Part 4 in 20 Atheist answers to questions they supposedly can’t
4. How did DNA and amino acids arise?
5. Where did the genetic code come from?
6. How do irreducibly complex enzyme chains evolve?
Short answer: Science!
Longer answer: These three questions neatly cover the calling cards of the intelligent design movement. The big idea is that standard evolutionary mechanisms (mutation and differential survival repeated loads and loads of times) is just inadequate to explain how we got genetic codes, or replicators in the first place, or particular biological structures which have this feature called ‘irreducible complexity’.
It’s worth pointing out that these ideas only really carry favour amongst groups of (fairly evangelical) Christians, and are rejected all-but-unanimously by practicing scientists, and that doubt in evolutionary theory declines with education in general and biological background in particular. But let’s not solely rely on social epistemology steers, and talk about the merits themselves.
The first two questions are not ones biology has an established answer to. It is unlikely the first replicators sprung into being with the central dogma of cell biology present, yet what the earliest replicators were remain controversial. A quick google suggests RNA worlds, PNA worlds, metabolism first models, and many more, all of which can marhsall some circumstantial evidence in their favour (for example, we know there are self catalysing RNA based reactions, the actual peptide bond forming bit of the ribosome is almost entirely RNA, and so on).
Yet the fact we have no complete theory (or even a settled theory) is a poor license for “it can’t have happened naturalistically – God must’ve been involved!” This seems fairly classic ‘God of the gaps’, and is likely to suffer the fate of God being gradually squeezed out as our naturalistic understanding grows, not to mention issues of God being a poorly descriptive (it offers little to explain why biological feature is this and not that, etc.) and explanatory ‘stopping’ explanation.
To have persuasive evidence of ‘It must have been god’, one needs more than just current explanatory inadequacy: one needs to show it is unlikely that there will ever be a good explanation. Given the track record of biological (and evolutionary) science has been pretty good at finding pretty good explanations, and the track record of those saying ‘evolution can’t possibly explain X’ has been one of being proven wrong, there doesn’t seem all that much for Naturalist to worry about.
Irreducible complexity is one way of trying to demonstrate something is beyond the explanatory scope of evolutionary mechanisms. The idea is that certain structures – a particular enzyme chain, a flagella, or whatever, are irreducibly complex: they only serve any beneficial role if all the parts are functioning as intended, and you can’t have simpler antecedents which serve any role. If so, the ID-proponent avers, such things are virtually impossible to occur via evolutionary mechanisms, as the entire system must spring into being ‘all at once’.
First problem is that irreducible complexity is hard to determine, and even if it were so it wouldn’t show the ‘all at once’ property. It is hard to be confident that a given system has no functional simpler antecedents, or that there is no possible path from simpler to more complex. Further, even if it was so, that wouldn’t be enough to say it had to happen all at once. The Mullerian two step (add a part, make it necessary) may apply: in the same way a building may be irreducibly complex insofar as it couldn’t be gradually built up without collapsing, it may still be built via including structures like scaffolding that are lost later. So too with biological structures: perhaps the irreducibly complex enzyme chain used to have other enzymes involved which used to be vital, become redundant, and were discarded.
Perhaps most telling is that the track record for irreducible complexity claims have as poor a track record as the God of the gap stuff mentioned earlier. It used to be claimed that the clotting cascade in man was irreducibly complex, only for us to find a simpler system in the whale (and an even simpler system in the lamprey). It used to be claimed the flagella was irreducibly complex, only for there to be found a simpler structure that serves a biological role. So that ‘I can’t for the life of me consider any way this system could have arisen from simpler antecedents’ is a very unreliable guide to whether that is true or not. So again, that Naturalist may not be able to supply these for any given biological facet needn’t trouble them.
The general rejoinders are collated on TalkOrigins.
More in depth understanding demands a fairly strong grasp of biology, so a good textbook would be a first port-of-call, and the research literature from there.