ONE of the essential ingredients of life on Earth may have arrived here in iron meteorites. These meteorites would merely have had to react with water to release phosphorus, an element crucial to the working of living cells.
Carbon-rich meteorites raining down on Earth about 4 billion years ago are thought to have supplied the hydrogen, carbon, oxygen and nitrogen needed for life to get started. But such meteorites are low in phosphorus, which is the fifth most common element in living cells and a vital component of DNA and adenosine triphosphate (ATP), cells’ main energy-carrying molecule. How life got hold of enough phosphorus has long been a mystery, because the element is relatively rare in nature.
Rocks on Earth do contain phosphorus, mostly in the form of phosphates in which each phosphorus atom is linked to four oxygen atoms. But converting phosphate into the triphosphate found in ATP requires so much energy that it could only happen at temperatures unrealistically high even for the early Earth.
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Now Dante Lauretta of the University of Arizona in Tucson has shown that triphosphates could have come from a phosphorus-rich mineral called schreibersite found in iron meteorites. He has previously shown that corrosion of metallic minerals in meteorites could concentrate phosphorus on their surface. Now, working with his student Matt Pasek, he has simulated what would have happened to these minerals in the atmosphere of early Earth by putting synthetic schreibersite into pure water in a sealed container.
As the mineral reacted with the water, a variety of chemically active phosphorus compounds formed, including P2O7, a diphosphate precursor to ATP. The reaction also released hydrogen, indicating that elemental phosphorus in the mineral had reacted with the water. The researchers presented their results on 25 August at the American Chemical Society meeting in Philadelphia.
While the reaction produced only traces of triphosphates, Pasek says that in producing diphosphate “we have gotten up one of two big steps”. Living cells store energy by converting diphosphate to the triphosphate of ATP. They can then release energy when it is needed by reversing the process.
Lauretta’s next step will be to test real iron meteorites to see if they too produce diphosphates and triphosphates.
If iron meteorites turn out to be a key part of the recipe, Lauretta says, it will mean that life required a planetary system that both makes and breaks planetesimals – small planet-like bodies – that are at least 500 kilometres in diameter. Bodies any smaller than this would not melt, and so would not have a nickel-iron core that could have produced iron-rich meteorites. Collisions would then have had to shatter these large objects in order to scatter meteorites throughout the asteroid belt, some of which would have hit Earth.