Rocks 2 Rockets is a mod-mod of Caveman 2 Cosmos. It has been trimmed down with the aim of getting it playable through the end of the game on a significantly less capable system than is required by C2C. It is still a large mod, just less demanding than C2C. There have been some adjustments for balance purposes. There have also been a few modifications for other reasons, and perhaps a bug fix or two that is not yet in C2C. The entire "galactic" era has been cut out, as has much of the "transhuman" era. Less drastic cuts have been made across the earlier eras. Features supplied by the game engine as of C2C version 26 are all present, including the viewport functionality, as well as many later bug fixes, AI improvements, and optimizations. R2R has a topic in the mod-mods subforum of the Caveman 2 Cosmos forum at the Civilization Fanatics' Center.
From your humble beginnings as a group of cavemen who have not quite gotten the hang of banging rocks together in a useful way you can work your way up to great metropolises.