Buzz
In the pursuit of extraterrestrial intelligence, some critics argue that we may be guilty of 'carbon chauvinism,' assuming that life elsewhere in the universe must share the same biochemical foundation as life on Earth. This bias influences how we approach the search for alien life. Below are 10 instances of both biological and nonbiological systems that challenge the conventional understanding of what constitutes 'life.'
10. Methanogens
In 2005, Heather Smith from the International Space University in Strasbourg and Chris McKay of NASA’s Ames Research Center co-authored a paper speculating about the potential for methane-based life forms, known as 'methanogens.' These life forms might be capable of consuming hydrogen, acetylene, and ethane, and releasing methane instead of carbon dioxide.
This concept suggests that life could thrive in cold environments, such as Saturn’s moon Titan. Titan’s atmosphere is primarily nitrogen, with a mixture of methane, and it is one of the few places in our solar system, along with Earth, to have large bodies of liquid—ethane and methane lakes and rivers. Titan, as well as other moons like Enceladus and Europa, also hosts underground bodies of water. Liquid is essential for the chemical processes of life, and while water is usually the focus, organic interactions could also occur in liquids like ethane and methane.
In 2004, the NASA-ESA Cassini-Huygens mission observed a mysterious, muddy world with temperatures as low as –179°C (–290°F), where water is frozen solid and methane flows through river channels and pools in polar lakes. In 2015, researchers from Cornell University, including chemical engineers and astronomers, proposed a theoretical cell membrane made from small organic nitrogen compounds that could function in Titan's liquid methane. They named this theoretical cell an 'azotosome,' meaning 'nitrogen body,' which exhibited the same stability and flexibility as an Earthly liposome. The main molecular compound was acrylonitrile azotosome, with acrylonitrile being a toxic, colorless organic molecule found in Titan’s atmosphere, used on Earth for making acrylic fibers, resins, and thermoplastics.
The potential implications for the search for extraterrestrial life are significant. Life might not only exist on Titan, but it could also be detected by observing the depletion of hydrogen, acetylene, and ethane on its surface. Methane-dominant atmospheres could be found on moons and planets around stars similar to our Sun, as well as around red dwarf stars with a broader habitable zone (since moons like Titan are opaque to blue and ultraviolet light but allow red and infrared light to pass through). If NASA launches the Titan Mare Explorer in 2016, we may have to wait until 2023 to learn more.
9. Silicon-Based Life
Silicon-based life is one of the most popular alternative biochemistries explored in science fiction, particularly exemplified by the Horta from Star Trek. This concept dates back to 1894, when H.G. Wells speculated, 'One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms—why not silicon-aluminium men at once?—wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace.'
Silicon is a favored element for such speculation due to its similarities with carbon, as it can form four bonds just like carbon, potentially allowing for an entirely silicon-based biochemical system. It is the second most abundant element in Earth's crust, after oxygen. In fact, certain algae on Earth incorporate silicon into their growth processes. However, silicon has a notable disadvantage when compared to carbon: while carbon forms more stable and diverse complex structures necessary for life, silicon-based molecules tend to break down more easily. Carbon also has the advantage of being extremely common throughout the universe and has been for billions of years.
Silicon-based life is unlikely to develop in an Earth-like environment, as most free silicon would be trapped in volcanic and igneous rocks composed of silicate minerals. While it is speculated that high-temperature environments might present different conditions, no supporting evidence has been found. An extreme world like Titan might offer the right conditions for silicon-based life, potentially serving as the foundation for the methanogens mentioned earlier, as silicon compounds like silanes and polysilanes closely resemble Earth’s organic chemistry. However, Titan's surface is mostly carbon-based, with the majority of silicon located deep beneath the surface.
NASA astrochemist Max Bernstein has proposed that silicon-based life could potentially exist on a very hot planet with a hydrogen-rich, oxygen-poor atmosphere, where complex silane chemistry could occur through reversible silicon bonds with selenium or tellurium. However, Bernstein considers this scenario unlikely and rare. On Earth, such organisms would reproduce very slowly, and their biochemistry would not pose a threat to ours. While they might 'slowly consume our cities,' Bernstein humorously notes, 'Presumably you could take a jackhammer to it.'
8. Other Alternate Biochemistries
Several other proposals for life systems based on elements other than carbon have been suggested. Like carbon and silicon, boron tends to form strong covalent molecular compounds, creating a variety of hydride structures where boron atoms are linked by hydrogen bridges. Similar to carbon, boron can bond with nitrogen to produce compounds with chemical and physical properties resembling alkanes, the simplest organic compounds. However, the primary challenge for boron-based life is the rarity of the element. Such life would be more feasible in environments where the temperature is low enough for ammonia to remain a liquid solvent, thus allowing chemical reactions to be more manageable.
One of the more intriguing hypothetical life forms that gained media attention is arsenic-based life. While Earth’s life is built from carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, in 2010, NASA reported the discovery of a bacterium named GFAJ-1 that could incorporate arsenic instead of phosphorus into its cellular structure without harm. GFAJ-1 thrives in the arsenic-laden waters of Mono Lake in California. Arsenic is toxic to almost all life forms, except a few microorganisms that can tolerate or use it. GFAJ-1 was the first known organism to use arsenic as a biological building block. However, independent studies later cast doubt on NASA's claims, finding no evidence that arsenic was incorporated into the DNA, only that arsenate was found clinging to the DNA's exterior. Nonetheless, the idea of arsenic-based biochemistry remains an intriguing possibility.
Ammonia has been proposed as an alternative to water for forming life-forms. Some theorists suggest a biochemistry based on nitrogen-hydrogen compounds, using ammonia as a solvent to build proteins, nucleic acids, and polypeptides. Any ammonia-based life would need to cope with the fact that ammonia only remains liquid within a narrow temperature range, and its solid form is denser than its liquid state, meaning it would freeze in extreme cold. While this is not a major problem for single-celled organisms, it would pose significant challenges for multicellular life forms. Nevertheless, the possibility of ammonia-based single-celled life exists on colder planets in the solar system, as well as gas giants like Jupiter.
Sulfur is thought to have played a key role in the origins of early metabolism on Earth, and organisms that metabolize sulfur instead of oxygen are known to survive in extreme Earth environments. On another world, sulfur-based life forms could have an evolutionary advantage. It is also believed that nitrogen and phosphorus might replace carbon under very specific conditions.
7. Memetic Life
Richard Dawkins argues that the fundamental principle behind life is that 'All life evolves by the differential survival of replicating entities.' For life to exist, it must have the ability to replicate (with variation) and be placed in an environment where natural selection and evolution can take place. In his book, The Selfish Gene, Dawkins illustrated that ideas and concepts form within the human brain and spread through communication between individuals. This process mirrors the behavior and adaptation of genes, leading him to coin the term 'memes.' Some even draw parallels between songs, jokes, and rituals shared within human societies and the earliest forms of organic life—free radicals swimming in the Earth's primordial oceans. These mental creations replicate, evolve, and compete for survival within the realm of ideas.
Memes existed long before humanity, evident in social bird calls and learned behaviors in primates. As humans developed the ability for abstract thought, these memes became more complex, influencing tribal dynamics and forming the foundation of early customs, culture, and religion. The invention of writing played a crucial role in accelerating the spread of memes, enabling them to propagate over vast distances and across time, much like how genes transmit biological information. While some view this as a mere analogy, others argue that memes represent the essence of a unique, albeit rudimentary and limited, form of life.
Some have expanded this idea further. George van Driem has introduced the theory of Symbiosism, which posits that languages are, in fact, life-forms in their own right. Older linguistic theories suggested that language was a parasite, but van Driem proposes that humans exist in a symbiotic relationship with the memetic entities residing in our minds. In this view, we rely on language organisms for survival—without them, we would be little more than feral creatures. Van Driem argues that the illusion of consciousness and free will emerges from the interplay between human animal instincts, desires, and the linguistic symbionts that reproduce through ideas and meaning.
6. XNA-Based Synthetic Life
Life on Earth relies on two key information-carrying molecules: DNA and RNA. This has sparked scientific curiosity about whether other similar molecules might exist. While any polymer can hold information, DNA and RNA have the unique ability to display heredity, encoding and transmitting genetic data, while also being capable of evolving through natural selection. These nucleic acids consist of chains of molecules called nucleotides, which are composed of three key elements: a phosphate group, a five-carbon sugar (deoxyribose in DNA or ribose in RNA), and one of five standard bases—adenine, guanine, cytosine, thymine, or uracil.
In 2012, an international team of scientists from England, Belgium, and Denmark succeeded in creating xeno-nucleic acid (XNA)—synthetic nucleotides that mimic DNA and RNA both functionally and structurally. By replacing the traditional deoxyribose and ribose sugars with various substitutes, they developed molecules that were not only structurally similar but also capable of replication and evolution. While similar molecules had been created before, this breakthrough marked the first time such molecules were proven capable of replicating and evolving. In DNA and RNA, replication relies on enzymes known as polymerases, which read, transcribe, and reverse transcribe the nucleic acid sequences. The team created synthetic polymerases to support the six newly created genetic systems—HNA, CeNA, LNA, ANA, FANA, and TNA.
One of the new genetic systems, HNA (hexitol nucleic acid), was found to be resilient enough to hold sufficient genetic information, potentially forming the foundation for biological systems. Another promising system, TNA (threose nucleic acid), is considered a potential candidate for the enigmatic primordial biochemistry that may have existed prior to the emergence of life itself.
This advancement offers a variety of potential applications. Ongoing research could improve models explaining the origins of life on Earth and enhance our understanding of biology. XNAs might have therapeutic potential, creating nucleic acid treatments capable of binding to specific molecular targets, and they may also degrade less rapidly than DNA or RNA. Furthermore, they could be the building blocks for molecular machines or even lead to the creation of an entirely synthetic form of life.
Before this becomes feasible, however, specialized enzymes tailored to each specific XNA would need to be developed. Some of these enzymes were successfully created in the UK in late 2014. There is also the potential risk that XNA could infiltrate the genetic makeup of RNA/DNA organisms, possibly causing harm. Therefore, strict safeguards must be established to prevent such occurrences.
5. Chromodynamic, Weak Nuclear Force And Gravitational Life
In 1979, scientist and nanotechnologist Robert A. Freitas Jr. proposed the idea of nonbiological life. He suggested that life forms could potentially exist based on the four fundamental forces—electromagnetism, strong nuclear force (also known as quantum chromodynamics), weak nuclear force, and gravity. Electromagnetic life represents the conventional biological life on Earth, as well as potential alien biological forms and even machine-based life-forms.
Chromodynamic life might exist based on the strong nuclear force, the most powerful of the fundamental forces, though only effective over extremely short distances. Freitas proposed that such an environment could exist on a neutron star—a dense, spinning object ranging from 10 to 20 kilometers (6 to 12 miles) in diameter, yet with the mass of an entire star. These stars possess incredibly strong magnetic fields and gravity 100 billion times that of Earth, as well as a 3-kilometer-thick (2 miles) crust of crystalline iron nuclei. Beneath this, a sea of intensely hot neutrons exists, alongside a mix of nuclear particles such as protons and atomic nuclei, and potentially neutron-rich “macronuclei.” These macronuclei could, in theory, form supernuclei analogous to organic molecules, with neutrons functioning as a liquid-like substance in a peculiar pseudo-biological system.
Freitas considers weak-nuclear-force life forms to be less likely, given that weak forces are effective only at sub-nuclear ranges and are not particularly powerful. Since they frequently appear in processes like radioactive-beta and free-neutron decay, a weak-force-based life form could exist by carefully controlling weak interactions in its environment. He envisioned organisms made up of atoms with an excess of neutrons, which become radioactive upon their death. Some speculate that certain regions of the universe may have stronger weak nuclear forces, thus enhancing the likelihood of such life forms.
Gravitational life forms could exist, as gravity is the most common and effective of the fundamental forces in the universe. These creatures might derive their energy directly from gravity, with massive gravitational beings feeding off collisions between black holes, galaxies, or other celestial bodies. Smaller entities might be powered by the rotational and orbital movement of planets, while even smaller life forms could derive energy from more common sources like waterfalls, wind patterns, ocean tides, or even earthquakes.
4. Dusty Plasma Life-Forms
Life on Earth is built on carbon-based molecules, and we've explored several biological alternatives to carbon. However, in 2007, an international team led by V.N. Tsytovich from the General Physics Institute of the Russian Academy of Sciences reported that under specific conditions, particles of inorganic dust can arrange themselves into helical structures. These formations can interact in a way similar to organic chemistry. This process takes place in plasma, a unique state of matter beyond solid, liquid, and gas, where atoms lose their electrons, leaving behind a mix of charged particles.
Tsytovich's team discovered that as electronic charges became separated and the plasma polarized, the particles within the plasma naturally organized into corkscrew-like helical structures. These structures were electrically charged and attracted to each other, and they could replicate, much like DNA, and induce changes in neighboring particles. Tsytovich remarked, 'These complex, self-organized plasma structures exhibit all the necessary properties to qualify them as candidates for inorganic living matter. They are autonomous, they reproduce and they evolve.'
Some critics remain skeptical, arguing that the suggestion these inorganic structures could be considered life is more of a public relations stunt than a genuine scientific claim. While the helical structures may resemble DNA in form, their similarity in appearance does not equate to a similarity in function. Moreover, the self-replication of these helices doesn't necessarily indicate life potential—clouds, after all, can also self-replicate. Furthermore, much of the research was based on computer simulations rather than real-world observations.
One researcher involved in the study admitted that, while the results appeared to mimic life, they were ultimately 'just a special form of plasma crystal.' However, if it's true that inorganic particles in plasma could evolve into self-replicating and evolving life forms, they could be the most common form of life in the universe, given the widespread presence of plasma and interstellar dust clouds in space.
3. Gaia Hypothesis
In 1975, Drs. James Lovelock and Sidney Epton coauthored a groundbreaking article for New Scientist titled 'The quest for Gaia.' While the traditional belief is that life flourished on Earth simply because the conditions were favorable, Lovelock and Epton argued that life itself plays an active role in shaping and sustaining those very conditions. They proposed that all living organisms on Earth—whether in the air, oceans, or land—form a single, interconnected system, much like a living super-organism, which actively regulates temperature and atmospheric composition to ensure its survival. They named this self-regulating system Gaia, after the Greek goddess of the Earth, whose role is to maintain a homeostasis that supports life on the planet.
Lovelock had been developing the Gaia hypothesis since the mid-1960s, with the idea that Earth’s biosphere consists of several natural cycles. When one cycle is disrupted, others compensate to keep the planet habitable for life. This concept helps explain why our atmosphere isn't dominated by carbon dioxide or why the oceans aren’t overwhelmingly salty. For instance, although volcanic eruptions initially created an atmosphere filled with carbon dioxide, bacteria that excrete nitrogen and plants that produce oxygen through photosynthesis gradually altered it. Over millions of years, the atmosphere evolved into its current, relatively mild state. Despite salt being carried by rivers into the oceans, oceanic salinity remains stable at 3.4 percent due to salt being removed through cracks in the ocean floor. These processes aren't conscious decisions but rather the result of feedback loops that help maintain a habitable environment.
Additional evidence supporting the Gaia hypothesis includes the fact that elements like methane and hydrogen would disappear from Earth's atmosphere within just a few decades if not for biological processes. Moreover, although the Sun's temperature has risen by 30 percent over the last billion years, the average global temperature has fluctuated by only 5 degrees Celsius (9°F). This is due to a regulatory mechanism that has removed excess carbon dioxide from the atmosphere, trapping it in fossilized organic matter and helping to maintain a stable climate.
When Lovelock first introduced his ideas, they were met with ridicule and dismissed as pseudoscience and New Age mysticism. However, over time, the Gaia hypothesis has significantly influenced how scientists think about Earth's biosphere. It has drawn attention to the interconnectedness of the biosphere and its components, and how they contribute to the overall stability of the planet. Today, the Gaia hypothesis is widely respected, though not universally accepted. Many view it as a valuable cultural framework that encourages scientific exploration, with a growing respect for the Earth as a cohesive global ecosystem.
Paleontologist Peter Ward has put forward the opposing Medea hypothesis, named after the tragic figure from Greek mythology, which posits that life is inherently self-destructive and suicidal. Ward highlights that historically, most mass extinctions have been brought about by life forms themselves, such as microorganisms or hominids in their actions, causing catastrophic shifts in Earth's atmosphere.
2. Von Neumann Probes
The concept of machine-based artificial life has become somewhat cliché, so let's instead explore the intriguing idea of Von Neumann probes. These were first conceived by Hungarian mathematician and futurist John Von Neumann in the mid-20th century. He proposed that to replicate the complex functions of the human brain, a machine would require mechanisms for self-control and self-repair. Von Neumann imagined self-replicating machines, inspired by how life forms increase in complexity through replication. He suggested that these machines should possess a universal constructor, enabling them to build replicas of themselves, and possibly even improved versions, fostering evolution and increasing complexity over time.
Other futurists like Freeman Dyson and Eric Drexler quickly expanded on these ideas, applying them to space exploration, and conceptualized the Von Neumann probe. The idea is that self-replicating robots could be the most efficient way to colonize space, potentially spreading throughout the entire Milky Way in less than a million years, even if limited by the speed of light.
As Michio Kaku explains:
A Von Neumann probe is a robot designed to travel to far-off star systems, setting up factories that can replicate themselves by the thousands. A lifeless moon, rather than a planet, is the ideal target for Von Neumann probes, as they can easily land and take off from such moons, which also remain unaffected by erosion. These probes would utilize the local resources, such as iron, nickel, and other naturally occurring materials, to create the components necessary for building a factory. They would then produce thousands of copies of themselves, which would disperse and search for additional star systems.
Over time, various iterations of the Von Neumann probe concept have been proposed, such as reconnaissance probes designed for discreet exploration and surveillance of alien civilizations, communication probes dispersed across space to enhance the detection of extraterrestrial radio signals, work probes intended to build massive cosmic structures, and colonization probes tasked with populating new worlds. There might even be uplift probes meant to help emerging civilizations progress into space. More concerning, there could be berserker probes aimed at eliminating all organic life they encounter, which might require the creation of police probes to protect against such threats. Given that Von Neumann probes are sometimes compared to a type of interstellar virus, we must carefully consider the consequences of advancing such technology.
1. iCHELLs
Professor Lee Cronin, the Gardiner Chair of Chemistry at the University of Glasgow's College of Science and Engineering, has a bold vision: to create living cells from metal. By using polyoxometalates, a group of metal atoms connected to oxygen and phosphorus, he has developed cell-like structures, which he refers to as inorganic-chemical-cells or iCHELLs.
Cronin’s team started by making salts from large metal oxides with negatively charged ions, combined with small, positively charged ions like hydrogen or sodium. This salt solution is then introduced into another solution containing large, positively charged organic ions bound to smaller negative ions. As the two solutions mix, they exchange components, causing the large metal oxides to bond with the large organic ions, forming a shell-like structure that is insoluble in water. By altering the metal oxide structure, the team can give these bubbles properties similar to biological cell membranes, enabling the selective passage of chemicals, much like the controlled chemical reactions that happen in living cells.
The team has gone further by creating nested bubbles, which imitate the internal structures of biological cells, and has made significant progress toward developing an artificial version of photosynthesis. This could eventually lead to the creation of artificial, plant-like cells. However, other synthetic biologists argue that these cells will not be truly life-like until they include some mechanism for replication and evolution, such as DNA. Despite this, Cronin remains hopeful that continued development will illuminate the path forward. Potential applications for the technology include creating materials for solar fuel devices (as the cells can also store electricity) and medical advancements.
As Cronin states, “The grand aim is to create complex chemical cells with life-like attributes that can help us understand the origins of life, and to use this approach to define a new technology founded on evolution within the material world—a form of inorganic living technology.”
