flOw is a free Flash game released in April 2006. Where "the player navigates a series of two-dimensional (2D) planes with an aquatic microorganism that evolves by consuming other microorganisms". The author/developer of the game was also involved in the development of Spore [1].
There are videos on YouTube showing the gameplay (which is circa an hour) of the PlayStation 3 version (released 2007) and PlayStation 4 version (released 2013).
Thesis about Flow in Games: http://jenovachen.com/flowingames/thesis.htm
If you read the Wikipedia article about the Flow video game you can also find a link the the archived version of the games' source code.
[1] "I wasn't hired for Spore just because of Flow. There's a lot of other game design philosophy involved in Spore, and they liked my philosophy, so I was hired to help with design.", https://v1.escapistmagazine.com/news/view/64984-Flow-Graduat...