Back in his Manhattan lab, McGovern sorts through hundreds of animal bones collected from various Greenland middens. By examining the bones and making careful note of which layers they were retrieved from, McGovern can tell what the people ate and how their diets changed over time. “This is a pretty typical set of remains for these people from this region and time period,” he says, leaning over a shiny metal tray of neatly arranged bone fragments. Some are the bones of cattle imported from Europe. Others are the remains of sheep and goats; still others of local wildlife such as caribou. Conspicuously absent, McGovern says, are fish of any kind.

“If we look at a comparable pile of bones from [Norwegian settlers of] the same time period, from Iceland, we see something very different,” McGovern explains. “We have fish bones and bird bones and little fragments of whale bones. Most of it, in fact, is fish—including a lot of cod.” It turns out that while the Greenland Vikings were guilty of Diamond’s third factor, failure to respond to the natural environment, their Icelandic cousins were not.

Like their cousins in Greenland, the Vikings who settled Iceland at about the same time were initially fooled into thinking that their newly discovered land could sustain their cow-farming, wood-dependent ways: Plants and animals looked similar to those back in Norway, and grasslands seemed lush and abundant. They cleared about 80% of Iceland’s forests and allowed their cows, sheep, and goats to chew the region’s grasslands down to nothing before finally noticing how profound the differences between Iceland and Norway actually were: Growing seasons in Iceland were shorter, both soil and vegetation were much more fragile, and because the land could not rebound quickly, cow farming was unsustainable.

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But once they saw that their old ways would not work in this new country, the Icelandic Vikings made changes. Not only did they switch from beef to fish, they also began conserving their wood and abandoned the highlands, where soil was especially fragile. And, as a result of these and other adaptations, they survived and prospered. The Icelanders responded to the limitations of their natural environment in a way that allowed them to meet present needs without compromising the ability of future generations to do the same—an approach known today as sustainable development.

Some of the most telling clues to the mystery of the Greenland Vikings’ demise come not from the Viking colonies but from another group of people who lived nearby: the Inuit. The Inuit arrived in the Arctic centuries before the Vikings. They were expert hunters of ringed seal—an exceedingly difficult-to-catch but very abundant food source. They knew how to heat and light their homes with seal blubber (instead of firewood). And they were experts at fishing.

Fishing is not nearly as labor intensive as raising cattle, and in the lakes and fjords of Greenland, it provides an easy, reliable source of protein. A comparison of Inuit and Viking middens shows that even as the Greenland Vikings were scraping off every last bit of meat and marrow from their cattle bones, the Inuit had more food than they could eat.

The Vikings might have learned from their Inuit neighbors; by adapting some of their customs, they might have survived the Little Ice Age and gone on to prosper as the Icelandic Vikings did. But excavations show that virtually no Inuit artifacts made their way into Viking settlements. And according to written records, the Norse detested the Inuit, who, on at least one occasion, attacked the Greenland colony; they called the Inuit skraelings, which is Norse for “wretches,” considered them inferior, and refused to seek their friendship or their counsel.

In addition to these hostile neighbors (Diamond’s fourth factor), the Greenland Vikings also suffered a loss of friendly neighbors (Diamond’s fifth factor). As the productivity of the Viking colonies declined, so did visits from European ships. As time wore on, it became apparent that the Greenland Vikings could expect very little in the way of trade; royal and private ships that had visited every year came less and less often. After a while, they did not come at all. For the Greenland Vikings, who depended on the Europeans for iron, timber, and other essential supplies, this loss proved devastating. Among other things, it meant that, as the weather grew colder, and food supplies dwindled, they had no one to turn to for help.