Me: You really do have a good question. My guess is the only answer that makes much sense is pure speculation: that the specific complex of environmental factors, mutations, etc that provided the impetus for the development of intelligence (or if you prefer, the evolving physiology of the brain that facilitated the emergent property called "mind") are fairly rare.
I wouldn't go so far as to say unique, however. An intriguing question (at least to me
) is what were the environmental factors that brought the evolution of chimp intelligence to a halt? Our nearest cousins are not only really close to us genetically, but are also almost frighteningly close to us mentally. A tiny increase in brain-body mass ratio and some cultural evolution, and
Pan could be as smart as we are. So IMO it looks like the rudiments of intelligence arose in our primate lineage, but stopped in the lineage that produced the great [edited to add "apes" here] at some point after the split between them and humans.
[This message has been edited by Quetzal, 09-02-2002]