Evolution does not say you keep getting better and better. We are not more intelligent than we were 500 years ago. We are not naturally more healthy. Our technological accomplishments and longevity are products of a better understanding of the world around us through scientific advancement.
One facet of evolution does predict that organisims that are better suited to the environment they live in are more likely to dominate over time but that is not guarenteed. That is not the same as saying we will all keep getting better and better.