Alien life may not be carbon-based, new study suggests

a swirling blend of planetary orbs and color gasses with math equations overlated transparently throughout
Life requires repetition of chemical reactions. Describing the kinds of reactions and conditions required for self-sustaining repetition — called autocatalysis — could focus the search for life on other planets. (Image credit: Betül Kaçar)

Self-sustaining chemical reactions that could support biology radically different from life as we know it might exist on many different planets using a variety of elements beyond the carbon upon which Earth's life is based, a new study finds.

On Earth, life is based on organic compounds. These molecules are composed of carbon and often include other elements such as hydrogen, oxygen, nitrogen, phosphorus and sulfur.

However, scientists have long wondered if alien life might evolve based on significantly different chemistry. For example, researchers have long speculated that silicon might also serve as a backbone for biology.

Related: The search for alien life 

"It's important to explore these possibilities so that we have an idea of what all forms of life can look like, not just Earth life," study senior author Betül Kaçar, an astrobiologist, bacteriologist and evolutionary biologist at the University of Wisconsin-Madison, told

A kind of chemical interaction that is key to life on Earth is known as autocatalysis. Autocatalytic reactions are self-sustaining — they can produce molecules that encourage the same reaction to happen again. Envision a growing population of rabbits. Pairs of rabbits come together, produce litters of new rabbits, and then the new rabbits grow up to pair off and make even more rabbits. It doesn't take many rabbits to soon have a lot more rabbits.

"One of the major reasons that origin-of-life researchers care about autocatalysis is because reproduction — a key feature of life — is an example of autocatalysis," Kaçar said. "Life catalyzes the formation of more life. One cell produces two cells, which can become four and so on. As the number of cells multiply, the number and diversity of possible interactions multiplies accordingly."

In the new study, researchers searched for autocatalysis beyond organic compounds. They reasoned that autocatalysis could help drive abiogenesis — the origin of life from lifelessness.

The scientists focused on what are called comproportionation cycles, which can generate multiple copies of a molecule. These products can be used as starting materials to help these cycles happen again, resulting in autocatalysis.

"Comproportionation is arguably unique because it is a single reaction that produces multiples of an output — it looks a lot like reproduction," study lead author Zhen Peng, an evolutionary biologist at the University of Wisconsin-Madison, told

Related: Fermi Paradox: Where are the aliens?

To find these reactions, the scientists analyzed more than two centuries of digitized scientific documents written in many different languages. "With effective language search and translation tools, we were able to design and conduct this first-of-its-kind assessment of the pervasiveness of autocatalytic cycles," study co-author Zach Adam, a geologist at the University of Wisconsin-Madison, told

Ultimately, the researchers discovered 270 different cycles of autocatalytic reactions. "Autocatalysis may not be that rare, but instead it might be a general feature of many different environments, even those that are really different from Earth," Kaçar said.

Most of the 270 cycles did not employ organic compounds. Some centered around elements that are absent or exceedingly rare in life on Earth, such as mercury, or the radioactive metal thorium. A number of cycles likely only happen under extremely high or low temperatures or pressures.

The researchers even discovered four autocatalytic cycles involving noble gases, which only rarely if ever chemically react with other elements. If even a relatively inert gas such as xenon could take part in autocatalysis, "there is good reason to guess that autocatalysis occurs more easily in other elements," Peng said.

Only eight of these cycles were relatively complex ones made up of four or more reactions. Most of the 270 cycles were simple, each consisting of just two reactions. 

"It was thought that these sorts of reactions are very rare," Kaçar said in a statement. "We are showing that it's actually far from rare. You just need to look in the right place."

The researchers noted that it's possible to combine multiple cycles together, even when they are very different from each other. This could lead to self-sustaining chemical reactions that generate a diverse range of molecules to produce a great deal of complexity.

"With so many basic recipes for autocatalysis on hand to draw from, a focus of research can now shift to understanding how autocatalysis, through comproportionation, may have more pronounced effects in shaping the chemistry of a planet," Kaçar said.

The scientists hope that future research can experimentally test this new cookbook they have created. 

"The cycles presented here are an array of basic recipes that can be mixed and matched in ways that haven't been tried before on our planet," Peng said. "They might lead to the discovery of completely new examples of complex chemistry that work in conditions where carbon- or even silicon-based cycles are too either combusted or frozen out."

It remains uncertain how plausible these cycles are, the researchers cautioned. "It is not guaranteed that all the examples we collated can be run in a lab or be found on other astronomical objects," Peng said.

In addition to the implications this work might have for the search for life in the universe and understanding the origins of life, this research may have practical applications, such as "optimizing chemical synthesis and making efficient use of resources and energy," Adam said. "Another is for using chemical compounds for interesting tasks such as chemical computation."

The scientists detailed their findings Sept. 18 in the Journal of the American Chemical Society.

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Charles Q. Choi
Contributing Writer

Charles Q. Choi is a contributing writer for and Live Science. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica. Visit him at

  • rod
    "In the new study, researchers searched for autocatalysis beyond organic compounds. They reasoned that autocatalysis could help drive abiogenesis — the origin of life from lifelessness."

    I read the paper, Ref - Assessment of Stoichiometric Autocatalysis across Element Groups,, 18-Sep-2023. “Abstract Autocatalysis has been proposed to play critical roles during abiogenesis. These proposals are at odds with a limited number of known examples of abiotic (and, in particular, inorganic) autocatalytic systems that might reasonably function in a prebiotic environment..."

    My note (from the paper), “Using this strategy, we surveyed the literature published in the past two centuries for reactions that can be organized into CompACs that consume some chemical species as food to synthesize more autocatalysts.”

    I recommend reading Charles Darwin letters on the warm little pond from 1871 and 1882. Charles Darwin in 1882 acknowledged there was no worthwhile evidence that life emerges from non-living matter, let alone spread around in the galaxy or among exoplanets. For all the efforts in science to demonstrate abiogenesis for the origin of life on Earth, no worthwhile evidence confirms this today or demonstrates a law of nature that describes abiogenesis, something Charles Darwin hoped would be established in science in his 1882 letter. We have Newton’s laws of motion, but no law of abiogenesis seen in nature and quantified as an example. The paper states about DNA (only 1 reference): “In contrast to autocatalytic cycles observed in biochemistry that may involve dozens of reaction steps and/or biomacromolecules (e.g., the Calvin cycle and DNA replication), CompACs are much simpler since they usually consist of only two or three reactions. Such simplicity could be important for a primitive, life-like autocatalytic system to emerge and persist. An autocatalytic cycle with fewer reaction steps tends to have a higher “carrying capacity”, (13) and it is arguably easier to find naturally occurring or laboratory-generated conditions that allow every reaction in a smaller autocatalytic cycle to occur.” My note, how this demonstrates non-living matter evolved into a single cell with DNA and grew and evolved into the tree of life we see on Earth today and in the fossil record, is a mystery to me.
  • rod also published reports on JWST seeing carbon on Europa and waiting to see Bennu asteroid sample with carbon, that could evolve into life on Europa and Earth. Now we have this article on abiogenesis where non-carbon-based life may arise somewhere else in the galaxy using a different paradigm. I agree with Charles Darwin in his 1882 letter. There is yet to be shown to the public, the worthwhile evidence that demonstrates life in any form (carbon based or other), evolved naturally from non-living matter. Since Charles Darwin 1871 letter where he documented the warm little pond, we are still waiting here, just like Charles Darwin was still waiting to show abiogenesis in 1882.
  • COLGeek
    While I haven't read the paper yet, I would offer a comment regarding the "little pond". In the cosmic span of time, from Darwin's observations to today, is not even the time of a "blink of the eye". Expecting life to form in the pond seems rather unrealistic given what we understand regarding the age of our planet and the universe.

    Regarding non-carbon life, aside from our "personal" experience, I am not surprised by the notion that life could be "something-based", other than carbon. The vastness of the universe and all contained within it are not something we remotely understand.
  • rod
    COLGeek expresses thinking I find common on the origin of life here or anywhere else in the Universe. The problem is how Charles Darwin approached this issue. He looked to see evidence for abiogenesis that confirmed his letters and acknowledged in 1882, there still was none. We can talk about the size of the solar system here or the vastness of the universe, does not show abiogenesis took place at all, not even one time let alone multiple times, over, and over again.

    As i read these different reports from on abiogenesis, I compare to Charles Darwin letters of 1871 and 1882 for a look back in science on the topic of the origin of life.,
    Charles Darwin hoped that someday evidence would be shown for life evolving from non-living matter but in his time, none was known that was *worth anything* and the *law of continuity* would provide this, also a general law of nature for abiogenesis. None of this in science is proven at present. There is no general law of abiogenesis seen operating in nature. So, here is a summary of four points in these letters that I learned. 1. A warm little pond is postulated for the origin of life on Earth but Charles Darwin thought if abiogenesis operating in a warm little pond in his time, perhaps such life evolving from non-living matter would be quickly destroyed by existing life and eaten. 2. No good evidence for abiogenesis taking place in Charles Darwin time seen in nature. 3. The law of continuity is needed for abiogenesis to work apparently, and 4. Someday a general law of nature developed to describe and show abiogenesis like other laws of nature, for example the laws of motion or law of gravity. Apparently all four I list here are missing in science today, even with natural law operating in nature in a uniform manner, i.e., *law of continuity*.

    There should be established via the scientific method, a general law of nature demonstrating how abiogenesis converts non-carbon-based elements into a different form of life, like silicon based, same applies to converting carbon in an asteroid to life on Earth with DNA. Charles Darwin hoped for this in 1882, still some are hoping here but not defining like Newton's laws of motion or gravity. Because of this situation, I consider such reports promoting a belief system seeking to be viewed as a real science paradigm, IMO.
  • Helio
    The idea of silicon-based life is not new. Star Trek, in the 60’s, had at least one episode involving this.
    Si seems to be a distant second in its ability to form both with other elements and itself. This greatly limits its chances to form necessary complex molecules, such as proteins.

    But, studying other forms only improves our understanding of complex forms, just like studying other planetary atmospheres will improve our knowledge of ours.

    Darwin never presented an abiogenesis theory. He did present a great theory that showed how an existing species, which often generate new varieties with reproduction, could, after many millions of years, evolve into new species. His Origins book was not on life’s origins (abiogenesis) but of one species originating from another species.

    He didn’t do much speculating on what bio process produced these changes to varieties in his original publication since Mendel’s work was unknown to him. He was not accepted into mainstream science due largely to this issue, though the more popular theories also suffered from this problem.

    In subsequent publications, he offered one suggestion that might explain trait consistency, but it came across as ad hoc, because it was. Thus, it’s his first book that has the highest regard.

    Abiogenesis , especially in his day, was known to be even more suppositional, and in no way was detrimental to his evolutionary model.

    Einstein’s SR (Special Relativity) theory never required the incorporation of gravity. GR gave us the bigger picture with gravity, just as genetics advanced Darwin‘s work. Just as Einstein was mostly unknown until after he was able to demonstrate GR, Darwin was not mainstream until genetics became understood.
  • rod
    Helio, interesting post#6. I went back and checked Origin of Species to see what Charles Darwin said about the origin of life in the work vs. his letters of 1871 and 1882. There clearly was a big change that took place.

    “…Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.”

    Ref - s/originofspecies00darwuoft/page/n539/mode/2up

    IMO, these views expressed about the origin of life in his work did not invoke abiogenesis and nature creating life all its own by natural processes. His 1871 and 1882 letters changed the paradigm, astrobiology and biology today went with that model change to explain the origin of life, whether carbon based, or silicon based.

    A better URL for Origin of Species,
    See pages 528-529.
  • Atlan0001
    I don't know if anyone else here has ever read E. E. "Doc" Smith Sci-Fi. His 'Lensmen' series has fascinated me ever since I first read it as a teenager ages ago. It still does fascinate me many decades later, especially his concept that life has body physicality but is not any particular body physicality based. That life is complex beyond body composition. Which is to say that life itself on its many levels of is multi-dimensionally 'complex' or 'complexity' based life structure . . . no more, no less.

    I know some here have heard the saying, "The thing (of whatever physicality) develops a life of its own!" Nothing could be truer . . . from a one celled creature -- and quantum physically lower -- up through a body of countless cells, up and down through whole universes having lives of their own (to include the living structures and infrastructures of dynamic frontier and static utopian civilizations (that I discussed -- to his delight -- for three hours with the Curator of the Korean Museum during a tour of Seoul, Korea. At the request of the Curator, the tour bus left me behind and came back by to pick me up later)).
  • Helio
    Thanks Rod, that's a great quote that I vaguely recall from reading most of his book.

    There is no small background story that could be imagined for his statement, especially given the emergence of scientific interests that may have started around 1800. Rev. William Paley's, Natural Theology, was one of Darwin's favorite books, as well as, a great call for everyone to go out and enjoy discovering stuff. For young Darwin, he excelled in his remarkable beetle collections.
    Paley applied scientific observations to religious views, arguing that if you stumbled upon a watch lying on the road, you would know it was made by intelligent beings. Likewise, the more complex eye has to have been made by someone even more intelligent (ie God). This is an example of why Darwin took many years to present enough in the way of arguments to present an alternative, at least one where the intelligence was more in the laws themselves that would allow even greater diversity of creation. The large book, Origins of the Species, was actually his small book. Once he got a letter from Wallace stating the same tenets he held (e.g. natural passive selection) he was highly depressed, but Wallace convinced him to publish, so we got his abbreviated book. IIRC, they were together in a premiere presentation.

    Also, recall that Darwin's plan was to become a parson, and likely would have become one had he not been invited to be company for the captain of the Beagle. His years of travel on that ship gave him focus on how varieties could progress to become new species. That was the heart of his extensive work, and not about even more challenging questions, which he at times addressed secondhandedly, like abiogenesis.
  • Unclear Engineer
    I think that there is likely to be a probabilistic element to how auto autocatalysis of various chemicals could lead to complex life forms. If carbon has a higher probability of autocatalyzing materials than silicon does, then carbon life forms would emerge instead of silicon life forms.

    But, the probability of that may depend highly on the parameters of the local environment. Higher temperatures and/or pressures might result in different chemistries having higher probabilities of succeeding in producing self-sustaining "life" forms.

    Because all chemistry seems to happen faster at higher temperatures and higher pressures, I would not be surprised if some sort of life form based on an element that is not carbon could evolve in a dry environment that is extremely hot by our Earthly standards.

    Such a high temperature life form visiting Earth would be somewhat like us Earthlings visiting our South Pole. Such extraterrestrials would probably need temperatures that would be lethal to us Earthlings. We would not be able to "shake their hands in welcome" without injuries to all involved.
  • billslugg
    Carbon has a big advantage in any race to form life at STP. When oxidized, carbon turns into CO and CO2, which are both gases, thus are ubiquitous in the environment.

    When silicon oxidizes it forms beach sand, extremely stable and unlikely to participate in further reactions. Yeah, I know, "sand gets everywhere" but that is not what I'm talking about.

    Perhaps silicon might work well in some high temperature, high pressure regime somewhere.