Among the phenomena which support the claim that life is the product of intentional, intelligent design is the sheer number of complex molecular machines that operate in each of the trillions of our body's cells to ensure that these cells carry out the functions that keep us alive.
One of these machines is the system of proteins that synthesizes adenosine triphosphate (ATP) from adenosine diphosphate (ADP). Here's a short video animation that describes how this machine, called ATP synthase, works:
There are thousands of such machines in the cell, all of which, on the standard Darwinian account, somehow developed - through random, undirected, processes - not only their structure, not only the coordination with other systems in the cell necessary for proper function, but also the genetic regulatory mechanisms that control how and when the machine operates.
David Hume, in his famous essay On Miracles, wrote that when we hear an account of a miracle we should ask ourselves whether it's more likely, given our experience, that a law of nature had been violated or that the witness was somehow mistaken. Hume argued that a mistaken witness is always more likely than that a law of nature had been violated, and we should always, he insisted, believe what's most likely. Applying the principle to the present case, when confronted with a structure like ATP synthase we should ask ourselves, what is the greater miracle, that such an astonishing thing came about by chance and luck or that it came about by intelligent engineering?
It seems to me that the only way one can assert the former is if they've already, a priori, ruled out the possibility of the existence of the intelligent engineer, but, of course, that begs the question. Whether the intelligent engineer exists is the very matter we're trying to answer by asking whether blind chance or intelligence is the best explanation for the existence in living things of such machines as ATP synthase.
If we allow the evidence to speak for itself rather than allow our prior metaphysical commitments to dictate what the evidence says then I'm pretty sure most people would say that the kind of specified complexity we see in this video points unequivocally to the existence of a designing mind.
If this video has whetted your interest here's another that pushes us toward the same conclusion. It's an animation of just a few of the structures and processes in a living cell. Note the amazing motor protein that carries the vesicle along the microtubule:
How does the motor protein "know" to carry the vesicle along the microtubule and where to take it? What regulates the process? How and why did such a complex system ever come about? Was it all just blind chance and serendipity or was it the product of intelligence?