Models and Standards of Proof in Cross-Disciplinary Science: The Case of Arsenic DNA

Abstract

Of all the conflicts possible within cross-disciplinary studies, none are more confounding than those that arise from different standards that different disciplines require for ‘‘proof.’’ Here, ‘‘proof’’ is not used as mathematicians use it, but rather it describes the collection of evidence sufficient to accept a discovery, stop experiments, and record a problem as ‘‘solved’’ (Davenas et al., 1988). Experiments can easily produce evidence that is sufficient proof for one scientific discipline but sufficient only to create further controversy for another. Indeed, the identical experimental evidence that is conclusive for one community and controversial for another might cause a third community to reject the very same conclusion entirely. Failure to understand the differences in these standards of proof can allow cross-disicplinary activities to produce flawed science on one hand and reject genuine innovation on the other. Astrobiology is quite cross-disciplinary. As such, astrobiology is expected to generate many such conflicts. As an example, a team of astrobiologists in 2010 reported evidence that they thought was sufficient to conclude that a microbe isolated from Mono Lake (GFAJ-1) contained DNA with some of its backbone phosphorus atoms replaced by arsenic atoms (Wolfe-Simon et al., 2010). The referees who reviewed the paper in the ‘‘planetary science’’ track at Science enthusiastically recommended publication of this conclusion. Physicists, also a major community within astrobiology, also found that the paper’s evidence strongly supported the conclusion of arsenate DNA (Kaku, 2010). However, biochemists and microbiologists, two other communities within astrobiology, found the very same data inadequate to conclude the presence of arsenic-substituted DNA (Redfield, 2010). Chemists went further, seeing in the very same data disproof of the hypothesized arsenic DNA (Drahl, 2010). How could the same data be interpreted so differently by communities that must work together within the discipline of astrobiology to consider life in the Cosmos? While the specific experiments applied to GFAJ-1 have been much discussed, we believe that the study can be examined as a living illustration of how disagreements over the nature of proof confound cross-disciplinary studies. Such examination is necessary if cross-disciplinary fields are to contribute to their many constituent scientific communities. It allows us to address a larger question: What is proof? We start with a simple aphorism: Only the unexpected needs explanation. Further, the more an observation is unexpected, the more explanations are needed. Captured in the aphorism of the late Carl Sagan (‘‘extraordinary claims require extraordinary evidence’’) (Sagan, 1990), we cannot understand what a community finds ‘‘extraordinary,’’ and what that community requires to meet a burden of proof, without understanding the expectations of the community. Considering GFAJ-1, let us begin with chemists, who found the report of arsenate DNA most unexpected and most extraordinary. Over the past two centuries, chemists have examined millions of compounds. Each is associated with a molecular structure, a model that describes the arrangement in space of constituent atoms held together by bonds. Associated with each compound are also measurements, often quite detailed, of its physical properties and molecular reactivities. These collections support ‘‘structure theory’’ in chemistry, which explains the properties and reactivities of all matter by making reference to molecular structures. Further, the structures in these collections are tightly and logically interconnected. Water is H2O, not H3O. If water were H3O, then the difference propagates across the collection by force of deductive logic. Thus, any claim that water is H3O is a claim that many of the structures in the entire collection must be wrong, and the observations that support those structures must also be revisited. That is, an enormous amount of data commonly viewed as true must be false if water turns out to be H3O. This makes a claim that water is H3O extraordinary, to a chemist. The cross-validation of structures in the chemist’s collection of compounds extends to reactivity. For example, modern databases of molecular structures contain many ‘‘arsenate esters,’’ molecules containing an arsenic atom surrounded by four oxygen atoms, with one, two, or three of the oxygen atoms attached to carbon chains. The carbon chains are different in different arsenate diesters, but the species react analogously. All known arsenate esters hydrolyze rapidly in water at any temperature where water is liquid. This leads chemists to expect that all arsenate esters will hydrolyze rapidly in water, even those not yet examined. Indeed, if one draws a structure of an as-yet unknown arsenate ester, a chemist will anticipate how fast it will