Mosses, lichens, heathers, coarse grasses - very meager fare. Yet such a diet used to support very large animals like reindeer, muskoxen and mammoths. So there is no reason to suppose that a suitably-engineered human could not subsist on such a diet.
Knut has been raised on it for a decade, but that was in the safety of a living module. The cold-weather plants were brought in regularly by flying machines, and the chill conditions were maintained artificially. All this time, Family have been outside in the warm and looking in.
Now the situation is reversed. The Family members, in their cradles, with all their delicate life-supports, are keeping warm in the modules of the flying machine, while Knut is outside, standing in the crisply-frosted grass of the tundra wilderness, beneath the vast cold grey and white sky. This is what he was brought up for, to take his place in nature.
Centuries ago there were herds of big animals here, which moved north and south as the seasons changed, wintering in the deep coniferous forests to the south and spending the summer on these wild plains. In those days, he was told, there were fierce hunting animals as well - animals that would harry and kill the gentle plant-eaters. Now there are none of those left either, and the whole landscape is his.
He looks down at the coarse little plants at his feet; they look the same as those he has been eating all his life. With the ice-hook developed from the nail on his big toe, he scrapes up a patch of moss, then he goes down onto his furry knees and scoops it up with his spade-like hand. Yes, it tastes just the same. He will survive here.
The whiteness that has been building up at one side of the sky descends. Chill flakes of snow begin to swirl past him, settling on his fine-curled fur over the layers of fat. In reaction he rolls up the ruff of fat around his neck and his face disappears into it. From the direction of the great flying ship behind him he can hear the clang and hiss of hatches and accesses closing and sealing. This is too much for Family. There is a sudden blast of warm air as the great vessel leaves the ground. They are going back to the cities where it is warm. Knut is left here, where he belongs.
Yet too much harsh weather will kill him, and the brief northern summer is over. He knows what he must do. As the sudden flurry of snow passes, he brings his face out from the folds of fat and turns it towards the south where away yonder are the huge coniferous forests and winter shelter. Like the great herds of grazing animals before him, he moves southwards with the season.
Yet, unlike the animals of the vast seething herds, he is alone (the only one of his kind) but this does not worry him. If he survives, and he has every intention of doing so, then the experiment will be a success. Others like him will be produced and together they will repopulate the chill northern wastes of the planet.