. While the heat of fire could protect them from the cold, the family at Chusang couldn’t shelter from an obvious yet insurmountable obstacle of living on the plateau: the air becomes thinner with every step towards the sky. At more than 4,000m (13,000ft) above sea level, each breath contains around a third less oxygen than the same breath far below. But deep inside each of their bodies, within their blood and DNA, an ancient and unique trick to surviving at altitude protected them from the thin air in which they built their home.
Humans have occupied the Tibetan Plateau for thousands of years – but the secrets to their survival are only just being discovered by scientists (Credit: Getty Images)
Any mountain climber will be able to describe the shortness of breath that normally comes with altitude. It’s not that the air has a lower percentage of oxygen – it’s around 21% wherever you stand in the world. But air pressure decreases the further you walk or fly from the sea’s surface, allowing the gas molecules to spread out in all directions, and a lung can only stretch so far to compensate.
There are ways to deal with this change in pressure, however. Over many hundreds of generations, people living on the Andean altiplano that extends from Peru into Bolivia, have evolved barrel-shaped chests that increase the volume of each of their breaths. And since the late 1800s, scientists have known that their blood is pumped full of red blood cells and haemoglobin, the oxygen-carrying molecules, that they contain.
When the air is thin, the blood thickens to increase the amount of oxygen it can shepherd to cells around the body. This hematopoietic (literally, “blood” and “to make” in Greek) response is also found in anyone who decides to hike up a mountain. “Compared at altitude to Andean highlanders, we are pretty similar,” says Cynthia Beall, an anthropologist from Case Western Reserve University in Ohio. “Not completely, but the general response is the same.”
And since virtually all research on high-altitude populations was focused in the Andes, haematopoiesis was seen as a universal response to low oxygen levels for nearly two centuries. It was only in the late 1970s and early 80s, after hiking to seven villages in Nepal, that Beall started to find that Tibetans didn’t fit this theory.