Daniel Potts, who worked with Glass at the Rocky Mountain Fur Company, wrote a letter to his friends in 1824 about an unnamed man who, after narrowly surviving a skirmish with a Native American war party, "was allso tore nearly all to peases by a White Bear and was left by the way without any gun who afterwards recover'd."
While Potts never mentions Glass by name in the letter, that sentence formed the backbone for what became one of the most popular and enduring tales of the time. But separating the facts from the myths about Glass's ordeal – assuming it ever happened in the first place – has been a matter of debate among scholars.
"We know Hugh Glass existed and we're pretty sure he got mauled by a grizzly bear," University of Notre Dame history professor Jon Coleman tells PEOPLE. But beyond that brief description in Potts' letter, Coleman says, "we don't know anything verifiable."
The mystery surrounding Glass, from his life to how he survived the attack, "might be why it's become such a fabulous story," Coleman explains. "There was plenty of room for invention from the very start." Any additional details from the story beyond Potts' letter come from second- and third-hand sources of varying reliability.
Warning: The following contains spoilers about the plot of The Revenant.
What is certain is that in 1823, a group of traders from the Rocky Mountain Fur Company were attacked by the Arikara Native Americans near the Missouri River in present-day South Dakota. The battle, reenacted in the beginning of The Revenant, killed roughly 15 fur traders and sent the rest into retreat.
After the battle, Glass, then 43, was supposedly sent ahead of the survivors to hunt for food and soon found himself face to face with a grizzly bear. "He managed to kill the grizzly but as far as his friends were concerned it killed him right back," Bruce Bradley, author of Hugh Glass, tells PEOPLE. "The bear had ripped his scalp half off, tore his throat open, ripped his back to shreds with its claws, broke his right leg, bit off a chunk of his right buttock and chewed up his left arm."
Convinced Glass would die from his injuries, and worried the Arikara were in pursuit, the men asked for two volunteers to stay behind and prepare a burial. The two volunteers are popularly believed to have been John Fitzgerald, played by Tom Hardy in the film, and an 18-year-old blacksmith named Jim Bridger, who would go on to become one of the most famous mountain men of the time.
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The two are said to have stayed with Glass for five days before Fitzgerald convinced a reluctant Bridger to leave him behind, fearing they'd be killed too if found by the natives. When Glass finally woke up, he was alone, badly wounded and lost in the cold wilderness.
Fitzgerald and Bridger left Glass with nothing to defend himself. "In those days no one would leave tools lying around for Native Americans to use so they had to take his rifle, his knife, his tomahawk, anything he would've needed to survive," Bradley explains.
The author reckons Glass began his roughly 300-mile trek from around what is now Wellen, South Dakota to the nearest fort, Fort Kiowa, not far from modern day Chamberlain, South Dakota. He set out on or about Aug. 23, 1823 and completed the journey around Oct. 11.
Along the way, Glass most likely relied on the contents of an emergency supply pouch, called a possibles pouch, and subsisted on everything from berries to rattle snake meat to buffalo marrow. Bradley believes Glass picked up survival skills from living with the Pawnee tribe, but Coleman contends he could find no evidence of Glass living with the Native Americans.
After arriving in Fort Kiowa, Glass rested for several days before making his way to Fort Henry, where he finally encountered Bridger. "When he got there, Jim Bridger was so eaten up with guilt that he thought Glass was a ghost, that's where 'the revenant' [a term for someone who returns from the dead] comes from," Bradley explains.
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But after "beating him up a little bit," Bradley says, Glass decided to forgive Bridger on account of his youth, and instead turned his sights on Fitzgerald. By that point, however, Fitzgerald had returned to civilization, and upon learning that Glass was alive and coming after him, he joined the Army.
When Glass finally caught up with Fitzgerald, "the Army wasn't about to let him get away with killing government property," Bradley says. "They did make Fitzgerald give him his rifle back, which was a big deal in those days."
What happened to Glass next, like most of his life story, is largely unknown. He was eventually killed, supposedly by the Arikaras, near the Missouri River in 1833.
As for the historical accuracy of Alejandro González Iñárritu's upcoming film, Coleman and Bradley are already skeptical about one plot point revealed in the trailers. "We see this whole back history where Glass was living with the Pawnees," Coleman says. "He has a Pawnee wife and they have a child who gets killed by Fitzgerald and that's the primary reason why he wants his revenge – that's all an invention of the filmmakers."
"I think that would've made its way into history had it actually happened," Bradley agrees. "It changes the whole story quite a bit I think, because in reality he ended up letting Fitzgerald go, which I'm not sure he would've done if the man had murdered his child."
Still, based on what they've seen, both historians believe Iñárritu has captured the atmosphere of the time. "I think they did a good job of recreating what it is was like to be a fur trapper working in the 1820s," Coleman says. "The main thing they get right is just how brutal it was."