The probability that evolution occurred may be low. But if you convert it to actual figures then all it needs is once.

For example if you throw 1 million eggs into the air, all you need is for 1 egg not to break when it lands, and you have a chicken. The more eggs you throw, the more chance of it occurring. But all it takes is that 1 egg for chickens to exist. When we're dealing in evolution, whose beginnings date back to bacteria. Then we're talking about so large a number I do not even know what it is called. It takes 1 hour for 1 bacteria to turn into over 100,000. If you begin with trillians and trillians plus some gazillions :P, then the likelihood of one of those bacteria accidently mutating into something that fares better in the environment it is in, then there's a realistic chance of that occurring.