The origin of the boy
At the start of the game while fleeing Earth, you can see the child you will eventually speak to running away from Husks. Before the part where you see Husks climbing up the wall (the first time you shoot at something), you can see the boy looking below, looking over the railing. When the Husks arrive and Anderson coaxes you to shoot them, watch the boy run into the building and lock the door. The building is blown up seconds later, but he apparently survives in the vent.