Well, this thought came to me in my Humanities class when we are questioning intelligence of animals other than humans.
Let's assume we have gone past the philosophical discussion (which like other philosophical things boils down to DEFINING intelligence) and accept the view that only we are intelligent animals on land ( supported by the fact that we can talk and write blogs to express our ideas :) )
So, the question is, why are other animals not intelligent??
I believe the answer lies with DARWIN.. Due to competition, we killed off other animals which competed with us due to their intelligence.. This is supported with the fact that 'we' defeated the nenderthal man.. But it might not just be restricted to them..This could have happened before in the path of evolution of the man too..
So, we killed all the other animals which were NEAR to us (in terms of intelligence) in the ENTIRE course of evolution leaving just us as the SUPERIOR!! ..
How to verify: Take a group of species which interact (have common ecosystem) and measure the intelligence of each species.. The minimum difference between the intelligence of ANY (??) two species is greater than some minimum..
Remark: As dolphins don't share the same environment as us, they were allowed to get intelligent. So, If you want to find an intelligent animal, go search a place devoid of humans and Dolphins...