I think I understand what happened. That floating entity was like the key to our evolution. At the beginning, when the monkey used that bone to kill the others, that thing appeared and allowed for the next step in our evolution. It was like they were somehow "worthy". At the end of the film, man (having made the incredible journey) was now "worthy" of evolving another step. The entity showed up yet again and hence the "star child" you see at the very end.