Lightning's role in the beginning of life

To Earth science now and the origins of life on our planet; and when Benjamin Hess was a student at Wheaton University in Illinois, he had no idea that a local storm would change the course of his career and produce a remarkable piece of geology that has shed light, or rather, lightning, on how life may have got started here. In a new study, which he explains to Phil Sansom, he’s shown how bolts of lightning may have first endowed the early Earth with one of the crucial building blocks of life: the element phosphorus...

Benjamin - Basically lightning struck somebody's back garden, their property nearby. They called up the department of geology at Wheaton College and were like, "Hey, there's this thing in our backyard, do you want to come check it out?" And some professors showed up and realised it was a lightning strike.

Phil - And when you say "it", what exactly was it?

Benjamin - It was a lightning strike glass called a fulgurite. So when lightning hits the ground, it heats it to thousands of degrees Celsius and melts it immediately. And then it reforms. So it was like this tree trunk-like structure under the ground, with branches coming off the bottom.

Phil - And the size of a tree trunk too, yeah?

Benjamin - Yeah. So the total mass that was collected was about 25 kilograms of material. So like a fair sized tree trunk under the ground basically.

Phil - Is that kind of geologist Christmas?

Benjamin - Yeah. I mean, it's rare to find something so interesting in the flat plains of the Midwest. Let me tell you that. And I just started prodding at it basically, with whatever analytical techniques I had access to at Wheaton College. And there were a few little small spheres of something, that I didn't know what they were, using the scanning electron microscope, which essentially tells you the elemental composition and the structures of the things you're looking at, that actually allowed us to finally figure out what these metal spherules were. And it turns out that they were an iron phosphide, and phosphide is a reduced form of phosphorus, meaning it's no longer bonded to oxygen, which is really weird because we live in a very oxygenated atmosphere. So it's hard to make that happen.

Phil - So a rare mineral?

Benjamin - Yeah. Something that you really would not see. I've never seen a phosphide before in any rock. When I first found it, I was kind of like, huh, that's strange. But when I started reading the literature on what this mineral is, it's a mineral called schreibersite. The most well-known locality where it shows up is in meteorites, because there's less oxygen out in space when meteorites are forming. And it turns out schreibersite is thought to be one of the most widespread sources for phosphorus for life on early Earth, the formation of life.

Phil - Wow. This is a big jump.

Benjamin - Yeah, that's what I thought too. I was like, this is crazy. What's this meteorite mineral doing in this lightning strike glass, and you know, meteorites and origins of life. And it's like, hold on. This could actually be a really big story.

Phil - How so? Where is phosphorus important to the picture and why is it such a big deal to try and get a hold of it?

Benjamin - Phosphorus is one of the essential elements for life. It forms a lot of structural and functional elements in cells. Like it makes up the backbone of DNA and RNA, the double helix structure. The problem is it's trapped in minerals that are common, but insoluble and unreactive. So you basically can't use the phosphorus. So the question is where do you get phosphorus, that's free to react and make molecules needed for life.

Phil - And it's this mineral?

Benjamin - Yeah, that's one of the best solutions scientists have come up with. They found this phosphide mineral, schreibersite, because it readily reacts with water to free up phosphorus. But we saw this in a lightning strike and we thought, wait a minute, maybe lightning could also be a source.

Phil - As opposed to meteors, you mean?

Benjamin - Not as opposed, but in addition.

Phil - How do you actually figure out then, whether this is a likely thing to have happened billions of years ago?

Benjamin - That is really the big question that took up the bulk of the time writing this study, trying to come up with a convincing estimate. The three big things are, you need to know how many lightning strikes are happening. You need to know how much phosphorus was in the rocks that they were striking, and you needed to know how much phosphorus each lightning strike turns into schreibersite or something similar that can be available for prebiotic chemistry. Based on our calculations, we think that lightning was providing about as much phosphorus as meteorites.

Phil - Doesn't it kind of remind you of a Frankenstein sort of situation? I mean, the popular idea of creating life involves a lightning strike. And it's crazy to imagine that this was partly what actually happened for us to come along.

Benjamin - Yeah, absolutely. And the cool thing about this mechanism is it can operate on other planets as well. After the solar system forms meteorites kind of get cleared out by planets. So there's kind of a cutoff window for when they can provide enough phosphorus for life. But if you have a stable atmosphere, lightning is a mechanism that can operate indefinitely.