Intelligence is a measure of the mental capacity of a brain. Mostly, that's a measure of its ability to deal with complexity. The consistent evidence from everything manmade is that a more complex construct requires greater intelligence to create it. Entities which are more complex, or more intelligent, than their creator are extremely rare, if they exist at all.
Man has become quite skilled at observing and evaluating and measuring the complexity of 'natural' objects: the laws of physics, life, and human thought. Man has never observed any of these being created. The most natural and obvious hypothesis is that they were created by some entity with an appropriate level of intelligence. That's the same hypothesis you use when you look at a car. That suggests God.
It's appropriate to look for other possible explanations. One common reason for doing so is to avoid even considering the possibility of the existence of a God to whom you are accountable. But that's not the only reason.
Starting with some brilliant insights by Darwin and his peers, evolutionary biology has provided an intellectual framework for observing how the complexity of an organism can increase incrementally over generations. We can see it at work. It doesn't violate the second law of thermodynamics, since each organism isn't a closed system, and there's plenty of energy being added.
The farther back you go in time, the harder it is to make the case for incremental development. The hardest cases to make are the first instance of something that qualifies as life, including the ability to reproduce itself. This applies most especially to the mechanism of DNA. Going farther back requires some mechanism for the 'accidental' formation of simple amino acids, the building blocks of proteins, see the famous Miller-Urey experiment.
The two main hypotheses thus are (1) it happened via natural processes, and (2) God did it. We can neither prove nor disprove either.