No, organisms do not always evolve into more complex forms. At first I was tempted to say yes, evolution leads to the more complex. I feel myself to be more complex than my prehistoric ancestors, but am I? In cognition, maybe, but physically maybe not. Evolution is essentially a process of trial and error. A mutation occurs that is favorable to the organism. The organism reproduces the mutation. Eventually changing that particular species. The organisms that are most adapted to their environment are the organisms that survive to make more of themselves. Is complexity a prerequisite for survival? No, it isn't just as intelligence isn't necessary for sex. The only prerequisite for survival is adaptability. Can the organism adapt to its environment?
Life forms that are less complex are able to multiply faster than more complex life forms. The numbers give simpler life forms an advantage. In fact, the statement that life evolves into more complex life is an illusion. To believe that is to ignore a very large group of organisms and focus only on a smaller group. Complexity is not the issue of evolution. It is diversity and diversity for the purpose of survival. The more diverse the life forms the more chances for the life forms to survive.
Now the earth is ruled by man, once upon a time it was ruled by dinosaurs. The dinosaurs had every advantage. They were complex,(perhaps not mentally, but who's to say that animal intelligence isn't as complex or even more complex than people)and formidable. Perhaps if a drastic environmental change hadn't occured, man would be reptilian, if we would have evolved at all. Evolution is an ever changing process on this planet. One era it favors giants, the next it favors the puny human.
Complexity does have its advantages. Humans were not fast, and they were not strong, but somehow they became the dominant species on this planet. Complexity has led to chaos though. The very environment that has allowed human life to thrive is now endanger of that very life form. Indeed, complexity can be a double edged sword. When the humans are gone, the bacteria, the parasites, and the fungus will still be here. All simpler life forms that had strength in numbers.
Complexity versus simplicity is not what evolution is all about. Evolution is about diversity. It is about the circle of life. After all, life and nature tend to run in cycles. Perhaps, life as we know it now is the pinnacle of that cycle and organisms will begin the process of returning to the primal ooze from which they came.