http://www.cambrianexplosion.com/
This is one of the coolest things I've seen on the internet recently. Anyone who is familiar with R. Dawkins' "Weasel", Greg Egan's work like "Crystal Nights" etc and the simulation argument might find it fascinating.
Addiction Warning: I've spent 8 hours playing with this in the last 2 days...
I currently have a species of facecrawling quadrupeds. The front legs have atrophied.
Because the one that got the furthest originally fell on its face and inched forward a bit. They move a bit like the worms.
I've ended up with a species whose back legs have atrophied, and they bound forward with their front legs while nearly executing a front flip and end up landing on their heads.