Life was hard for most people in the society depicted in A Single Shard but especially for young orphans like Tree-ear. In the absence of any kind of social safety net, unless orphans could find someone to take care of them, they were pretty much on their own.
That's the unfortunate situation in which Tree-ear finds himself. As an orphan boy in twelfth-century Korea, he has no one to take care of him. And so he has no choice but to survive by foraging for scraps of food. Most of this scavenging takes place at one of the village rubbish heaps, where well-fed villagers often leave behind carrot-tops and chicken bones. On one occasion, however, Tree-ear is able to scavenge a large bag of rice that has fallen off the back of a cart.
It isn't just food that Tree-ear needs, of course, but shelter, which he finds beneath a bridge. It is here that Tree-ear lives with his friend Crane-man. Crane-man came to live under the bridge as he too had no one to take care of him. He had outlived all his family, and unable to work, he was forced to sell off all his worldly possessions, including his house. That's how he came to live under a bridge.