There's some prehistory and even some predicted future. Indeed, the publisher and developer have made it clear that the lead designer of Age of Empire, Rick Goodman, left Ensemble to start up Stainless Steel Studios. It seems that Empire Earth will be the most ambitious RTS to make it to store shelves. The characters from this period will actually look a bit like cavemen, and will fight each other with primitive weapons. As in Age of Empires, things start to get a bit more interesting during this time.
Weapons become more prevalent as well as tools.
As the name implies, the pervasion of copper was widespread during this time period. The ability to create better weapons will mean that we can probably expect more fighting during this period. Also, the agricultural ability allowed people to settle down, and civilizations begin to crop up. Finally, this section of the game will introduce sea vessels. This period of the game will introduce swords and the almighty Phalanx.