By Barrie Winn
Part 1 of this series focused on the critical role that phosphates play in the biochemistry of all living organisms. In Part 2 we will address some of the challenges in identifying the source of phosphates for the first living organisms on Earth.
Phosphorus Necessary for Life’s Origin
Research into naturalistic explanations for the origin of life has long focused on how the “building blocks of life” (amino acids, nucleotides, sugars, and fatty acids) could have formed as the necessary precursors for life. An article by two preeminent origin-of-life researchers1 describing possible synthesis routes for building blocks of life from simple precursors makes no mention of a precursor containing phosphorus. This is a serious omission. Charles Darwin himself knew that there had to be such a source. In speculating about “the first production of a living organism” he wrote: “But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts,–light, heat, electricity &c present, that a protein compound was chemically formed . . . ”2 then perhaps life could have gotten the start it needed.
Phosphorus is the eleventh most abundant element in the Earth’s crust, forming about 0.1% of the crust.3 Although this level makes it a “trace” element, the Earth is nevertheless an unusually phosphorus-rich planet. The concentration in our Sun is “higher than the average abundance found anywhere else in the universe—except Earth, which is an unusually phosphorus-rich planet.”4
Right Kind of Phosphorus Needed
Most of the phosphorus in the Earth’s crust occurs as fluorapatite or carbonate-fluorapatite, which are both orthophosphate minerals.5 As noted in Part 1, there are vast deposits of sedimentary, igneous, and island apatites on the Earth. However, origin-of-life scenarios can only assume the presence of igneous phosphates since sedimentary and island phosphates are derived from living organisms. Assuming igneous apatite was the source of phosphorus for the origin of life presents a challenge: it is a relatively insoluble mineral and not likely to provide soluble orthophosphate ions unless in an acidic environment. Even if it dissolved in a locally acidic environment, synthesis routes to biomolecules would have had to compete with a number of other possible reactions, in particular the precipitation of inorganic phosphates (salts of calcium, magnesium, iron, and aluminum). Also, many biomolecules would not have been stable in the low pH and high temperature required to dissolve apatite.
The bond energy released when adenosine triphosphate (ATP) is converted to adenosine diphosphate (ADP) is the energy storage mechanism used in all living cells. Researchers have been challenged to develop an explanation for the formation of such an energy-rich bond under prebiotic conditions. Carl Sagan and colleagues suggested that the energy could have come from ultraviolet radiation in the early Earth’s atmosphere, and they successfully demonstrated the step-wise conversion of adenine to ATP under simulated early Earth conditions using ultraviolet radiation.6 However, the nucleoside phosphates did not form when the source of phosphate was phosphoric acid (an orthophosphate); they only formed when the source was ethyl metaphosphate, a non-orthophosphate source.
Delivery by Meteorites?
Recognizing the challenges of explaining how fluorapatite could be the source of phosphorus for prebiotic chemistry, researchers have considered the possible delivery by meteorites, which have been found to contain several phosphorus-bearing minerals.7 The predominant form in iron meteorites is the phosphide mineral schreibersite (Fe,Ni)3P, while in stony meteorites it is primarily present in orthophosphate minerals. Origin-of-life researcher Matthew Pasek has estimated that during the Late Heavy Bombardment, iron meteorites could have “deposited between 106 and 1010 kg of phosphorus as reactive schreibersite across the surface of the Earth per year.”8
Research on the chemistry of schreibersite corrosion in aqueous environments has shown that:
Phosphide corrodes in water to form three major species—hydroxyl, hydrogen, and phosphite radicals. The recombination of these three radicals produces the orthophosphate, phosphite, and hypophosphate species. Secondary radical propagation reactions produce pyrophosphate and many minor P species. Interactions of these radicals with organic radicals in turn forms organophosphorus compounds, with P–C and P–O–C linkages. . . . Radical recombination reactions overcome the energetic difficulties traditionally associated with origin of life studies. The reaction of a P radical with an organic radical or organic compound produces an organic P compound without the need for condensing agents, elevated temperatures, or geologically rare P compounds.9
Researchers have reported the successful phosphorylation of adenosine and uridine in aqueous solutions with synthetic analogs of schreibersite.10 However, a critical review of their results raises serious questions as to whether the controlled lab conditions required for successful phosphorylation could have operated with any significant yields on the early Earth.11
While this line of research may eventually yield a credible pathway for the incorporation of phosphorus in the first biomolecules, an origin-of-life scenario that depends on the delivery of phosphorus in meteors poses a major challenge to a fully naturalistic description. The date for the earliest evidence of life on Earth is approaching 3.8 billion years ago, close to the end of the Late Heavy Bombardment.12 This date allows a very narrow time window for chemical evolution from simple molecules to the first living cell. And, coupled with the controlled conditions that origin-of-life researchers have shown to be necessary for their postulated pathways, the scenario presents a huge challenge for naturalistic evolution. Instead, these conditions attest to the level of intelligence needed for the first living cell to develop, which is strong evidence for a Creator!