OMG, I cannot believe the ignorance here.
Folks, those were NOT aliens at the end. Watch it again and pay attention to the dialogue.
They are very advanced robots. When the humans died out, all that was left was the AI/robots. The AI advanced on it's own and reached a very advanced and self aware state. They were doing archaeological digs to find their past. To them, finding the boy was a link to their past. And the humans are their creators. The boy was a link for them to understand their creators.