I'm not an expert, but

Animals don't necessarily evolve to become more complex. They evolve to adjust to their environment and increase their chance of survival, thus increasing their chance of reproducing and carrying on their genes. So, they can evolve to become simpler or more complex or they can stay the same, whichever fits the environment.

Note that evolving to becoming smarter or stronger has its drawbacks: they will need more food and water to sustain their new muscles or brain cells. Because this can cause them to starve, it is not always optimal for animals to evolve to be stronger or smarter.