Travel on the California Trail, the route heading west from the middle of the country, followed a tight schedule. Emigrants needed to begin their journey late enough in the spring for there to be grass available for their pack animals, but also early enough so they could cross the treacherous western mountain passes before winter. The sweet spot for a departure was usually sometime in mid to late-April, yet for unknown reasons, the core of what became the Donner Party didn’t leave their jumping-off point at Independence, Missouri until May 12. They were the last major pioneer train of 1846, and their late start left them with very little margin for error. “I am beginning to feel alarmed at the tardiness of our movements,” one of the emigrants wrote, “and fearful that winter will find us in the snowy mountains of California.”

After reaching Wyoming, most California-bound pioneers followed a route that swooped north through Idaho before turning south and moving across Nevada. In 1846, however, a dishonest guidebook author named Lansford Hastings was promoting a straighter and supposedly quicker path that cut through the Wasatch Mountains and across the Salt Lake Desert. There was just one problem: no one had ever traveled this “Hastings Cutoff” with wagons, not even Hastings himself. Despite the obvious risks—and against the warnings of James Clyman, an experienced mountain man—the 20 Donner Party wagons elected to break off from the usual route and gamble on Hastings’ back road. The decision proved disastrous. The emigrants were forced to blaze much of the trail themselves by cutting down trees, and they nearly died of thirst during a five-day crossing of the salt desert. Rather than saving them time, Hasting’s “shortcut” ended up adding nearly a month to the Donner Party’s journey.

Despite the Hastings Cutoff debacle, most of the Donner Party still managed to reach the slopes of the Sierra Nevada by early November 1846. Only a scant hundred miles remained in their trek, but before the pioneers had a chance to drive their wagons through the mountains, an early blizzard blanketed the Sierras in several feet of snow. Mountain passes that were navigable just a day earlier soon transformed into icy roadblocks, forcing the Donner Party to retreat to nearby Truckee Lake and wait out the winter in ramshackle tents and cabins. Much of the group’s supplies and livestock had already been lost on the trail, and it wasn’t long before the first settlers began to perish from starvation.

Like most pioneer trains, the Donner Party was largely made up of family wagons packed with young children and adolescents. Of the 81 people who became stranded at Truckee Lake, more than half were younger than 18 years old, and six were infants. Children also made up the vast majority of the Donner’s Party’s eventual survivors. One of them, one-year-old Isabella Breen, would go on to live until 1935.

On December 16, 1846, more than a month after they became snowbound, 15 of the strongest members of the Donner Party strapped on makeshift snowshoes and tried to walk out of the mountains to find help. After wandering the frozen landscape for several days, they were left starving and on the verge of collapse. The hikers resigned themselves to cannibalism and considered drawing lots for a human sacrifice or even having two of the men square off in a duel. Several members of the party soon died naturally, however, so the survivors roasted and consumed their corpses. The gruesome meat gave them the energy they required, and following a month of walking, seven of the original 15 made it to a ranch in California and helped organize rescue efforts. Historians would later dub their desperate hike “The Forlorn Hope.”

During the “Forlorn Hope” expedition, the hiking party included a pair of Indians named Salvador and Luis, both of whom had joined up with the Donner emigrants shortly before they became snowbound. The natives refused to engage in cannibalism, and Salvador and Luis later ran off out of fear that they might be murdered once the others ran out of meat. Indeed, when the duo was found days later, exhausted and lying in the snow, a hiking party member named William Foster shot both of them in the head. The Indians were then butchered and eaten by the hikers. It was the only time during the entire winter that people were murdered for use as food.

As their supplies dwindled, the Donner emigrants stranded at Truckee Lake resorted to eating increasingly grotesque meals. They slaughtered their pack animals, cooked their dogs, gnawed on leftover bones and even boiled the animal hide roofs of their cabins into a foul paste. Several people died from malnutrition, but the rest managed to subsist on morsels of boiled leather and tree bark until rescue parties arrived in February and March 1847. Not all of the settlers were strong enough to escape, however, and those left behind were forced to cannibalize the frozen corpses of their comrades while waiting for further help. All told, roughly half of the Donner Party’s survivors eventually resorted to eating human flesh.

Of the five months the Donner Party spent trapped in the mountains, nearly half of it took place after they had already been located by rescuers. The first relief parties reached the settlers in February 1847, but since pack animals were unable to navigate the deep snowdrifts, they only brought whatever food and supplies they could carry. By then, many of the emigrants were too weak to travel, and several died while trying to walk out of the mountains. Four relief teams and more than two-and-a-half months were eventually required to shepherd all the Donner Party survivors back to civilization. The last to be rescued was Lewis Keseberg, a Prussian pioneer who was found in April 1847, supposedly half-mad and surrounded by the cannibalized bodies of his former companions. Keseberg was later accused of having murdered the other emigrants for use as food, but the charges were never proven.

Perhaps the most famous of the Donner Party’s saviors was John Stark, a burly California settler who took part in the third relief party. In early March 1847, he and two other rescuers stumbled upon 11 emigrants, mostly kids, who been left in the mountains by an earlier relief group. The two other rescuers each grabbed a single child and started hoofing it back down the slope, but Stark was unwilling to leave anyone behind. Instead, he rallied the weary adults, gathered the rest of the children and began guiding the group singlehandedly. Most of the kids were too weak to walk, so Stark took to carrying two of them at a time for a few yards, then setting them down in the snow and going back for others. He continued the grueling process all the way down the mountain, and eventually led all nine of his charges to safety. Speaking of the incident years later, one of the survivors credited her rescue to “nobody but God and Stark and the Virgin Mary.”

Of the 81 pioneers who began the Donner Party’s horrific winter in the Sierra Nevada, only 45 managed to walk out alive. The ordeal proved particularly costly for the group’s 15 solo travelers, all but two of whom died, but it also took a tragic toll on the families. George and Jacob Donner, both of their wives and four of their children all perished. Pioneer William Eddy, meanwhile, lost his wife and his two kids. Nearly a dozen families had made up Donner wagon train, but only two—the Reeds and the Breens—managed to arrive in California without suffering a single death.