Empire Earth, need a little info

[PolyPill]

I just got Empire Earth, and I started a skermish type game from the beginning. The prehistoric period. I'm at the Imperial period and I have no clue what I need to do to be able to get to the next period, I've upgraded everything, built at lease 1 of every unit, and I've got way more than enough resources, at least 5,000 in every catigory but the option to upgrade isn't popping up. I tried looking for some thing that said what was required for each period but i can't find anything. I'm not going to play it until I figure this out because 2 out of the 4 computer plays have already upgraded and with me just sitting on my ass not knowing what to do will cause me to get really far behind.

Has anyone played this and know what I'm supposed to do?

[PolyPill]

Well incase anyone acually plays this game and wants to know how. You need to build another town center thing, This is done by telling 5 of your workers to "populate" the building then it turns into one. You can then just rebuild your 5 workers that you just lost.