As an astrophysicist specialized in problems related to dust and its evolution in the Galaxy but also as an "astrobiologist" active in "pre-prebiotic" chemistry (formation of organic molecules in extraterrestrial environments and their delivery to primitive Earth), I, like many others, believed the publicity surrounding the possible significance of the Wolfe-Simon et al, (2010), to be unwarranted. Did they discover or create another form of life? Did this life-form really switch arsenic for phosphorus? The answers appears to be "no."

First there is the problem raised by the absence of straight evidence for the incorporation of arsenate bridges within the genetic material of the studied bacteria (why has this DNA strand not been purified and studied with conventional methods to accurately prove the point? Then there is the fact that these authors seem to ignore that they live in a universe (at least in a galaxy) whose physical and chemical evolution is driven by a major fact: the observation of the cosmic abundances of the elements. Taking this into consideration, any astronomer knows that P is 3000 times more abundant that As and that, in the crust of the Earth, this ratio is still around 500. No wonder then that life as we know it, has chosen P instead of As to build the phosphate bridges so important in the DNA structure. Naturally, one might argue at length but inconclusively, that As being more soluble than P, may have been somewhere in the past more available for the first biochemical reactions. However, this answer is subject to much debate due in large part to the factor of 500 in the relative abundances of these elements.

However the hypothesis that life may have been started using As rather than P, if As was more readily available for a short primitive period, is certainly interesting. However, if true, then clearly "modern" organisms switched to P probably since long. The claim of Wolfe-Simon et al, (2010) is, if true, important but does not change by much all other considerations we may proclaim about the origin of life as we know it, since all other elements (H, O, C, N and S) remain the same.

The possibility of replacing As by P will give simply more flexibility and adaptability of life to different environments but it is impossible "a priori" to drive any firm conclusions on the true extension of this flexibility as life-based. As will still need the presence of free carbon (organic molecules including chemical functions using H, O, N, S) and water, with its very special deterministic properties, and must remain liquid over relatively large ranges of physical (T, P) parameters.

This simple notion of "abundance and availability" of elements (here P and As) can be extended to rationalize somewhat a few basic principles that must be considered when life's origins, "forms" and ubiquity are concerned. Indeed, this Wolfe-Simon et al, (2010) paper does support a rather widespread belief that totally different life forms should exist based on other heavier elements just because of their relative proximity on the Mendeleev's periodic table of the elements. If P is just above As allowing some chemical similarities between both elements, one may easily note that O is above S, C above Si and N above…P. Can we really start to imagine building life forms on basically "heavier" elements ignoring for the moment their different chemical properties? after all, C and Si are very far from forming the same chemical network which is enormously richer for C than for Si.

I believe the answer is plainly and simply: no!

The reason lies in one of the most important properties of our Galaxy and the chemical elements which are directly related to the evolution of the universe and to the nucleosynthesis of stars; a process which is really well known and dominated the field of astrophysics of in middle of the 20th century (see Tolstikhin and Kramers, 2008 and references therein). Cosmic abundances are well measured from the solar photosphere with a very high precision and, not totally surprisingly, correspond to abundances measured in very primitive meteorites, the so-called CI chondrites (albeit for a few light and not readily condensable elements like H and He, to the first order of this discussion). Note in passing that these cosmic abundances measured from the solar photosphere date back actually from the time of the Sun formation because the Sun itself does not produce any high mass elements apart from He. It may be, after all, not so surprising that our life and any life on other putative planets has just chosen the most abundant elements (H, O, C, N, S, P) not only because these elements are abundant but more importantly, because their relative abundances allow them to react almost only with each other. In super giants AGB stars, the real factories of solid interstellar matter (grains), less abundant elements such as Si, Mg, Fe, Al… do indeed consume all the oxygen they can to form well known minerals that are refractory elements and become thus lost for the gas phase. O, C, N and S at least partly remain in the gas phase (2/3 for O, 1/2 for C, almost 1 for N and S, according to depletions of the elements as observed in the gas of the diffuse interstellar medium (Greenberg, 1974). Thus, these atoms remain in the gas and are somewhat "forced" to react just by the fact that they are the only ones to survive in the gas. Besides, molecules then formed happen to be, besides H₂, CO, H₂O, N₂, NH₃, CH₃OH, H₂CO…for the simplest ones, found indeed in the gas of molecular clouds but also in the solid phase (the so-called and famous interstellar ices), permanently exchanging between gas and solid and, thanks to the extraordinary and unique nature of C, creating an extremely rich chemistry.

As well known, organic chemistry involves millions of different molecules. Some chemists even believe that the total number of organic molecules is virtually "infinite". By contrast, Minerals, although comprised of complicated and various structures, display a comparatively very modest diversity, and the number of known minerals is only around 7500. If life and life sustaining processes needs diversity, no wonder that the basic building blocks of life are just organic molecules containing the already cited elements; elements which made the origins of life possible.

To push the demonstration a bit further, imagine that O and C would be even slightly less abundant than Si, Mg, and Fe. Then all O will be found in minerals, all C in carbides. Water would even not exist, neither in the gas, nor in the solid and, more importantly not in the liquid form. Tetravalent C would simply be useless in the form of carbides and the world would be mineral, with the useful elements, those who are semi-refractory and easily soluble in water, lost in a thermodynamic dwell that indeed a solid is.

Lessons from basic astrophysics just tell us that life, massively based on other elements, is probably impossible. A last and important word of caution though: we must distinguish between the origins of life, the evolution of life, and the adaptability of life the later of which may not require or need all the elements from Mendeleev's table--but that is obviously a completely different story.