I think the Intelligent Design folks will put this in the category of irreducible complexity. And it would seem to be a valid argument at this early stage…

First, cool! This is great stuff and I am happy that the authors waited to publish their report in Science, rather than in a newspaper or online.

My first impression from reading an online blurb from my Facebook friend was that this was a fundamentally different lineage of life. Now that I have partly read the article, I am not so sure. First the authors claim that it is a bacterium, which to me would put it within one of the three known domains of life. That implies that the use of arsenic instead of phosphorus is a derived trait that is unique to this fascinating new lineage, but one would expect that it should still have retained plenty of historical baggage that would place it as a particular lineage within known bacterial diversity. It is important whether or not this microbe had ancestors with more conventional use of phosphorus for its genetic material and its equivalent of proteins. We definitely need to figure out how this microbe is related, if at all, to other known living organisms on Earth.

If this discovery is further confirmed (and science is quite good at revealing false claims), AND in the future it is possibly shown that this life form is outside the “clade” of previously known life, this discovery would assume entirely different significance. We already knew that all previously known life could be traced back to a single common microbial ancestor that was already quite a sophisticated life form, with cell membranes, DNA and RNA, plus the universally shared machinery to produce proteins on ribosomes, thousands of genes still found more than three billion years later in the genomes of contemporary living organisms. However, will the discovery of this new life form help us say anything new about life prior to the last common ancestor of all previously known life? The previous last universal common ancestor (was “LUCA” and could it become “PLUCA”?) has been as far as we could go back in time using the conventional methods of phylogenetic analysis, where we find that two lineages have more in common, due to shared ancestry, than a third lineage. We even were able to argue that two of the previously-known domains of life, Archaea and Eukarya, probably more recently shared a common ancestor than the third domain, Bacteria, although the history is also complicated by rather rampant swapping of genes. Ordinarily, phylogeneticist would not have been able to sort out the very deep relationships among these three known domains of life because there was no known outgroup for the LUCA clade. Outgroups are lineages outside of the clade of interest, and these are the primary way that a phylogeneticist can “root” a tree, in this case the tree of all previously known life. Fortunately, scientists have been able to figure out that some genes had already been duplicated pre-LUCA, so it proved feasible to use some gene trees to be effectively “rooted” with the inclusion of a related gene tree. This is a bit complicated but is essential for many interpretations about how cellular life forms have evolved.

To study the pre-LUCA origin of life, a fundamentally different approach was needed. This often involved coming up with a scenario for how pre-LUCA life might have gotten to that point of considerable sophistication, and then demonstrating (if possible) that the steps in such a progression are at least plausible. If the discovery of an arsenic-based life form is later found to be outside of the LUCA clade, then this might or might not help phylogeneticists push back the history of pre- or post-LUCA life. That would depend on whether the newly discovered life form was truly an independent lineage of life. It would not help at all if it were a completely independent life form. To me, it might be most interesting if was either a fourth domain of life, effectively inside the LUCA clade, or else is a more deeply diverging branch, indicating a deeper shared ancestor of all life on Earth. If it were revealed, instead, as a completely independent life form, this would of course be fascinating for different reasons.

Remembering the big splash “NASA” made about fossil “life forms” inside Martian rocks, I personally am going to remain skeptical. I am happy that NASA-funded astrobiologists are using the cost-effective approach of better exploring our own backyard. We still know very little about extreme environments right here on Earth and I do applaud NASA-funded astrobiologists for their efforts to explore these more thoroughly. I especially hope this discovery helps promote further exploration of deep sea habitats, where fundamentally different extreme environments are known to exist.

The Harvard chemist Frank Westheimer wrote a memorable paper explaining “why nature chose phosphates” which can be found here. It explains based on basic principles of chemistry why phosphates rather than arsenates or silicates have been chosen by life. Must read for those who think alternative life-forms could be common.

So I would not call it “arsenic based DNA” just yet; the researchers themselves admit that they don’t know that the arsenic is replacing the phosphorus in DNA in exactly the right places.