Know Your LotRO Lore: Origins of Man
Just as with the Elves and Hobbits (because they were derived from Man), Men were created by the One God Ilúvatar. They awoke in the Years of the Sun, which was the last of the great time periods of Arda, when Middle-earth was created. Since the Elves awoke before any of this, they refer to Men as the Afterborn.
Men are said to be blessed by Ilúvatar with the Gift of Men. In Lord of the Rings Online, there is actually a trait or characteristic given to any Man player, which gives them improved Fate above other races. In Tolkien's world however, this is explained a bit more thoroughly.
The Gift of Men was given to set Men apart from the Elves, with more than just a lack of ear points. Men are not bound to the Earth, as are the Elves who are considered immortal because they arise to Valinor after being slain. Men grow old and die much faster than most beings, because their spirits seek something greater outside of Arda. This gives Men a deeper sense of fate, motivating them to create great legacies. Whether these legacies are for good or for evil, depends on the cultural divide.
There is a great tale among Men about the origins of the Gift of Men. Some believe that it was not given outright, but was bestowed as punishment for Men turning away from Ilúvatar's teachings and worshipping Morgoth. It's not hard to see the relationship this story has with many Judeo-Christian beliefs along the same lines, as Tolkien was an avid Christian himself.