Did lightning spark life on Earth?

A quintillion lightning strikes over a billion years may have helped kickstart life on Earth, according to scientists from Yale University, US, and the University of Leeds, UK.

The earliest undisputed signs of life show up around 3.5 billion years ago, although researchers have good reason to suspect that the whole affair of living could have got going much earlier. But there’s a small problem: life didn’t have the right ingredients yet.

The organic molecules that make up the building blocks of life – including sugars, enzymes, protein and DNA – didn’t exist naturally on the very early Earth, and the elements needed to make them (like carbon, nitrogen, hydrogen and oxygen) were bound up in the rocks, atmosphere and early oceans.

Now, a new study published in Nature Communications suggests that lightning could have “freed” one such element: phosphorous.

“We propose that under the conditions on early Earth, phosphorus reduction via lightning strikes is a more significant process than previously appreciated,” the authors write in their paper.

Phosphorous has long been known to be fundamental to all living things, essential to the formation of DNA, cell membranes and more. Today, it enters the planet’s systems through the weathering of rocks over time, but the phosphorous on early Earth was locked up in insoluble minerals.

Since many meteorites contain phosphorous in a highly reactive form called schreibersite, scientists first thought that meteorites brought enough in usable form to the surface; however, the frequency of impacts was estimated to be too low.

But schreibersite has a terrestrial source, too, as a separate team of researchers discovered in 2009. It’s found in fulgurites, which are glassy veins of melted sand, soil, or rock formed when lightning strikes the ground. This prompted this new study to consider just how much phosphorous could be created by lightning strikes.

The team, led by Benjamin Hess from Yale, used computer modelling to estimate that the ancient Earth experienced far more lightning than we do now.

The Earth today sees about 560 million flashes of lightning per year, while the early Earth saw 1–5 billion flashes, with between 100 million and 1 billion of these striking the ground.

Over a billion years, those kinds of numbers add up: potentially, a quintillion (a billion billion) strikes could have struck the ground and helped release usable phosphorous.

“Unlike meteorite impacts, which are extremely destructive, lightning strikes would provide a relatively non-destructive, continual source of reactive phosphorus species that would not interfere with the delicate evolutionary steps required for complex prebiotic synthesis,” the authors write in their paper.

Their models also suggest that lightning would have been more prevalent on land masses in tropical regions, potentially creating areas of concentrated phosphorous.

“This work helps us understand how life may have formed on Earth and how it could still be forming on other, Earth-like planets,” says Hess. “It makes lightning strikes a significant pathway toward the origin of life.”