Phosphorus is an essential element for all life on Earth. Without it, we would not have the DNA that stores our genetic information, the ATP that provides energy for chemical reactions, or the phospholipids that make up our cell membranes. While phosphorus is abundant in Earth’s rocks, it is present in an oxidized form that makes it unreactive, and is therefore inaccessible to biological organisms. So how did early life on Earth gain access to phosphorus?
Until now, the prevailing theory has been that meteorites brought phosphorus to Earth in the form of the mineral schreibersite, a phosphorus-rich compound that dissolves in water, yielding reduced phosphorus. The difference between this reduced phosphorus and the oxidized phosphorus in many of Earth’s rocks is the charge on the atom, which makes this reduced atom able to react to form biomolecules while the oxidized atom cannot.
There are a couple of problems with this meteorite theory. First, the frequency of meteorite strikes was declining as life emerged on Earth. Second, the force of a meteorite impact could kill off any nearby lifeforms or vaporize the phosphorus-rich schreibersite before it could be used.
A study published in Nature earlier this year suggests an alternative source of reduced phosphorus for early life: lightning. Lightning strikes can form glasses called fulgurites, which this study found to contain schreibersite, particularly in clay fulgurites with high concentrations of graphitic carbon. Fulgurites can also contain reduced phosphorus as part of other compounds, even when there is no schreibersite present.
The researchers created a mathematical model to determine how much reduced phosphorus would have been produced by lightning strikes versus meteorites over time. They used available data on historic concentrations of CO2 to predict the frequency of lightning strikes. This correlation occurs because CO2 increases the temperature of the atmosphere, allowing more water vapor to collect. Lightning is formed by charge separation within fast-moving clouds, with water vapor as the fuel for this process. Thus, as an oversimplification of a phenomenon that climate scientists are actively studying, more CO2 means more water vapor means more lightning strikes. Combining CO2 data with their fulgurite analysis, the researchers were able to determine how much reduced phosphorus was produced by lightning strikes and compare that to what previous studies had determined was produced by meteorites. They concluded that lightning strikes were a source of reduced phosphorus as significant in size as meteorites when life was emerging some 3.5 billion years ago.
This research has important implications in understanding the development of life on Earth as well as informing our search for life on other planets. Lightning provides a source of phosphorus that remains constant over time and is far less destructive than meteorite strikes, enabling the development of life in shallow water environments such as those on early Earth and those being studied on Mars.
This article was edited by Noah Schwab and Lydia Guertin.