What I have understood is that you evolve because you are better as a result of natual selection.
To add to Nosy's comments, evolution does not proceed from worse-to-better, and is not a linear process.
As examples, we are poorly "evolved" for life above the Arctic Circle, or at the bottom of the ocean near a deep sea vent. Other forms of life have evolved to survive well under those conditions. That does not mean that humans, polar bears, or thermophilic bacteria are any more or less evolved than the others - they are simply best suited to their environments.
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