One thing is certain: life on Earth emerged very early in the history of planet Earth. There are few concrete answers to the questions of how and where the first organic molecules were formed. One popular theory assumes that the breeding ground for life was hydrothermal vents deep under the sea. Researchers are proposing a new plausible scenario for the origin of life on Earth: Meteorites. The iron they contain could have played a decisive role in the formation of the first building blocks of life.
Researchers at the Max Planck Institute for Astronomy and the Ludwig Maximilian University of Munich have used experiments with meteorites and volcanic ash to show a new way in which organic molecules could have formed under the conditions on early Earth. The key role here is played by iron particles from meteorites and volcanic ash, which act as catalysts. Catalysts are substances whose presence speeds up specific chemical reactions, but which are not consumed in the process. In that way, they are akin to the tools used in manufacture, for example, to build not just one bicycle but several.
In this case, it is plausible that these iron particles could have contributed to the formation of the first organic molecules from the carbon dioxide-rich primordial atmosphere, including hydrocarbons, acetaldehyde or formaldehyde. These substances are in turn building blocks for fatty acids, nucleobases (themselves the building blocks of DNA), sugars and amino acids. These organic molecules are the building blocks of more complex organisms. Their formation was the first early step in a sequence of events that brought life to Earth. It took about 2 billion years for the first (eukaryotic) cells to form.
From industrial chemistry to the beginnings of the Earth
Key inspiration for the research came, of all things, from industrial chemistry. It is known that carbon monoxide and hydrogen can be converted into hydrocarbons with the help of metallic catalysts. The process behind this is called the Fischer-Tropsch process. Oliver Trapp, professor at the Ludwig Maximilian University of Munich and Max Planck Fellow at the Max Planck Institute for Astronomy, wondered whether this process could not also have taken place on an early Earth with an atmosphere rich in carbon dioxide: "When I looked at the chemical composition of the Campo-del-Cielo iron meteorite, consisting of iron, nickel, some cobalt and tiny amounts of iridium, I immediately realized that this is a perfect Fischer-Tropsch catalyst", Trapp explains. The logical next step was experiments to test the cosmic version of Fischer-Tropsch.
Dmitry Semenov, a staff member at the Max Planck Institute for Astronomy, brought volcanic ash into play: "When Oliver told me about his idea, my first thought was that we should also study the catalytic properties of volcanic ash particles. After all, the early Earth must have been geologically active." There should have been plenty of fine ash particles in the atmosphere and on the Earth's first land masses.
Early Earth in the Laboratory
Trapp's doctoral candidate, Sophia Peters, carried out the necessary experiments as part of her PhD work. For access to meteorites and minerals, as well as expertise in the analysis of such materials, she reached out to mineralogist Rupert Hochleitner, an expert on meteorites at the Mineralogische Staatssammlung in Munich. For their experiments, they used iron particles from an iron meteorite, an iron-bearing stony meteorite or volcanic ash from Mount Etna. The iron particles were then mixed with various minerals, as they were supposed to have been present on the early Earth. These minerals served as a support structure, since catalysts usually accumulate as small particles on a suitable substrate.
Particle size matters. The fine ash particles produced by volcanic eruptions are typically a few micrometers in size. In the case of iron-rich meteorites falling through the atmosphere of the early Earth, atmospheric friction would ablate both micrometerand nanometer-sized iron particles, while the iron would evaporate in the intense heat and later re-solidify in the surrounding air.
The researchers aimed to reproduce this variety of particle sizes in two different ways. By dissolving the meteorite material in acid, they produced nanometre-sized particles from their prepared material. And by putting either the meteorite material or the volcanic ash into a ball mill for 15 minutes, the researchers mechanically produced larger, micrometre-sized particles. Such a ball mill is a drum that contains both the material and steel balls. The drum is rotated at high speed, in this case more than ten times per second, with the steel balls grinding up the material.
Producing organic molecules under pressure
Since the original Earth's atmosphere contained no oxygen, the team carried out chemical reactions that removed almost all of the oxygen from the mixture. The scientists then brought the mixture into a pressure chamber filled mainly with carbon dioxide (CO2) and hydrogen molecules. Unlike today, the atmosphere back then consisted mainly of CO2 and water vapour and exerted almost a hundred times the atmospheric pressure on the Earth's surface. "Since there are many different possibilities for the properties of the early Earth, I tried to experimentally test every possible scenario", says Sophia Peters. "In the end, I used fifty different catalysts, and ran the experiment at various values for the pressure, the temperature, and the ratio of carbon dioxide and hydrogen molecules."
Under the conditions of a young Earth simulated here, the ancient atmosphere reacted to produce a considerable amount of organic compounds such as methanol, ethanol and acetaldehyde, as well as formaldehyde, thanks to the iron dust. Acetaldehyde and formaldehyde in particular are the compounds for important building blocks of life: fatty acids, nucleobases, sugars and amino acids. The result is a strong indication that such reactions may indeed have taken place on the early Earth - largely independent of the exact composition of the Earth's atmosphere at that time, which we do not currently know.
Adding a scenario to the portfolio of possible mechanisms
With these results, there is now a new contender for how the first building blocks of life were formed on Earth. Joining the ranks of "classic" mechanisms such as organic synthesis near hot vents on the ocean floor, or electric discharge in a methane-rich atmosphere (as in the Urey-Miller experiment), and of models that predict how organic compounds could have formed in the depth of space and transported to Earth by asteroids or comets (see ), there is now another possibility: iron particles that rained down on Earth during the early bombardment by meteorites or fine volcanic ash. These very likely acted as catalysts in an early, carbon dioxide-rich atmosphere, heralding the origin of life on Earth.
As in real life, it is likely that not only one path leads to the goal, but several. With this new process, a wide range of possibilities is available. Chances are good that further research into the primordial atmosphere and the physical properties of the early Earth will shed light on which of the various mechanisms yields the highest yield of life building blocks under realistic conditions. The role of iron as a catalyst has a special feature: the origin of this element lies inside huge stars, the cosmic kitchens of galaxies. At the end of their lives, these stars enrich interstellar gas with the very elements that were created inside them through massive supernova explosions. Among them is iron, a potential catalyst for life that could have become active not only on Earth.