A new study (ref.) has found that some chemical reactions are capable of supporting a biology radically different from ours. There could be life on other planets that uses a variety of elements beyond carbon, which is the basis of life on Earth.
On our planet, life is built on organic compounds. These molecules are composed of six main atoms: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. However, scientists have long wondered if alien life could evolve with a different chemistry. For example, researchers have long hypothesized that silicon could also serve as a backbone for biology.
“It is important to explore these possibilities to get an idea of what all forms of life could look like” said senior author Betül Kaçar, astrobiologist, bacteriologist, and evolutionary biologist. A type of fundamental chemical process for life on Earth is autocatalysis. Autocatalytic reactions are self-sustaining. They can produce molecules that encourage the repetition of the same starting reaction.
“One of the main reasons researchers are interested in autocatalysis is because reproduction is an example of autocatalysis” Kaçar said. “Life catalyzes the formation of other life. A cell produces two cells, which can become four and so on. As the number of cells multiplies, the number and diversity of possible interactions multiply accordingly”.
In the study, researchers looked at autocatalysis, thinking that this process could help drive abiogenesis, the origin of life from the absence of life. Scientists focused on so-called disproportionation cycles, which can generate more copies of a molecule. These products can be used as starting materials to promote the repetition of these cycles, resulting in autocatalysis.
“Disproportionation is probably unique because it is a single reaction that produces multiples of an output; it closely resembles reproduction” said the study’s lead author, Zhen Peng, an evolutionary biologist. To find these reactions, scientists analyzed more than two centuries of digitized scientific documents.
“With effective search tools, we were able to design and conduct this first-of-its-kind assessment of the pervasiveness of autocatalytic cycles” said study co-author Zach Adam, a geologist. In the end, they discovered 270 different autocatalytic reaction cycles. “Autocatalysis may not be so rare, but it may instead be a general feature of many different environments, even those very different from Earth” Kaçar said.
Most of the 270 cycles exclude carbon, on which terrestrial life is based. Some were focused on elements absent or extremely rare in our biology, such as mercury or thorium, a radioactive metal. It is likely that some number of cycles only occur in extreme conditions that disrupt the chemistry we know. Indeed, researchers found that four autocatalytic cycles involved noble gases, which rarely chemically react with other elements. If even an inert gas like xenon could take part in autocatalysis, “there are good reasons to believe that autocatalysis might occur more readily with other elements” Peng said.
What to Look For?
Only eight of these cycles were relatively complex and comprised four or more reactions. Most of the 270 cycles were simple, each consisting of only two reactions. “These types of reactions were thought to be very rare” Kaçar noted. “We are showing that they are anything but rare. You just have to look in the right place”. Researchers noted that it is possible to combine multiple cycles together, even when they are very different from each other. This would lead to self-sustaining chemical reactions that generate a wide range of molecules to produce great complexity.
“With so many basic recipes for autocatalysis available, the focus of research can now shift to understanding how autocatalysis, through disproportionation, can have more pronounced effects in shaping the chemistry of a planet” Kaçar said. “These are a set of basic recipes that can be mixed and matched in ways never before tried on our planet” Peng said. “They would lead to the discovery of new examples of complex chemistry (life) that work in conditions where carbon-based cycles are blocked”.
The plausibility of these cycles remains uncertain, the researchers cautioned. “It is not guaranteed that all the examples we have collected can be analyzed in the laboratory or found on other astronomical objects” Peng said. This work could have a huge impact on the search for life in the universe and understanding how it formed. It could also have practical applications, such as “optimizing chemical synthesis and efficient use of resources and energy” Adam said.