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Sim Life AGA is the fourth game in Maxis's famous “Sim” series, after the excellent Sim City and Sim Earth and the okay-ish Sim Ant.
The basic concept of Sim Life AGA is to indeed simulate life. You've got a large world map made of rectangular blocks. You can then populate this world map with tens, even hundreds of animals and plants, and see how they live their entire lives in front of your eyes. Sounds boring? It might be, if that was all there is. But Sim Life also simulates evolution and mutations, so the lifeforms' genome changes as time goes by. As well as that, you can make up your own lifeforms.
In practice, Sim Life AGA is not that exciting. It will be completely boring to all except a select few biology-buffs. The graphics look nice, but are hardly spectacular. Game comes in two versions, AGA and non-AGA, but they don't look all that different. Sound effects are plentiful, but simple. They are too generic to give the illusion that your animals are actually communicating, and just serve as reminders.
If you like games that have you on the edge of your seat, trying hard to achieve a result, don't download Sim Life, it will fail to deliver your expectations.