Is there any species who's evolutionary history shows that they have become less intelligent over time?
Well, intelligence is a difficult thing to measure, so that's hard for us to say, but I'd say it's probable.
There's a lot of discussion about surviving, but evolutionary success is about a lot more than surviving. It's about leaving descendants. Sometimes the most effective strategy for doing so involves dying, so, not only is it easy to come up with a scenario a less intelligent creature could survive better (one in which instinctual response matters more than planning), it's too simplistic just to ask who will survive in some arbitrarily invented scenario as if that defines evolutionary success.
Intelligence doesn't necessarily lead you to make useful decisions for the propagation of your genes, and it's not free. Our brains consume a considerable amount of energy which requires us to consume more; the size of our heads makes birth difficult and dangerous; and the level of development our brains need leave us helpless as infants for much longer than most animals. These disadvantages to our big brains have obviously been outweighed by the advantages in our evolutionary history, but it's not clear that they always would be.
While it's difficult to declare a clear link between brain size and intelligence, there have been cases of species evolving smaller brains. The article I originally read on this subject was about bats, and about Homo floresiensis, but I can't find it now. Possibly, they just stumbled across a more efficient way of being just as intelligent with a smaller brain capacity. But I don't think we can dismiss the idea that, in certain environments, the greater energy efficiency of a smaller brain outweighed the advantages of being a bit cleverer.
You mentioned eyes later on, but they are also often reduced or lost in specific environments where the costs of eyes outweigh their advantages.
On the subject of life elsewhere, there's one trait I can't see anyone mentioning yet, which I think would be likely to evolve given the existence of animals - heads.
Heads have evolved several times independently in different animal lineages, and it seems to make sense to have your major sensory equipment clustered close together with the bits you use to process sensory information. Once you've developed animals, I think heads aren't going to be too far behind.
However, I don't know if animals are at all likely. Prokaryotic life may well be common as muck, and many evolutionary changes we can expect to happen over a given period of time. But the development of the eukaryotic cell seems to be a very contingent, unusual event, which only happened once here on earth. Obviously, I've no idea how to calculate it's likeliness, and in the vast stretches of the cosmos maybe there's enough space for something similar to have happened over and over again. But it wouldn't surprise me if complex, multicellular life is exceedingly rare.
The question to me is not which is better, but whether or not intelligence will be an emergent property of evolution under sufficiently appropriate ecological conditions.
My belief is that it is such a property, that if conditions are suitable on another planet that it will arise.
Oddly enough, I'm actually partially coming round to jar's point, once he started explaining it and stopped writing posts that were only two lines long.
The question isn't whether intelligence as such will arise. The question is whether intelligence that gives rise to something we would recognise as a technological civilisation will arise. I think that intelligence in the form of problem-solving abilities and social cunning might well be an expected outcome of animal-like life; but it might require a very contingent and specific set of circumstances to lead to an organism which builds things similar to us.
The universe is very, very big, though - so even the staggeringly unlikely may have happened many times.
Oni's definition of intelligence is an arbitrary point somewhere near the top of the scale, possessed by only one species. If all life has some common ancestor then the seeds of that intelligence should be possessed by all life.
You mean like the seeds of flight, or the seeds of vitamin c synthesis, or the seeds of coprophagy?