Nine years ago, the head of a farm in Maçanet de la Selva (Girona) observed something very curious. Due to construction work, a septic tank that had contained pig manure for years had to be cleaned. The operation uncovered a magnificent formation of crystals that lined the concrete walls of the pit. The wise farmer, fascinated by the unique phenomenon, collected numerous samples and, thinking of the potential interest of the crystals, sent several to our laboratory.
We identify them as struvite, magnesium ammonium phosphate. Other than the size of the crystals, there was nothing strange. Struvite is common in oxygen-poor environments, with ammonia and organic matter. If you find crystals or crystalline grit in a can of canned fish it could be struvite formed after canning. Do not worry: it is harmless and does not imply a bad conservation.
This anecdote gained interest after a few years. It was during discussions about the chemistry of the origin of life, which we held in the NSF-NASA Center for Chemical Evolution. We wanted to understand an old problem of the origin of life: how was phosphate incorporated into chemical evolution (processes of synthesis, assembly and molecular selection that lead to biochemical complexity and life)? This could have happened on Earth about 4.2 billion years ago and phosphate is one of the keys.
That Girona grave was inspiring in our research on the subject.
Phosphate: the carrier of the book of life
Perhaps the first thing that comes to mind when you think of phosphorus and life are bones, made up of calcium phosphate. But, if we travel through the molecular world, we see that phosphate is key in cellular communication and regulation, in metabolism and energy. It also forms the framework for DNA and RNA.
Phosphate connects letters in DNA, and is the ideal carrier for genetic information. It favors the formation of the double helix, the folding of structures such as the ribosome, it is essential in the interaction between DNA and proteins and in basic processes of molecular biology such as replication. Phosphate is the binding of a very flexible book, on which information can be written so that it can be read, copied and corrected.
We do not know of any viable alternative to phosphate that allows evolution as we know it. Therefore, we think that in the process that gave rise to life there was a decisive moment in which phosphate entered from the mineral environment. But phosphate tends to form very insoluble minerals and, in addition, it is difficult for it to react with the organic precursors of life.
This difficulty was called “the phosphate problem”, and we were interested in exploring possible solutions.
Some meteorites are rich in schreibersita (iron phosphide), a very rare form of phosphorus on our planet. In the early Earth, subjected to intense meteoric bombardment, this phosphide must have been much more frequent. Our colleague Matthew Pasek observed something very interesting: Schreibersite weathers, releasing active phosphorus species, which easily form organic phosphate compounds. Perhaps the meteorites were the key to the phosphate problem.
However, it seemed difficult that schreibersite was effective in driving chemical evolution, since it is only a minor part of a small percentage of total meteorites. Lightning strikes in phosphate soils have been shown to give rise to phosphide fulgurites, which would increase their abundance.
We have a different idea: phosphate is the most abundant form of phosphorus. In addition, phosphate tends to concentrate in volcanic environments, forming for example the phosphate tephras with minerals such as apatites or merrillita. In a volcanic area, in which pools form that seasonally dry and flood, alteration minerals and organic compounds formed in what we call prebiotic chemistry.
If this occurs on phosphate soil, will RNA precursors form?
It was inevitable to recall the letter that Charles Darwin wrote to JD Hooker in 1871, in which he imagined a “little hot pool” containing phosphate and ammonia, where, by the effect of light, heat and electric discharges, the organic matter that life preceded. We think that this primordial “hot pool” must have resembled our Girona septic tank, rich in urea and organic matter. In addition, it must have contained other relevant components in the early Earth, such as cyanide and its derivatives.
When we conducted the “urea pool” experiment on phosphate mineral, beautiful struvite crystals formed. Some scientists thought that struvite is a mineral associated with life. In fact, in the septic tank, the bacterial decomposition of urea creates the conditions for its formation. We saw that their formation is possible in the absence of life. In addition, the combination of struvite and urea promotes, among others, the formation of RNA precursors.
Thus, we do not depend on meteorites and lightning to explain how phosphate entered prebiotic evolution. The planet’s own geochemistry would suffice. Of course, both processes could have occurred simultaneously, contributing to the formation of phosphate compounds.
The chemical processes that gave rise to the precursors of life could also change the rocks, contributing to their weathering and forming minerals such as struvite. At the origin of life, it is not only necessary to take into account molecules such as RNA. The geological context and minerals are also very important. Therefore, exploring the geology and mineralogy of Mars is important to understand how life originated. If minerals related to struvite were found on Mars, they could be a prebiotic marker that would indicate to us that the planet could have started the path to life.