Born under a bad sign
Been down since I began to crawl
If it wasn't for bad luck
You know, I wouldn't have no luck at all
—Booker T. Jones and William Bell
One of my favorite blues tunes is "Born Under a Bad Sign," a song about someone who just can't catch a break.
If there is a scientific discipline that is characterized by "havin' bad luck all of [its] days," it's origin-of-life research. This trend of bad luck continues for a collaborative team from the University of South Florida (USF) and the Georgia Institute of Technology (GT) who are seeking to identify a chemical process that could produce organic phosphates on early Earth, a necessary step in any origin-of-life scenario. Ironically, in their attempts to support a naturalistic origin-of-life scenario, these researchers have demonstrated the critical role an intelligent agency must play in life's genesis.
The emergence of organic phosphates stands as one of the most significant challenges facing any naturalistic origin-of-life scenario. Organic phosphates include DNA; RNA; the biomolecules that form cell membranes; and ATP, the compound that serves as the cell's energy currency.
The Problem of Organic Phosphates
Phosphorylation reactions are the chemical processes that generate organic phosphates in the cell. Phosphorylation occurs when enzymes catalyze the addition of phosphate groups to a target molecule. One conceivable way that phosphorylation could have happened during abiogenesis is through the direct addition of a phosphate group to prebiotic molecules. However, this reaction doesn't take place readily in water—it requires dehydrating conditions and relatively high temperatures.
Origin-of-life researchers aren't sure where these types of conditions would exist on early Earth. Additionally, they are concerned that high temperatures would have caused fragile prebiotic materials to break down. Another problem origin-of-life researchers have identified with this chemical route relates to phosphates' solubility. These compounds tend to be highly insoluble in the presence of calcium and magnesium ions, both of which would have been abundant in early Earth's oceans. The insolubility of calcium and magnesium phosphates would have rendered these compounds unavailable for any prebiotic reactions. (For a more complete discussion of the problems associated with generating organic phosphates on early Earth see my book Creating Life in the Lab.)
Have Researchers Found a Solution?
A few years ago, the team from USF proposed a way around these problems. They suggested that organic phosphates could be produced from the iron phosphide and iron-nickel phosphide composing schreibersite (a mineral found in meteorites).1 The USF scientists speculated that abundant schreibersite would have been delivered to early Earth when the planet was pummeled with asteroids during its early history. To confirm their suspicion, these researchers analyzed carbonate minerals from a geological formation in Australia that dates to around 3.5 billion years ago. The team identified phosphites in the carbonate minerals at levels that indicated these minerals would have been a prominent species in early Earth's oceans. Phosphites do not have a biological origin and the phosphites in the carbonate minerals were most likely generated from the phosphides in schreibersite.
Phosphites are much more chemically reactive than phosphates and can phosphorylate organic materials in water. This makes them—and schreibersite—a potential source of phosphorus for phosphorylation reactions on early Earth.
To confirm that schreibersite could, indeed, phosphorylate organic compounds, the researchers heated an aqueous solution of glycerol and schreibersite to 150°F for two days. Afterwards, they found phosphite in the solution along with low levels of glycerol phosphate.
In a follow-up study, the USF team, in collaboration with researchers from GT, assessed whether or not schreibersite could phosphorylate adenosine and uridine nucleosides. The phosphorylated forms of these molecules comprise two of the four building blocks of RNA.2 These building block materials factor significantly into the RNA world hypothesis, one of the most important origin-of-life scenarios. The scientists showed that these two nucleosides could be phosphorylated when heated with schreibersite for several days at 175°F, when the solution was slightly alkaline. They even showed that this reaction would proceed in the presence of magnesium ions.
Based on these two studies, the researchers posit that they have made significant strides towards understanding how organic phosphates formed on early Earth and provided support for chemical evolution and abiogenesis:
The reactions we observed in our experiments have shown that the necessary prebiotic molecules were likely present on the early Earth and that the Earth was predisposed to phosphorylated biomolecules. Our results suggest a potential role for meteoric phosphorus in the development and origin of early life.3
Why the Proposed Solution Doesn't Hold Up
Careful analysis of these two studies identifies significant problems with their conclusion. First, the yields of these reactions are low, raising questions about the significance of schreibersite-mediated phosphorylation reactions. When schreibersite was incubated with glycerol, the yield was only 2.5 percent; and when incubated with adenosine and uridine nucleosides, the yields were only 1 to 6 percent. Second, both studies were conducted under chemically pristine conditions, in which the researchers carefully excluded materials that would compete with the desired phosphorylation reactions. Other compounds would have likely been present on early Earth—many at relatively high levels—that could take part in phosphorylation reactions. These competing side reactions would dramatically reduce the already low yields of the desired products.
Selectivity of these reactions also raises concern. In biological systems, nucleosides are phosphorylated at a specific site (the 5' position) in the molecule, but in the laboratory studies, the 2' and 3' positions were also phosphorylated. This lack of selectivity is problematic and further reduces the yield of desired phosphorylation products.
Finally, phosphorylation of nucleosides by schreibersite is pH dependent. The researchers discovered that unless the reaction mixture was alkaline, phosphorylation would not occur. Unfortunately, early Earth's oceans were acidic. This fact alone makes it unlikely that schreibersite-mediated phosphorylation could have ever occurred to any appreciable extent on early Earth.
The USF and GT researchers have identified a chemical process that could, in principle, yield key organic phosphates. However, they failed to show that this process would operate efficiently enough on early Earth to contribute to a naturalistic origin of life.
If it were not for the researchers' careful design and the controlled lab conditions, the schreibersite-mediated phosphorylation reactions wouldn't have been successful. In other words, the researchers acted as intelligent agents. As Simon Conway Morris pointed out in his book Life's Solution, "Many of the experiments designed to explain one or other step in the origin of life are either of tenuous relevance to any believable prebiotic setting or involve an experimental rig in which the hand of the researcher becomes for all intents and purposes the hand of God."4