One of these digressions had to do with tolerance and genetics and evolution; Johnson describes how frequently drinking water rather than some kind of alcohol was a fairly recent thing in the 19th century. For hundreds, thousands, years before, people who settled in towns, villages and cities drank alcohol, because - due to their fixed and substantial populations, procuring clean water was not feasible. But evidently, a tolerance to alcohol had to be acquired over generations; as Johnson points out, "alcohol... is a deadly poison and notoriously addictive." He tells us that "early agrarians lacked that trait [a gene producing an enzyme allowing humans to process alcohol] and thus were genetically incapable of 'holding their liquor'." Thus, over many long years, humans with that gene came to be dominant in urbanized/settled regions as humans lacking that gene died without reproducing (this is how evolution works). The key here is urbanization and agriculture; shifting away from hunting-gathering, with is almost by definition migratory, humans stayed in one place and began mucking up those places with their waste products, which end up in the water supply and deliver brutal gifts like dysentery and cholera to the inhabitants who drink that water. Thus, alcohol, which kills off most of those waterborne bugs.
This is when Johnson gets interesting. He writes:
The descendants of hunter-gatherers - like many Native Americans or Australian Aborigines - were never forced through this genetic bottleneck, and so today they show disproportionate rates of alcoholism. The chronic drinking problem in Native American populations has been blamed on everything from the weak "Indian constitution" to the humiliating abuses of the US reservation system. But their alcohol intolerance most likely has another explanation: their ancestors didn't live in towns.
Johnson doesn't give sources for his comments on genetics, and I also lack the time to really thoroughly research this (I suspect it's not as easy or solid a claim as Johnson makes it appear, though I don't really know). But it's an intriguing possibility. I haven't done more than dip a few toes into anything like serious Native American scholarship and criticism, but you don't have to read much fiction (Sherman Alexie's Absolutely True Diary of a Part-time Indian will suffice) to know that alcoholism and alcohol have been catastrophic for Native Americans. A genetic explanation doesn't, of course, get us anywhere in terms of a way of solving this problem or aiding Native populations, and it's perilously similar to the "weak constitution" claim, which makes the Natives seem at blame, or at fault, for alcoholism. And having an alcohol intolerance doesn't mean you should have been naturally-selected out of existence; it simply means that Native populations either kept their water supply clean, had other kinds of tolerances, or moved around enough to maintain uncontaminated water, and thus had no biological imperative to shed non-genetically-tolerant people.
Ultimately, the genetic basis for specifically Native alcoholism isn't terribly productive in any ways other than intellectual or historical ones. The problem is what alcohol abuse and alcohol intolerance does to individuals and their families (who are also frequently impacted by the "humiliating abuses" of the reservation system).
But it's an interesting claim, and one I have not come across elsewhere. It makes me rethink the dreadful representations (from the 18th and 19th century) of the alcoholic "savage" in a very different light; it makes the way Native people were taken advantage of by traders, settlers, explorers, etc using alcohol as payment and lure, seem even more appalling than it did (a difficult accomplishment).