The only replayability lies in doing nearly the exact same thing from Cell to Space stage with purple elephantmen instead of your previous red lizardmen:
Cell stage: You either eat microscopic plankton or kill other organisms for their 'meat'. Doing the former is more boring since all you have to do is look around for plankton while evading larger organisms. You could attack other ones of your size, but in that case it'd be basically acting as carnivore without eating the meat of those you kill. So, not many choices here. The type of mouth you finish the stage with determines whether you're herbivore or carnivore (or omnivore, if you have two different mouths) in the Creature stage.
Creature stage: Two choices here: either slaughter species by using mediocre, MMORPG-like combat, or befriend him using childish Simon Says-like interaction. Body parts provide different attacks or 'social moves'. If you think about it, combat and socializing is pretty alike, given in the former you go through very similar mechanics to bite the other creature instead of dancing for it. No more choices than that.
Tribal stage: RTS-like part, instead of third-person light-hearted action. Same two choices as before: ally other tribes through another Simon Says-like mechanic (somewhat more complex, given you need instruments you build in your settlement, and you get to unlock more types as you conquer/befriend others) or destroy them through a barebones RTS combat system (Dune 2 and Warcraft I were far deeper than this) using members of your tribe equipped with three kinds of weapons instead of three kinds of instruments. You unlock weapons just like you do instruments. As in the Creature stage, destroying everything is way easier than playing nice. Again, you're stuck with two choices.
Civilization stage: Still RTS, considerably shallower than even the first RTS games. This time you have three (yes, THREE) choices. Well... no, not really. According to your behaviour in the previous stages, you end up either a military, economic or religious civ. So you can only follow the path imposed on you. Economic is the most boring, given all you do is accumulate cash, establish trade routes with other cities and wait till they're available for purchase. Military and religious are pretty much the same thing: you either literally destroy stuff with your land, air and sea units (of which you have one type: one tank, one ship and one plane, if you designed them that way, and that's it) or bombard them with religious propaganda in pretty much the same way you'd fire your guns at them. You have several 'powers' according to your orientation, which get progressively get unlocked as you take over cities (sounds familiar?). The last one is basically an insta-win button, which automatically conquers all the remaining cities on the planet. LAME. But then, doing the exact same thing you've been doing to conquer/purchase cities any longer would be just as atrocious. This is the worst stage of them all.
Space stage: Back to third-person. This time you have several small-scale options, like travelling around, explore, terraform, settle and such using your species' spaceship design (again, just one). However, on the empire scale, your only option is to try and ally every other race you meet, since the more wars you get into, the more you will be harrassed with attacks.
For making money, pretty much the only option is selling spice to other races (for which you need to have acceptable relations with them), either the one your planets provide or what you can buy low from other race. There are missions you can carry out for your people or the other races, but they get repetitive pretty fast and only offer pocket change, compared to spice trading.
Back to harrassing, it's the biggest issue you have. Every 5 minutes, you've to drop whatever you were doing to defend your colonies (or your allies') from enemy aliens or pirates, or prevent some ecodisaster which, if left unchecked, can totally cripple a planet's ecosystem.
This stage will hold your attention the longest, but eventually you'll find yourself doing the same stuff over and over and over. Eventually, it gets boring. There's Earth somewhere, but it's nothing more than an easter egg. And there's also the galactic core, which you can reach after a stupidly, unimaginatively challenging voyage. You get a special item there. In order not to spoil anything, I'll just say it could be something way more revolutionary than the thing you get.
The Space stage is the best of the five, but it's not good enough to redeem the dumbness of the rest. I got bored of it a while ago, and haven't touched the game ever since. It's just not replayable unless you enjoy doing the exact same stuff (with marginal differences) with a graphically different race.
There are stupid achievements all over the place, symptom of the terminal disease known as consolitis. Doing the exact same thing 10,000 times to get an idiotic badge is not my idea of replayability nor, above all, FUN.