tl;dr: a better Spore
From the site:
Thrive is a free, open-source game about the evolution of life.
From the lowly tidepools of your home planet, you will rise to cosmic dominance. At every step, powerful creation tools and deep simulation mechanics will aid your species’ development. Revolutionary Games is the open-source team of dedicated volunteers aiming to make Thrive a reality.
Our team seeks to accomplish two major goals: create engaging, compelling gameplay that respects our players’ intelligence, and remain as accurate as possible in our depiction of known scientific theory without compromising the former.
Yup.
Sui Generis is being worked on simultaneously. Anything developed for Exanima is used in Sui Generis.
It’s one of my favourite games despite not being finished yet, but it will probably gain more popularity once we get co-op and a multiplayer arena mode.