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.

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.

Since natural selection requires a function to select, an irreducibly complex biological system, if there is such a thing, would have to arise as an integrated unit for natural selection to have anything to act on.

In order to say that some function is understood, every relevant step in the process must be elucidated.

Despite some remaining puzzles, there's no reason to doubt that Darwin had this point right, that all creatures on earth are biological relatives

As can be seen even by this limited number of examples proteins carry out amazingly diverse functions.

Throughout history there have been many other examples, similar to that of Haeckel, Huxley and the cell, where a key piece of a particular scientific puzzle was beyond the understanding of the age.