It was easy to imagine Chris McCandless making the same mistake as the Indian woman and becoming similarly incapacitated. From all the available evidence, there seemed to be little doubt that McCandless-rash and incautious by nature-had committed a careless blunder, confusing one plant for another, and died as a consequence. In the Outside article, I reported with great certainty that H. mackenzii, the wild sweet pea, killed the boy. Virtually every other journalist who wrote about the McCandless tragedy drew the same conclusion.

But as the months passed and I had the opportunity to ponder McCandless s death at greater length, the less plausible this consensus seemed. For three weeks beginning on June 24, McCandless had dug and safely eaten dozens of wild potato roots without mistaking H. mackenzii for H. alpinum; why, on July 14, when he started gathering seeds instead of roots, would he suddenly have confused the two species?

McCandless, I came to believe with increasing conviction, scrupulously steered clear of the toxic H. mackenzii and never ate its seeds or any other part of the plant. He was indeed poisoned, but the plant that killed him wasn’t wild sweet pea. The agent of his demise was wild potato, H. alpinum, the species plainly identified as nontoxic in Tanaina Plantlore.

The book advises only that the roots of the wild potato are edible. Although it says nothing about the seeds of the species being edible, it also says nothing about the seeds being toxic. To be fair to McCandless, it should be pointed out that the seeds of H. alpinum have never been described as toxic in any published text: An extensive search of the medical and botanical literature yielded not a single indication that any part of H. alpinum is poisonous.

But the pea family (Leguminosae, to which H. alpinum belongs) happens to be rife with species that produce alkaloids- chemical compounds that have powerful pharmacological effects on humans and animals. (Morphine, caffeine, nicotine, curare, strychnine, and mescaline are all alkaloids.) And in many alkaloid-producing species, moreover, the toxin is strictly localized within the plant.

“What happens with a lot of legumes,” explains John Bryant, a chemical ecologist at the University of Alaska in Fairbanks, “is that the plants concentrate alkaloids in the seed coats in late summer, to discourage animals from eating their seeds. Depending on the time of year, it would not be uncommon for a plant with edible roots to have poisonous seeds. If a species does produce alkaloids, as fall approaches, the seeds are where the toxin is most likely to be found.”

During my visit to the Sushana River, I collected samples of H. alpinum growing within a few feet of the bus and sent seed pods from this sample to Tom Clausen, a colleague of Professor Bryant’s in the Chemistry Department at the University of Alaska. Conclusive spectrographic analysis has yet to be completed, but preliminary testing by Clausen and one of his graduate students, Edward Treadwell, indicates that the seeds definitely contains traces of an alkaloid. There is a strong likelihood, moreover, that the alkaloid is swainsonine, a compound known to ranchers and livestock veterinarians as the toxic agent in locoweed.

There are some fifty varieties of toxic locoweeds, the bulk of which are in the genus Astragalus-a genus very closely related to Hedysarum. The most obvious symptoms of locoweed poisoning are neurological. According to a paper published in the Journal of the American Veterinary Medicine Association, among the signs of locoweed poisoning are “depression, a slow staggering gait, rough coat, dull eyes with a staring look, emaciation, muscular incoordination, and nervousness (especially when stressed). In addition, affected animals may become solitary and hard to handle, and may have difficulty eating and drinking.”

With the discovery by Clausen and Treadwell that wild potato seeds may be repositories of swainsonine or some similarly toxic compound, a compelling case can be made for these seeds having caused McCandless s death. If true, it means that McCandless wasn’t quite as reckless or incompetent as he has been made out to be. He didn’t carelessly confuse one species with another. The plant that poisoned him was not known to be toxic-indeed, he’d been safely eating its roots for weeks. In his state of hunger, McCandless simply made the mistake of ingesting its seed pods. A person with a better grasp of botanical principles would probably not have eaten them, but it was an innocent error. It was, however, sufficient to do him in.

The effects of swainsonine poisoning are chronic-the alkaloid rarely kills outright. The toxin does the deed insidiously, indirectly, by inhibiting an enzyme essential to glycoprotein metabolism. It creates a massive vapor lock, as it were, in mammalian fuel lines: The body is prevented from turning what it eats into a source of usable energy. If you ingest too much swainsonine, you are bound to starve, no matter how much food you put into your stomach.

Animals will sometimes recover from swainsonine poisoning after they stop eating locoweed, but only if they are in fairly robust condition to begin with. In order for the toxic compound to be excreted in the urine, it first has to bind with available molecules of glucose or amino acid. A large store of proteins and sugars must be present to mop up the poison and wring it from the body.

“The problem,” says Professor Bryant, “is that if you’re lean and hungry to begin with, you’re obviously not going to have any glucose and protein to spare; so there’s no way to flush the toxin from your system. When a starving mammal ingests an alkaloid-even one as benign as caffeine-it’s going to get hit much harder by it than it normally would because they lack the glucose reserves necessary to excrete the stuff. The alkaloid is simply going to accumulate in the system. If McCandless ate a big slug of these seeds while he was already in a semi-starving condition, it would have been a setup for catastrophe.”

Laid low by the toxic seeds, McCandless discovered that he was suddenly far too weak to hike out and save himself. He was now too weak even to hunt effectively and thus grew weaker still, sliding closer and closer toward starvation. His life was spiraling out of control with awful speed.

There are no journal entries for July 31 or August 1. On August 2, the diary says only, “TERRIBLE WIND.” Autumn was just around the corner. The temperature was dropping, and the days were becoming noticeably shorter: Each rotation of the earth held seven fewer minutes of daylight and seven more of cold and darkness; in the span of a single week, the night grew nearly an hour longer.

“DAY 100! MADE IT!” he noted jubilantly on August 5, proud of achieving such a significant milestone, “BUT IN WEAKEST CONDITION OF LIFE. DEATH LOOMS AS SERIOUS THREAT. TOO WEAK TO WALK OUT, HAVE LITERALLY BECOME TRAPPED IN THE WILD.-NO GAME.”

If McCandless had possessed a U.S. Geological Survey topographic map, it would have alerted him to the existence of a Park Service cabin on the upper Sushana River, six miles due south of the bus, a distance he might have been able to cover even in his severely weakened state. The cabin, just inside the boundary of De-nali National Park, had been stocked with a small amount of emergency food, bedding, and first-aid supplies for the use of backcountry rangers on their winter patrols. And although they aren’t marked on the map, two miles even closer to the bus are a pair of private cabins-one owned by the well-known Healy dog mushers Will and Linda Forsberg; the other, by an employee of Denali National Park, Steve Carwile-where there should have been some food as well.