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.

As the number of unexplained, irreducibly complex biological systems increases, our confidence that Darwin's criterion of failure has been met skyrockets toward the maximum that science allows.

For example, both humans and chimps have a broken copy of a gene that in other mammals helps make vitamin C. ... It's hard to imagine how there could be stronger evidence for common ancestry of chimps and humans. ... Despite some remaining puzzles, there's no reason to doubt that Darwin had this point right, that all creatures on earth are biological relatives.

In many biological structures proteins are simply components of larger molecular machines.

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

Irreducible complexity is a problem for Darwinian evolution. Whenever we see these complex functional systems we realise that they have to be designed.