A common misunderstanding of evolutionary theory founders on misunderstanding of the word 'fittest,' as in 'survival of the fittest.' Whole philosophies, like social Darwinism, are founded on this misunderstanding.
This does not mean that the fittest as in the biggest, fastest, most intelligent etc. It merely means those most suited to their environment. So evolution is not a ladder that creatures climb, getting fitter and fitter (like they've been working out) until they become some Arnie-like uber-critter with great pecs, big hair, a good smile, and natural camera skills.
Nope. The ones that survive are those most well suited to their environment - and that environment changes constantly (which you'll have noticed if you've been paying attention lately).
The best 'fit' for one period will not be best for another period and therefore, by definition, creatures don't just get bigger/better/faster/smarter on some kind of endless developmental ladder. Sometimes they get smaller (dinosaur to bird). Sometimes all the big boys die (e.g. mass extinctions), and smaller/weaker animals take over.
A few things flow from this, mainly that we are not the evolutionary crown of creation sitting at the top of this developmental ladder, just the creature best suited to the way things are now.
One side-effect of the way our intelligence works is that we're changing the way things are now, and are therefore making the world less supportive of creatures like us and furry mammals in general. In an extreme case, we may trigger catastrophic climate change and render the world uninhabitable for ourselves--at which point some other creature will be better suited to the new environment; cockroaches have long been a popular candidate. But if it happens, it doesn't mean the cockroach is better than us, just as our current ascendance doesn't mean we're better than the cockroach.