How did the building blocks of life originate?
The question has long vexed scientists. Early Earth was dotted with pools of water rich in chemicals—a primordial soup. Yet biomolecules supporting life emerged from the mixtures, setting the stage for the appearance of the first cells.
Life was kickstarted when two components formed. One was a molecular carrier—like, for example, DNA—to pass along and remix genetic blueprints. The other component was made up of proteins, the workhorses and structural elements of the body.
Both biomolecules are highly complex. In humans, DNA has four different chemical “letters,” called nucleotides, whereas proteins are made of 20 types of amino acids. The components have distinct structures, and their creation requires slightly different chemistries. The final products need to be in large enough amounts to string them together into DNA or proteins.
Scientists can purify the components in the lab using additives. But it begs the question: How did it happen on early Earth?
The answer, suggests Dr. Christof Mast, a researcher at Ludwig Maximilians University of Munich, may be cracks in rocks like those occurring in the volcanoes or geothermal systems that were abundant on early Earth. It’s possible that temperature differences along the cracks naturally separate and concentrate biomolecule components, providing a passive system to purify biomolecules.
Inspired by geology, the team developed heat flow chambers roughly the size of a bank card, each containing minuscule fractures with a temperature gradient. When given a mixture of amino acids or nucleotides—a “prebiotic mix”—the components readily separated.
Adding more chambers further concentrated the chemicals, even those that were similar in structure. The network of fractures also enabled amino acids to bond, the first step towards creating a functional protein.
“Systems of interconnected thin fractures and cracks…are thought to be ubiquitous in volcanic and geothermal environments,” wrote the team. By enriching the prebiotic chemicals, such systems could have “provided a steady driving force for a natural origins-of-life laboratory.”
Brewing Life
Around four billion years ago, Earth was a hostile environment, pummeled by meteorites and rife with volcanic eruptions. Yet somehow among the chaos, chemistry generated the first amino acids, nucleotides, fatty lipids, and other building blocks that support life.
Which chemical processes contributed to these molecules is up for debate. When each came along is also a conundrum. Like a “chicken or egg” problem, DNA and RNA direct the creation of proteins in cells—but both genetic carriers also require proteins to replicate.
One theory suggest sulfidic anions, which are molecules that were abundant in early Earth’s lakes and rivers, could be the link. Generated in volcanic eruptions, once dissolved into pools of water they can speed up chemical reactions that convert prebiotic molecules into RNA. Dubbed the “RNA world” hypothesis, the idea suggests that RNA was the first biomolecule to grace Earth because it can carry genetic information and speed up some chemical reactions.
Another idea is meteor impacts on early Earth generated nucleotides, lipids, and amino acids simultaneously, through a process that includes two abundant chemicals—one from meteors and another from Earth—and a dash of UV light.
But there’s one problem: Each set of building blocks requires a different chemical reaction. Depending on slight differences in structure or chemistry, it’s possible one geographic location might have skewed towards one type of prebiotic molecule over another.
How? The new study, published in Nature, offers an answer.
Tunnel Networks
Lab experiments mimicking early Earth usually start with well-defined ingredients that have already been purified. Scientists also clean up intermediate side-products, especially for multiple chemical reaction steps.
The process often results in “vanishingly small concentrations of the desired product,” or its creation can even be completely inhibited, wrote the team. The reactions also require multiple spatially separated chambers, which hardly resembles Earth’s natural environment.
The new study took inspiration from geology. Early Earth had complex networks of water-filled cracks found in a variety of rocks in volcanos and geothermal systems. The cracks, generated by overheating rocks, formed natural “straws” that could potentially filter a complex mix of molecules using a heat gradient.
Each molecule favors a preferred temperature based on its size and electrical charge. When exposed to different temperatures, it naturally moves towards its ideal pick. Called thermophoresis, the process separates a soup of ingredients into multiple distinct layers in one step.
The team mimicked a single thin rock fracture using a heat flow chamber. Roughly the size of a bank card, the chamber had tiny cracks 170 micrometers across, about the width of a human hair. To create a temperature gradient, one side of the chamber was heated to 104 degrees Fahrenheit and the other end chilled to 77 degrees Fahrenheit.
In a first test, the team added a mix of prebiotic compounds that included amino acids and DNA nucleotides into the chamber. After 18 hours, the components separated into layers like tiramisu. For example, glycine—the smallest of amino acids—became concentrated towards the top, whereas other amino acids with higher thermophoretic strength stuck to the bottom. Similarly, DNA letters and other life-sustaining chemicals also separated in the cracks, with some enriched by up to 45 percent.
Although promising, the system didn’t resemble early Earth, which had highly interconnected cracks varying in size. To better mimic natural conditions, the team next strung up three chambers, with the first branching into two others. This was roughly 23 times more efficient at enriching prebiotic chemicals than a single chamber.
Using a computer simulation, the team then modeled the behavior of a 20-by-20 interlinked chamber system, using a realistic flow rate of prebiotic chemicals. The chambers further enriched the brew, with glycine enriching over 2,000 times more than another amino acids.
Chemical Reactions
Cleaner ingredients are a great start for the formation of complex molecules. But lots of chemical reaction require additional chemicals, which also need to be enriched. Here, the team zeroed in on a reaction stitching two glycine molecules together.
At the heart is trimetaphosphate (TMP), which helps guide the reaction. TMP is especially interesting for prebiotic chemistry, and it was scarce on early Earth, explained the team, which “makes its selective enrichment critical.” A single chamber increased TMP levels when mixed with other chemicals.
Using a computer simulation, a TMP and glycine mix increased the final product—a doubled glycine—by five orders of magnitude.
“These results show that otherwise challenging prebiotic reactions are massively boosted” with heat flows that selectively enrich chemicals in different regions, wrote the team.
In all, they tested over 50 prebiotic molecules and found the fractures readily separated them. Because each crack can have a different mix of molecules, it could explain the rise of multiple life-sustaining building blocks.
Still, how life’s building blocks came together to form organisms remains mysterious. Heat flows and rock fissures are likely just one piece of the puzzle. The ultimate test will be to see if, and how, these purified prebiotics link up to form a cell.
Image Credit: Christof B. Mast
