People believe in God because the world is very complicated and they think it is very unlikely that anything as complicated as a flying squirrel or the human eye or the brain could happen by chance. But they should think logically and if they thought logically they would see that they can only ask this question because it has already happened and they exist. And there are billions of planets where there is no life, but there is no one on those planets with brains to notice. And it is like if everyone in the world was tossing coins eventually someone would get 5,698 heads in a row and they would think they were very special. But they wouldn't be because there would be millions of people who didn't get 5,698 heads.
And there is life on earth because of an accident. But it is a very special kind of accident. And for this accident to happen in this special way, there have to be 3 conditions. And these are
1. Things have to make copies of themselves (this is called Replication)
2. They have to make small mistakes when they do this (this is called Mutation)
3. These mistakes have to be the same in their copies (this is called Heritability)
And these conditions are very rare, but they are possible, and they cause life. And it just happens. But it doesn't have to end up with rhinoceroses and human beings and whales. It could end up with anything.
And, for example, some people say how can an eye happen by accident? Because an eye has to evolve from something else very like an eye and it doesn't just happen because of a genetic mistake, and what use is half an eye? But half an eye is very useful because half an eye means that an animal can see half of an animal that wants to eat it and get out of the way, and it will eat the animal that only has a third of an eye or 49% of an eye instead because it hasn't got out of the way quick enough, and the animal that is eaten won't have babies because it is dead. And 1% of an eye is better than no eye.
And people who believe in God think God has put human beings on the earth because they think human beings are the best animal, but human beings are just an animal that will evolve into another animal, and that animal will be cleverer and it will put human beings into a zoo, like we put chimpanzees or gorillas into a zoo. Or human beings will all catch a disease or make too much pollution and kill themselves, and then there will only be insects in the world and they will be the best animal.