Flint The Time Detective (aka: Space-Time Detective Genshi-kun, 1999).
It involved a time travelling... buggy manned by a caveman,
Flint is a Time Detective who was resurrected from a prehistoric fossil. I don't know what you're considering a "buggy". He's got a little vehicle of sorts, but not what I think of when I hear the word "buggy".
who went in search of monster... eggs in each episode.
His job was to go back in time and convince time-shifters to join him. He seems to find them in egg-shaped fossils.
He'd then use this hammer to revive the eggs into talking creatures.
He has a sentient hammer which acts as a weapon and adviser.
These creatures when they were in danger could...evolve I suppose... into larger creatures to protect them.
The Time-Shifters evolve into either an good or evil larger form.
It sounds (and was) pretty crappy but I think it was on Cartoon Network.
I can't find exactly which channel it was on, but it apparently aired from March until November, 2000. There was a big anime push on Cartoon Network around that time.