Lightning and life

Gregory Nemec

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Benjamin Hess, a graduate student in Yale’s Department of Earth and Planetary Sciences, was studying fulgurite—that reticulated glass that sometimes can be found in soil after a lightning strike—that had formed in the clay-rich earth in the flats of northern Illinois. While analyzing the fulgurite for the impact of thermal shock on minerals, Hess was surprised to discover large quantities of schreibersite, a rare mineral whose presence raised an intriguing possibility: might lightning strikes have been central to the rise of life on Earth?

One of the fundamental elements of life as we know it is phosphorous. It forms the structural backbone of DNA and RNA; it is essential in the generation of cell membranes. And though phosphorous is abundantly present on Earth, most of it is inaccessible, chemically bound up in hard minerals.

Schreibersite is different, because it carries phosphorous in a soluble form. When schreibersite gets wet, the phosphorous washes out and, with exposure to sunlight, can go on to form the foundational organic molecules from which life emerges.

This, of course, raises a difficult question: where does schreibersite come from?
For years, the answer has been meteorites, which often carry the mineral, and which pummeled Earth with great frequency during the planet’s infancy. “But when we found large amounts of schreibersite in this fulgurite—more than anyone had reported before—it got us thinking,” says Hess. “Maybe lightning was a more significant mechanism for life on Earth than previously believed.”

He and two colleagues from the University of Leeds used computer models of Earth’s early atmosphere to estimate the number of annual lightning strikes that were occurring roughly 3.5 billion years ago, when life is believed to have emerged. This, in turn, provided an estimate of how much phosphorous lightning may have produced. The estimate—thousands of kilograms per year—was comparable to the amount of phosphorous produced by meteorites.

Hess noted that this finding rests on two key assumptions. First, and most important, there remains a fair amount of uncertainty about Earth’s atmospheric conditions billions of years ago. The number of lightning strikes is contingent on carbon dioxide concentrations; if the actual CO2 levels were very different from what the model predicted, then the results would be off. Second, there’s no clear understanding of how much exposed land there was on Earth when life first emerged. If there was land enough for lightning strikes to create plentiful phosphorous, life may have first emerged terrestrially, in swampy ponds. If not, then the competing theory is that life emerged beside deep-sea hydrothermal vents.

Given favorable conditions, though, perhaps the seeds of life were carried down not from the stars, but from the clouds; not by meteorites, but by electricity.