The oldest evidence for life on Earth dates back over 3.5 billion years, but there is still considerable debate about how those first organisms came to be, and what conditions they might have faced. A new study suggests that micropores in early ocean floors could have acted like crucibles, providing the conditions for precursor molecules to change into nucleic acids that eventually became capable of self-replication. Dieter Braun of Ludwig-Maximilians-Universität München in Munich is senior author of the paper, which was published in Nature Chemistry.
The idea that biomolecules were in an environment that allowed them to change into the precursors of life is not a new one, but there have been some flaws in those theories. Primarily, the concentration of molecules believed to be needed for those reactions to occur is considerably higher than what was available at the time. Braun’s team believes they have found a solution to this problem, which they propose would have allowed the necessary chemical reactions to take place with limited ingredients, by trapping them and concentrating them enough to react with one another.
The early Earth was a hot and hellish place, with constant bombardment from asteroids and comets as well as tremendous amounts of volcanic activity. The oceans did not have large quantities of biomolecules, and what was present was quickly being depleted; not exactly picturesque conditions for delicate life to emerge. However, molecules trapped in pores on the seafloor could have been incubated by temperature fluctuations of hot rocks located below Earth’s crust. This heat could have contributed to chemical reactions that resulted in the first RNA.
“The key requirement is that the heat source be localized on one side of the elongated pore, so that the water on that side is significantly warmer than that on the other,” Braun explained in a press release.
The location of the pores facilitate what is known as thermophoresis, which alters how molecules in a solution behave in differing temperatures. The temperature gradient in the pores caused charged particles to congregate at the top of the pore where it is cooler. While new material could still get in from the water above, longer polymers were trapped in the bottom, where they could perform the necessary reactions over time and evolve into nucleic acids. In order to test the feasibility of this, Braun’s team performed proof-of-concept experiments that mimicked environmental conditions at the dawn of life.
“We used tiny glass capillary tubes to construct an analog of the natural pores found in rock, heated the pore from one side and allowed water containing dissolved fragments of linear DNA of varying lengths to percolate through it. Under such conditions, the long strands are indeed trapped within the pore,” described Braun. “Pores that were exposed to heat are frequently found in igneous rock formations, and they were certainly common in rocks of volcanic origin on the early Earth. So this scenario is quite realistic. And the temperature effect is enhanced by the presence of metal inclusions within the rock, which conduct heat at rates 100 times higher than water.”
The DNA in the experiment began to cycle throughout the pore, with the temperature gradient facilitating its replication. The warm bottom portion denatured the DNA, “unzipping” the two strands. As the single strand of DNA came to the cooler top, the raw ingredients from above connected to it, rebuilding the complimentary strand. This process also led to different strands of DNA becoming connected, lengthening the sequence. Eventually, the DNA becomes too long and escapes from the pore.
“Life is fundamentally a thermodynamic non-equilibrium phenomenon. That is why the emergence of the first life-forms requires a local imbalance driven by an external energy source – for example, by a temperature difference imposed from outside the system,” Braun concluded. “That this can be achieved in such a simple and elegant way was surprising even to us.”
