The Arctic treeline is the frontline of climate change, where the trees have been creeping towards the pole for fifty years already. These vast swathes of forests, which encircle the north of the globe in an almost unbroken green ring, comprise the world's second largest biome.

When our ancestors came down from the trees, they brought the trees with them and remade the world. How did the descendants of small arboreal primates manage to stand on our own two feet, become top predators and take over the world?