Evolution isn't a race to some cosmic finish line. No species is more evolved than the next. Every living thing is descended from the same common ancestor. All the different lineages have been evolving for exactly the same length of time. True, humans are better than pine trees at doing the things humans do, but we can't stand outside in the sun and soak up energy — something that pine trees do very well. The reason life is so different is that different environments select for different outcomes.
Neither is evolution a climb to the top of some life-form ladder on which the "higher" orders take over the top rungs (we humans are at the tippy-tippy top) and the "lowlier" creatures hang around the base. In fact, not all evolution results in more complex life forms. This point may seem like a small one, but it's actually quite important and is easily lost when most people think of evolution in terms of the "monkey-to-man" graphic — the one that shows the evolution of man in a series of stages, from monkey to ape to caveman to investment banker. Although you can make an argument that the caveman gave way to the investment banker and therefore forms a valid time series, other primates are still around and are just as evolved as humans are.
Evolution can lead to greater complexity, but it doesn't always. Over the history of the earth, since the first single-celled life forms, there was really nowhere to go but up in terms of size and obvious physical complexity. But as soon as larger, more complex critters evolved, the possibility existed that some would evolve simpler forms. Parasites, for example, have lost many of the functions that they can scam off their hosts. The eyes of cave-dwelling organisms constitute another example. Absent the need for the complex structure of the eye, mutations that cause a reduction in the eye can pile up.
P.S: Just between you and me, I do sometimes think of myself as being a bit more evolved than a bacterium — but then I think of the incredible biochemical diversity that bacteria are capable of, and I realize the error of my ways.
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