Possible life forms from a distant planet may look nothing like life on Earth, but there are only a limited number of chemical ingredients in the universe's pantry, and only a finite number of ways to mix them together to create life.
A team led by scientists at the University of Wisconsin-Madison has exploited these limitations to create a cookbook of hundreds of chemical combinations that could potentially explain the origin of life.
Their ingredient list could focus the search for life elsewhere in the universe by pointing to the most likely conditions - planetary versions of mixing techniques, oven temperatures, and cooking times - for the ingredients to come together and start the right reactions.
The process of progression from basic chemical ingredients to the complex cycles of cellular metabolism and reproduction that define life, the researchers say, requires not only a simple beginning but also repetition.
Self-Catalysis and the Origin of Life
"The origin of life is really a process that comes from nothing," says Betül Kaçar, a NASA-funded astrobiologist and professor of bacteriology at UW-Madison. "But that something cannot happen just once. Life depends on chemistry and conditions that can generate a pattern of self-replicating reactions."
Chemical reactions that produce molecules that allow the same reaction to be repeated over and over again are called autocatalytic reactions. In a new study published in the Journal of the American Chemical Society, Zhen Peng, a postdoctoral researcher in Kaçar's lab, and his collaborators compiled 270 combinations of molecules, involving atoms from every group and series of the periodic table, with the potential for sustained growth through autocatalysis.
"These kinds of reactions were thought to be very rare," says Kaçar. "We are showing that they are anything but rare. You just have to look in the right place."
The researchers focused their research on so-called comproportionation reactions. In these reactions, two compounds containing the same element with different numbers of electrons, or reactive states, combine to form a new compound in which the element is in the middle of the initial reactive states.
"To be autocatalytic, the result of the reaction must also provide starting materials for the reaction to repeat itself, so the output becomes a new input," explains Zach Adam, co-author of the study and a UW-Madison geoscientist who studies the origins of life on Earth. The compounding reactions result in multiple copies of some of the molecules involved, providing materials for the next steps of autocatalysis.
"If the conditions are right, you can start with relatively few of these outcomes," Adam continues. "Every time you go around the loop, you give off at least one extra result that speeds up the reaction and makes it go even faster."
Autocatalysis is like a growing population of rabbits. Pairs of rabbits mate, produce litters of new rabbits, and then the new rabbits grow to mate and produce even more rabbits. It does not take many rabbits to have many more rabbits.
But searching the universe for floppy ears and furry tails is probably not a winning strategy. Instead, Kaçar hopes chemists will take ideas from the new study's list of recipes and test them in pots and pans that simulate alien kitchens.
"We will never know exactly what happened on this planet that led to the origin of life. We don't have a time machine," says Kaçar. "But in a test tube, we can create different planetary conditions to begin to understand how the dynamics can evolve to support life."
Kaçar leads a NASA-supported consortium called MUSE, for Metal Utilization & Selection Across Eons. His lab will focus on reactions involving the elements molybdenum and iron, and he is excited to see what others will prepare from the more exotic and unusual parts of the new cookbook.
Zhen Peng et al, Assessment of Stechiometric Autocatalysis across Element Groups, Journal of the American Chemical Society (2023). DOI: 10.1021/jacs.3c07041
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