The "eureka moment" for Australian archaeologist Dr Tim Maloney came as he hunched over the skeletal remains of a Stone Age hunter-gatherer who lived 31,000 years ago.
It had been backbreaking, painstaking work, sweeping away a quarter-teaspoon of soil at a time to expose an immaculately preserved skeleton.
The body had been buried about one-metre-deep inside an extremely remote limestone cave in the east of Borneo, only accessible by boat at certain times of the year.
But what the small team of archaeologists stumbled on was the find of a lifetime.
It made every meticulous stroke, and the days-long journey upriver into deep rainforest in the East Kalimantan region, worthwhile beyond their wildest dreams.
Lying inside the grave they were clearing was a skeleton where the young buried person's legs had been placed in a unique, resting flex position.
All the bones of the right leg, down to each of their five toes, was in perfect condition.
And so was the left, except for one glaring exception.
"I'll never forget the moments of brushing away at that area and thinking something is very exceptional and unusual here," Maloney exclusively told 9news.com.au.
"The left foot was completely absent.
"It was just an absolute perplexing, exciting mystery."
Paleo-pathological experts later reached an extraordinary conclusion to explain the missing foot, and the unusual distal bony growths on the remaining shin bones.
"It was deliberately amputated," Maloney said.
"So that was essentially the eureka moment."
It was evidence, Maloney said, of advanced Stone Age surgery, and it completely turned any assumed knowledge of ancient surgical procedures on its head.
"It changes the known history of human medicine," he said.
The findings, published today in Nature, suggested advanced surgical procedures were happening in tropical Asia tens of thousands of years earlier than previously recorded.
Before the Borneo cave find, the oldest known complex operation happened to a Neolithic farmer from France about 7000 years ago, whose left forearm had been surgically removed and then partially healed.
The surgeon who conducted the Borneo amputation, 31,000 years ago in the Upper Paleolithic period, must have had detailed knowledge of limb anatomy and surgical hygiene, and considerable technical skill, researchers said.
Before modern clinical developments, such as antiseptics, most people undergoing amputation surgery died from blood loss, shock or infection.
Regular amputation success was only achieved by medieval surgeons in Europe around the turn of the last century.
Based on trauma patterns on the bones found by Maloney, the study concluded the lower left leg had been amputated, probably when the person who researchers anointed TB1, was a child, aged in their pre or early teens.
Researchers couldn't determine the sex of TB1, but believed they survived the surgery and lived for another six-nine years, likely dying in their early 20s.
It was remarkable, Maloney said, that TB1 made it through surgery and went on to live for years in the mountainous, rainforest terrain; it showed a high degree of post-op care.
He said there was a "compelling case" that antiseptic and pain medications had been used by the surgeon, who likely operated with a very sharp lithic edged rock to cut the limb off.
The surrounding rainforest is "home to some of the highest biodiversity on planet Earth," Maloney said, "a great many of which have medicinal properties".
The team began to excitedly speculate as soon as they saw a foot was missing.
While continuing to carefully excavate the skeleton, recording, photographing and laser-scanning the remains, an animal attack and other theories were discussed.
"It's an area which is home to sun bears, crocodiles and anacondas," Maloney said.
"And there's a long list of potential causes of injury in this mountainous rainforest area that we were all speculating on at the time."
But amputation also got discussed, he said.
"Oh, that'll be outrageous if it was that, that'll change the course of known human medicine," is how Maloney remembered the conversation going.
"And that's exactly what it was, and represents."
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Later in a lab, scientists concluded trauma patterns on the left leg were similar to "a large metal blade or mechanical processes" found in modern amputations.
Blunt force trauma from an accident or animal attack typically caused crushing factures, a feature which was "absent" from TB1's left leg bone, the study said.
Instead, tell-tale bony growths showed signs of post-operative healing, researchers said.
There was also no evidence of infection in the limb, further pointing to antiseptic aftercare and ruling out the probability of an animal attack, such as a crocodile bite.
Researchers are still trying to ascertain exactly what kind of surgical tool was used to amputate the foot, but Maloney concedes it's unlikely they will ever know for sure.
Sharpened bamboo and marine shells were considered, but the study settled on flaked metamorphic stone, which has extremely sharp lithic edges and is found in abundance in the area, as the most likely tool.
Even today a small number of surgeons still use obsidian blades – a type of volcanic glass which has a cutting edge many times finer than even the best steel scalpels.
"Stone Age surgery is in fact very much alive and well," Maloney said.
He described the discovery as a "dream" and "the crème de la crème of finds".
Led by Griffith University's Maloney, Dr India Dilkes-Hall from University of Western Australia and Indonesian archaeologist Andika Priyatno, the team camped inside Liang Tebo for several weeks in February 2020, when they found and excavated Tebo1.
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They had been drawn there after remarkable cave art, dating back at least 40,000 years, was discovered on the rock walls, several years earlier.
That art, described by Maloney as "very compelling and captivating", was a clue that the cave could be holding something very special.
When TB1 died, Borneo was part of the Eurasian supercontinent Sunda.
That eastern edge of Sunda was the closest coastline for the movement of people towards Australia and the archipelago of the tropics.
So remote is the cave, the team had no idea a strange virus called COVID-19 was beginning to sweep the globe while they were uncovering TB1.
They completed the excavation "before a frantic evacuation", Maloney said, where the team made it back to their homes in Jakarta, Brisbane and Perth just 24 hours before international borders closed.
