Short answer: No. There has been unicellular life for at least 3.8 billion years here and our microbes are most likely as dumb as they were then.

Not enough space to make enough connections to reach the complexity level of any brain. Also, at this level, the ampunt of energy that would consume a microscopic "thinking" brain would be enough to fry it instantly.

Ask yourself if it would be evolutionary useful? The purpose of intelligence is to be able to build strategies or tools to adapt to our environment where our body is unsufficient. Those microbial life-form live for how long before splitting or dying? And what is the use of a functioning brain if you don't have the time to learn / teach anything? There would simply be no yuse to it. It would be a waste of energy.