In order to build an intelligent design such as a bridge or a mechanical watch, you need a series of non useful steps.
As far as I understand, in random evolution theory, the "intelligence" involved is the process of natural selection.
The problem with this is that natural selection requires that each step be advantageous right away. If I want to build a bridge, I need many steps which would be disadvantageous until the final product which would be advantageous. This would be like trying to write a computer program where each line of code must be advantageous to the program. An intelligent design just cannot be done like this.
How does modern biology deal with this problem? To say it all happened at once, is not possible since the probabilities against this are simply outside the capabilities of a random system. Furthermore even if by some freak monumental event it happened, it would need to happen again many times since there is a huge diversity of intelligent designs in life forms such as lungs in birds vs gills in fish, or sexless reproduction in bacteria versus male/female reproduction in animals (which are fundamentally different).
(Also, as side question has there been ANY experiment demonstrating that such a thing is even possible? I mean for example, to have a computer simulation of random self-replicating molecules, then having an intelligent human being picking out the best version and eliminating the others, then repeating, etc. until you have an intelligent design.)