Thrive [Official Site] is a game I came across years ago, a game about the evolution of life with you starting as a tiny Microbe and eventually working up to something more complex.
That idea might sound familiar and for good reason, as it was originally inspired by the game Spore. However, they're attempting to go a little further by being scientifically accurate and have the evolution play-out across both you and everything around you.
It's not a finished game and development has been taking a while (it's also pretty rough), but to my surprise they actually put out a major new release back in December with multiple smaller updates since then so it seems very much alive again.
I'm not actually aware of many games that have attempted something like this, likely as it's a very complex type of game to create. Especially considering all the different stages of life, and balancing the realism with fun in such a thing can be pretty tricky. So it's good to see one that's open source, at least people can keep it going no matter what happens or fork it and take it in a different direction.
I love the idea, I just hope they can make the game fun. It's very rough in the current form and the controls really don't feel good. Still, there's promise and so I shall be following it along properly now.
Want to give it a try? You can find out more on the official site, with the code up on GitHub under the GPL. They also have an easy to use launcher to download it and keep it up to date, which you can find on GitHub.