Silmarillion Blues : Quenta Silmarillion : XII. Of Men

And at the start of the new Age, the Valar rested.  We've got a bit of a pattern here, but on the other hand, we have the equivalent of children or teenagers running the planet.  Immortality, powers of creation, and honestly little to do but entertain themselves besides the occasional scurry to pick up all the dirty socks off the floor does not do a whole lot for developing a work ethic or a sense of responsibility.

So with little notice or fanfare, the Younger Children of Iluvatar awake in Middle Earth.  The Elves greeted Men like an unwanted sibling, and the Valar are the absent parent.  The metaphor isn't perfect, but it's close enough.

With the Sun in the sky checking Morgoth's power, Men and Elves become great allies.  Together they  strive against the hosts of Morgoth, though Men were more fragile than the Elves, vulnerable to sickness and more easily slain.  But in time, division grows between the races, Morgoth triumphs, and the Elves wane while Men flourish.  However, before it comes to that, Men learn and grow from their association with the Elves, and also is the pairing of Eärendil and Elwing, the parents of Elrond... all three of whom I have a sneaking suspicion we'll be hearing more about in depth as the book goes forth.