Crood Awakening

As part of Aardman Animation's "five-picture deal" with DreamWorks Animation, an animated film called Crood Awakening was in the early works at Aardman. Written by John Cleese and Kirk DeMicco

Plot
it would've been loosely based on the Roald Dahl book The Twits but set in the stone age.

Why it was cancelled
The film was cancelled due to Aardman and DreamWorks' separation in 2007 following the release of Flushed Away

Results
Ultimately, both studios would end up making an animated movie set in the stone age: DreamWorks in 2013 with The Croods (where Cleese and DeMicco were heavily involved) and Aardman in 2018 with Early Man.