There's rarely time to write about every cool science-y story that comes our way. So this year, we're once again running a special Twelve Days of Christmas series of posts, highlighting one science story that fell through the cracks in 2023, each day from December 25 through January 5. Today: Swedish forensic artist Oscar Nilsson combined CT scans of frozen mummified remains with skull measurements and DNA analysis to reconstruct the face of a 500-year-old Inca girl.

In 1995, archaeologists discovered the frozen, mummified remains of a young Inca girl high in the mountains of Peru, thought to have died as part of a sacrificial ritual known as Capacocha (or Ohapaq hucha). In late October, we learned how she most likely looked in life, thanks to a detailed reconstruction by Swedish forensic article Oscar Nilsson. A plaster bust of the reconstruction was unveiled at a ceremony at the Andean Sanctuaries Museum of the Catholic University of Santa Maria in Arequipa, Peru, where the girl's remains (now called Juanita) have been on near-continuous display since her discovery.

"I thought I'd never know what her face looked like when she was alive," archaeologist Johan Reinhardt told the BBC. Reinhardt had found the remains with Peruvian mountaineer Miguel Zárate at an altitude of 21,000 feet (6,400 meters) during an expedition to Ampato, one of the highest volcanos in the Andes. "Now 28 years later, this has become a reality thanks to Oscar Nilsson's reconstruction."

According to Reinhardt, Spanish chroniclers made reference to the Inca practice of making offerings to the gods: not just statues, fine textiles, and ceramics, but also occasionally human sacrifices at ceremonial shrines (huacas) built high on mountain summits. It's thought that human sacrifices of young girls and boys were a means of appeasing the Inca gods (Apus) during periods of irregular weather patterns, particularly drought. Drought was common in the wake of a volcanic eruption.

During those periods, the ground on summits would unfreeze sufficiently for the Incas to build their sites and bury their offerings. The altitude is one reason why various Inca mummified remains have been found in remarkable states of preservation.

Earlier discoveries included the remains of an Inca boy found by looters in the 1950s, as well as the frozen body of a young man in 1964 and that of  a young boy in 1985. Then Reinhardt and Zárate made their Ampato ascent in September 1995. They were stunned to spot a mummy bundle on the ice just below the summit and realized they were looking at the frozen face of a young girl. The body was surrounded by offerings for the Inca gods, including llama bones, small carved figurines, and bits of pottery. Juanita was wrapped in a colorful burial tapestry and wearing a feathered cap and alpaca shawl, all almost perfectly preserved. Reinhardt and Zárate subsequently found two more ice mummies (a young boy and girl) the following month, and yet another female mummy in December 1997.

It was a bit of a struggle to get Juanita's body down from the summit because it was so heavy, the result of its flesh being so thoroughly frozen. That's also what makes it such an exciting archaeological find. The remains of meal of vegetables were in her well-reserved stomach, although DNA analysis from her hair showed that she also ate a fair amount of animal protein. That, and the high quality of her garments, suggested she came from a noble family, possibly from the city of Cusco.

There were also traces of coca and alcohol, likely administered before Juanita's death—a common Inca practice when sacrificing children. A CT scan of her skull revealed that Juanita had died from a a sharp blow to the head, similar to the type of injury made by a baseball bat, causing a massive hemorrhage. This, too, was a common Inca sacrificial custom.

Nilsson was able to draw upon those earlier analyses for his reconstruction, since he needed to know things like her age, gender, weight, and ethnicity. He started with the CT scan of Juanita's skull and used the data to 3D print a plastic replica of her head. He used wooden pegs on the bust to mark out the various measurements and added clay to mold the defining details of her face, drawing on clues from her nose, eye sockets, and teeth. The DNA indicated the likely color of her skin. "In Juanita's case, I wanted her to look both scared and proud, and with a high sense of presence at the same time," Nilsson told Live Science. "I then cast the face in silicone [using] real human hair [that I] inserted hair by hair."