Tiny cracks in rocks may have concentrated chemicals needed for life

In some ways, the origin of life is looking much less mystifying than it was a few decades ago. Researchers have figured out how some of the fundamental molecules needed for life can form via reactions that start with extremely simple chemicals that were likely to have been present on the early Earth. (We've covered at least one of many examples of this sort of work.)

But that research has led to somewhat subtler but no less challenging questions. While these reactions will form key components of DNA and protein, those are often just one part of a complicated mix of reaction products. And often, to get something truly biologically relevant, they'll have to react with some other molecules, each of which is part of its own complicated mix of reaction products. By the time these are all brought together, the key molecules may only represent a tiny fraction of the total list of chemicals present.

So, forming a more life-like chemistry still seems like a challenge. But a group of German chemists is now suggesting that the Earth itself provides a solution. Warm fluids moving through tiny fissures in rocks can potentially separate out mixes of chemicals, enriching some individual chemicals by three orders of magnitude.

Feeling the heat (and the solvent)

Even in the lab, it's relatively rare for chemical reactions to produce just a single product. But there are lots of ways to purify out exactly what you want. Even closely related chemicals will often differ in their solubility in different solvents and in their tendency to stick to various glasses or ceramics, etc. The temperature can also influence all of those. So, chemists can use these properties as tools to fish a specific chemical out of a reaction mixture.

But, as far as the history of life is concerned, chemists are a relatively recent development—they weren't available to purify important chemicals back before life had gotten started. Which raises the question of how the chemical building blocks of life ever reached the sorts of concentrations needed to do anything interesting.

The key insight behind this new work is that something similar to lab equipment exists naturally on Earth. Many rocks are laced with cracks, channels, and fissures that allow fluid to flow through them. In geologically active areas, that fluid is often warm, creating temperature gradients as it flows away from the heat source. And, as fluid moves through different rock types, the chemical environment changes. The walls of the fissures will have different chemical properties, and different salts may end up dissolved in the fluid.

All of that can provide conditions where some chemicals move more rapidly through the fluid, while others tend to stay where they started. And that has the potential to separate out key chemicals from the reaction mixes that produce the components of life.

But having the potential is very different from clearly working. So, the researchers decided to put the idea to the test.

You gotta keep ’em separated

Their tests focused on a very simplified system, one that doesn't include all the complexities of fluid flow through rocks. Instead, they started with a simple system of two chambers connected by a small bit of lab tubing. So, there wasn't the potential for physical differences in the fluid flow in different cracks or changes in the chemical environment as the fluid flowed across different rock surfaces. Instead, the key difference was temperature, with the fluid flowing from a warm source to a cooler destination.

Even in that simplified system, however, a difference of 15° C was enough to get chemicals to travel between the two chambers at different paces. This resulted in some chemicals being purified by 30 percent relative to the original mixture they started out in. Others reached over 140 percent purification—all driven by nothing more than their different mobility across a temperature gradient. Starting with a mix of all 20 amino acids, a few of them ended up purified by 80 percent in this system.

Obviously, natural systems in rocks are far more complex than two chambers connected by a crack. To test something more elaborate, the researchers set up a single chamber linked to two separate ones, all held at different temperatures. Again, starting with a mix of all 20 amino acids resulted in a variety of purifications, ranging from a low in the area of fourfold, and a high of over twentyfold.

Using this data, they built a computer model of an even more elaborate system where fluid flowed through 20 individual chambers, again with temperature gradients across the system. Here, enrichment could reach 2,000-fold, although it didn't get much above 20-fold at the low end. This means that these systems can result in areas where individual chemicals are over 95 percent pure and, for some chemicals, over 99.9 percent pure.

As a final demonstration, they showed that these systems can bring together two reactants to boost a reaction between them that was otherwise rare. In this experiment, the levels of the reaction product were increased by up to five orders of magnitude.

What’s this mean?

This is likely to be a low estimate of the amount of purification that's possible, since the experiments took place in standard plastic labware. A more complicated chemical environment, like the one provided by rocks, could potentially change the mobility of chemicals even further.

That said, it's worth remembering that when given a mix of amino acids, they ended up concentrated in different locations. While this might be great for separating out a key biological molecule from a reaction mixture, it doesn't guarantee that a bunch of biologically relevant chemicals will all end up in the same location. So, it'll probably take a while to get some better ideas about what this might mean for the chemistry that might have led up to life.

But those caveats shouldn't be viewed as taking away from the work here. "Our results show the simultaneous but spatially separated, heat-flux-driven purification of more than 50 prebiotically relevant organic compounds," the authors write. And that's an impressive collection of results.

Nature, 2024. DOI: 10.1038/s41586-024-07193-7  (About DOIs).