Thanks for asking such insightful questions. Here's a few points to help you with your search for some answers:
1. Humans are just one of 10 million or so other species. All species are totally reliant on a suite of other species for their survival. Other species provide us with the necessary goods and services for the continuation of life. For example, you are totally reliant on the colonies of bacteria that live within your gut, and you need other species of bacteria to dispose of all the wastes that you produce, and you need to take in energy by eating various other species. Humans are just one part of a complex web that links all species to all other species.
The concept of "survival of the fittest" is widely misunderstood. In evolutionary terms it relates to the likelihood of any individual passing its genes on to the next generation. It is a mathematical model, where the "fittest" individuals have a greater chance to pass on more of their DNA than do their conspecifics. Fitness, thus defined, refers to how well an individual animal is able to obtain and utilise the resources available to it from its environment. It is a measure of how well it is adapted to its environment. It has little to do with physical fitness (although it can mean that in some instances).
2. Humans are not the current pinnacle of evolution. Evolution is a genetic response to environment conditions. How evolution proceeds in the future is totally dependent on environmental forcings. A meteor could slam into the Earth next month and wipe out every animal bigger than a mouse. Only the animals and plants who are fortunate enough to have the right genotypes to enable them to survive in that new environment will survive, all the others will join the overflowing ranks of the extinct. Just such a scenario has happened at least 5 times in Earth's history. The species alive today represent the tiniest fraction of all the species who have ever lived. It is the fate of every species to suffer extinction. And humans will be no different. It is hubristic to think otherwise.
3. Evolution is a continuum. Every organism alive today is a another link in the long unbroken chain that stretches back through 3.5 billion years of history to the first life form. Every organism is slightly different to its parents, and it's offspring differs slightly to it. The fossil record preserves snapshots of various stages along that unbroken chain. If every organism became fossilised at death than this "family album" would show all these changes. You wouldn't spot the differences between individuals only a few generations apart but as more generations went by the changes would become noticeable. Fossils are a record of the process of evolution in action.
You are correct in assuming that evolution is observable in small, short-lived species. In large animals and plants, evolution can be observed by its effects, but in bacteria it can be observed in action. Laboratory experiments have developed new species of bacteria in a matter of months. In bacteria, this time frame represents thousands of generations and allows the accumulation of genetic differences to allow speciation to occur.