Actually, it's up to you in Spore, an unbelievably great-looking game coming out this fall. Designed by "Sims" guru and overall genius Will Wright, the game puts you in control of a single cell organism that you propel around a microscopic world in search of food pellets while avoiding becoming a food pellet to other predators. And if that were all there was to it, the game would be kinda cool. But that's not all there is to it. Not by a money shot.
That's because you get to reproduce, at first asexually, by laying an egg. Click on the egg and up comes a design interface. You get to choose which evolutionary improvements your little character gets. Swap out flagellum for fins for better hunting and faster fleeing. Slap a needle-like horn over your proboscis to increase the food you can access and the predators you can defend against.
As you evolve, your universe will grow around you -- from a dab of primordial goo, to a tidal pool, to an ocean and eventually to land. In all cases, your world is populated by other players' creations, which are carefully selected from a database by the game's logic engine to achieve ecological balance.
Pretty soon, your critter will be complex enough to crudely communicate, create music and use simple weapons. That's when the game switches from a single-player control to an RTS, allowing you to select groups of your creatures to hunt, dance and defend against other attacking villages.
As evolution continues, you will advance technologically. Sticks and spears make way for guns and lasers and all sorts of cool shit. Soon, you're traveling around the globe and even throughout the galaxy. You can visit another planet and battle the indigenous life forms (Created by other players) or terraform an uninhabited planet (watch flora spread, moss-like, from your view far above the planet) and drop off some of your creatures there to evolve anew.
What's so cool about this game is that it is built by players in a logical, tactical way. It's evolution that you have a hand in designing, or at least guiding.
I had a friend years back who was writing a book that detailed an alternate evolution. The premise was complex, but went essentially like this: Someone goes back in time to the primordial ooze and removes several organisms that were ultimately responsible for life forms as we today know them. Without these alpha critters, other organisms that would otherwise have lost the race evolved -- bizarre-looking creatures that nonetheless had to be biologically viable and logically formed. His book was to include pictures of the creatures and stats about them. This game is basically that, but you're the designer, author and, to an extent, the creature.
Check out the video walkthrough!