I find the idea of common descent (that all organisms share a common ancestor) fairly convincing, and have no particular reason to doubt it.

The question is: exactly how did life get here? Was it by natural selection and random mutation or was it by something else? Everybody - even Richard Dawkins - sees design in biology. You see this design when you see co-ordinated parts coming together to perform a function - like in a hand. And so it's the appearance of design that everybody's trying to explain. So that if Darwin's theory doesn't explain it we're left with no other explanation than maybe it really was designed. That's essentially the design argument.

It is often said that science must avoid any conclusions which smack of the supernatural.

Skin is made in large measure of a protein called collagen.

The basic structure of proteins is quite simple: they are formed by hooking together in a chain discrete subunits called amino acids.

In the 19th century the anatomy of the eye was known in great detail and the sophisticated mechanisms it employs to deliver an accurate picture of the outside world astounded everyone who was familiar with them.

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.