A system is irreducibly complex if it has a number of different parts that all work together to accomplish the task of the system, and if you were to remove one of the parts, the system would no longer work.
1. Darwin said in his Origin of Species, "If it could be demonstrated that any complex organ which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."
2. There, the complex molecular micromachines that make up cells must have been developed a bit at a time, small step by small step. For instance, using the example of the mousetrap, maybe first there was just the platform, which evolved over time into a platform with a hammer, then a platform with a hammer and a spring, and so on.
3. Darwin's theory of natural selection, or "survival of the fittest," says that the systems that work the best are the ones that survive and develop further. And a mousetrap that is just a flat wooden platform doesn't work. So according to natural selection, it wouldn't develop further; it wuld become extinct.
4. Like the mousetrap, molecular machines are irrdecuibly complex. Without all their parts in the right places, they don't work.
Since natural selection chooses systems that are already working, any incomplete molecular machines would bite the evolutionary dust, not develop bit by bit. And, these micromachines are too complex for all of their parts to have come together all at one by random process.
