A research team led by scientists has exploited the limitations of chemical combinations to compile a cookbook containing hundreds of recipes that could potentially give rise to life. Life on distant planets - if it exists - may be completely different from life on Earth. But there are only so many chemical ingredients in the universe's storehouse, and only so many ways to mix them. A team of researchers led by scientists at the University of Wisconsin-Madison took advantage of these constraints and compiled a cookbook of hundreds of chemical recipes that could potentially give rise to life.
Life requires the repetition of chemical reactions. Describing the types of reactions and conditions required for self-sustaining repetitions, called autocatalytic reactions, could allow us to focus our search for life on other planets. Source: Betül Kaçar
Their ingredient lists put the recipe together by pointing out the most likely conditions—a planetary version of mixing techniques, oven temperatures, and baking times—that put the focus on the search for life elsewhere in the universe.
From basic chemical ingredients to complex cycles of cellular metabolism and reproduction, life is defined not just by a simple beginning but by repetition, researchers say.
Betül Kaçar, a NASA-supported astrobiologist and professor of bacteriology at the University of Washington, Madison, believes that the origin of life is indeed a process from scratch. But "creating something out of nothing" cannot happen just once. Life ultimately depends on chemistry and conditions that produce a self-replicating reaction pattern."
Chemical reactions that produce molecules that cause the same reaction to happen repeatedly are called autocatalytic reactions. In a new study published September 18 in the Journal of the American Chemical Society, Zhen Peng, a postdoctoral researcher in Cachar's lab, and collaborators compiled 270 molecular combinations involving atoms in all groups and series of the periodic table that have the potential for sustained autocatalysis.
"These types of reactions are thought to be very rare," Kaçar said. "Our research shows that this reaction is actually not uncommon. You just have to look in the right places."
The researchers' study focused on so-called proportional reactions. In these reactions, two compounds containing the same element but different numbers of electrons or reaction states combine to form a new compound with the element in the middle of the initial reaction state.
To be autocatalytic, the result of a reaction also needs to provide the starting material for the reaction to happen again, so the output becomes a new input, said study co-author Zach Adam, a geoscientist at the University of Wisconsin-Madison who studies the origin of life on Earth. Ratio reactions produce multiple copies of some related molecules, providing material for the next step in the autocatalytic reaction.
"If the conditions are right, you can start with a relatively small amount of output," Adam said. "Every time you do a loop, you spit out at least one additional output, which speeds up the reaction and makes it happen faster."
Autocatalysis is like a growing herd of rabbits. Pairs of rabbits come together and give birth to a litter of new rabbits, and then the new rabbits grow up, pair up on their own, and give birth to more rabbits. It doesn't take too many rabbits and there will soon be more.
However, searching the universe for floppy ears and furry tails may not be a winning strategy. Instead, Kaçar hopes chemists will take some ideas from the new study's recipe list and try them out on the pots and pans of a simulated alien kitchen.
"We will never know exactly what happened on this planet to give rise to life. We don't have a time machine," Kaçar said. "But, in the test tube, we can create a variety of planetary conditions to understand how the forces that sustain life first evolved."
Kaçar leads a NASA-backed consortium called MUSE, the Metals Utilization and Options for a Future Alliance. Her lab will focus on reactions involving elements such as molybdenum and iron, and she's excited to see what others cook up from the strangest and most unusual sections of the new recipe book.
Carl Sagan said that if you want to bake a pie from scratch, first you have to create the universe, Kaçar said. "I think if we want to understand the universe, first we have to bake some pies."