I read this a few years ago online: a man creates some kind of simulated world using a quantum computer with creatures that end up evolving to look 'crab-like'. He's able to fast-forward the simulation; after fast-forwarding to where they're of human-level intelligence and grieving over their dead, he is emotionally unable to let them continue dying. That generation lives forever and (I believe) he stops them from being able to reproduce, so the population is entirely stagnant immortals. At some point he establishes contact with a crab-being, but only one.
Does anyone else remember reading it, what it was called, or what the website it was hosted on could have been?