If nutritionists can’t easily determine how one dietary factor affects human health, evolutionary biologists can’t tell what affected the survival of long dead animals.
In the abstract, it might be tempting to imagine that irreducible complexity simply requires multiple simultaneous mutations – that evolution might be far chancier than we thought, but still possible. Such an appeal to brute luck can never be refuted... Luck is metaphysical speculation; scientific explanations invoke causes.
The conclusion of intelligent design flows naturally from the data itself – not from sacred books or sectarian beliefs. Inferring that biochemical systems were designed by an intelligent agent is a humdrum process that requires no new principles of logic or science. It comes simply from the hard work that biochemistry has done over the past forty years, combined with consideration of the way in which we reach conclusions of design every day.