Life is Complicated-Literally, Astrobiologists Say
There have always been false positives in the search for extraterrestrial life, when scientists believe they’ve found life but they don’t have a convincing case.
It is because of NASA’s twin Viking landers which, in the mid-1970s, delivered controversial evidence of life on Mars that the archetypal example can be drawn. Three other life-detection experiments carried by the landers only produced null results despite the smell of radioactive carbon wafting from the soil. Scientists discovered what may have been microbial microfossils in an Antarctic meteorite containing what could have been Martian life in 1996. But subsequent studies showed that several other entirely abiotic routes may have been capable of producing the putative microfossils. Researchers have noticed significant amounts of phosphine in the Venusian atmosphere, a gas that is primarily produced on Earth by microorganisms. The measurement of the gas was questioned soon after, as other scientists suggested that the gas was the remnant of some strange, but lifeless, Venusian volcanism.
Each of these cases showed a similar pattern: initial interest, followed by skepticism, and eventually dismissal. Astrobiologists are frustrated over and over again by finding only inconclusive evidence of alien life-so-called biosignatures. Astrobiologists search for the simplest, most robust forms of life that seem possible in harsh environments outside our own, and the chemical and structural elements that make such organisms on Earth possible can often be produced abiotically. We might also observe entirely different chemistry on alien planets than on earth. Can we look at it differently?
Nature Communications published a new theory that says there is. The assembly theory works by embracing life’s fundamental complexity rather than searching for simple chemical biosignatures. Life’s information is encoded in complex molecules that are distinguishable from lifeless matter in any form of biology found anywhere in the universe.
The Assembly Theory is a landmark for the field, says study co-author Sara Walker of Arizona State University, since it presents the first complexity measure that can be tested in a laboratory. It also shows us we can link philosophical ideas about the nature of life to empirically observed phenomena, she says.
A recent trend has been to appeal to complexity in astrobiology. Scientists have developed theoretical models and definitions of life based on more sophisticated processes-metabolism, adaptation, replication, evolution-that could help us distinguish living systems from nonliving ones due to the ambiguous results that can result from chemical research. For instance, NASA adopted a complex definition of life in 1994: “Life is a self-sustaining chemical system able to evolve according to Darwinian principles.” However, the key concepts behind advanced frameworks are hard to test and quantify. Five different evolutionary biologists will probably give you five slightly different definitions of “Darwinian evolution,” for example. “I can’t build an instrument that will detect evolution, reproduction, or metabolism,” says NASA’s chief scientist, Jim Green.
Life, whether familiar or alien, may be better understood via assembly theory. Using physical complexity and abundance as two basic ideas, it states that, as the physical complexity and abundance of any object in any given environment increase, the chances of the object having an abiotic origin decrease. By determining the amount of steps required for assembly, abundance measures how often an object appears in a context, whereas complexity measures how often an object occurs in an environment. If you look at a seashore littered with water-worn pebbles, you might attribute this to an inert, lifeless process, but a seashore adorned with intricately sculpted seashells could be well attributed to a living process.
Researchers examined how the universal theory applies to molecules, perhaps the most fundamental building blocks of biology, both in space and in the lab.
Using an algorithm, a mass assembly number (MA) was assigned to different kinds of molecules according to their complexity based on a mass assembly index created for molecule complexity. In our proof-of-concept, we used this approach to index and rank a widely used chemistry database. The MA of 1 denotes that the molecule is simple and likely to originate from an abiotic source; molecules with a greater degree of complexity are assigned a higher MA number. Due to having only one atom of phosphorus and three atoms of hydrogen, phosphorus gas, the putative Venusian biosignature, has an MA of only 1. A second option is tryptophan, which is an amino acid with a MA of 12 due to its elaborate structure, comprised of 11 carbon atoms, twelve hydrogen atoms, two nitrogen atoms, and two oxygen atoms.
This study revealed that at a certain threshold-around MA 15-the probability of an abiotic molecule producing on Earth becomes almost impossible. Lead researcher Lee Cronin of the University of Glasgow led this study. Cronin says the probability is less than one in about 600 quadrillions. Life will almost always make molecules ranked 15 or higher.
Therefore, does this mean that MA 15 represents everyone’s life everywhere? Certainly not. A molecule as simple as a molecule of ozone can be a biosignature-for example, the structureless oxygen emitted into Earth’s stratosphere by photosynthetic organisms. Because of this, assembly theory, while it may decrease the possibility of false positives, may also correspondingly raise that of “false negatives,” which would allow genuine biosignatures to slip through investigative cracks. Cronin says the threshold for life might differ in other planetary environments even though MA 15 seems to be the threshold on Earth. According to Cronin, the trick is to use assembly theory to determine which chemical combinations are produced abiotically as opposed to those created by living systems.
Cronin and colleagues used mass spectrometry fragmentation to confirm their theoretical calculations of complexity by examining a large sample of molecules and substances broken up into their constituent parts to determine how many chemical steps are needed to assemble them. The experiments were close to theoretical predictions, distinguishing among a wide range of living, nonliving, and dead substances, such as E. bacteria, yeast cells, alkaloids from plant leaves, coal, granite, limestone, and even beer.
Cronin’s collaborator and study co-author is Heather Graham, a NASA astrobiologist who works at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Scientists from Graham’s lab tested the theory with blind samples. An ancient biological specimen was preserved in these fossils. It also contained organic carbon compounds (though they were not alive), which were derived from a sample taken from a Murchison meteorite that fell to Earth in 1969. Murchison material held many complex molecules according to Cronin, but it was classified below MA15, rendering it lifeless. The fossil, in contrast, contained evidence of life.
The distinction between a complex sample and a complex molecule became apparent to all involved at this stage of the research for study co-author and NASA astrobiology postdoctoral fellow Cole Mathis. Strange chemical mixtures like those present in Murchison may lead one to believe that life can be found there, but it is actually the complex molecule, which shows how chemistry works, which seems to be of critical importance.
Initial excitement was sparked by the publication of these results. He and his colleagues are “extremely enthusiastic” about assembly theory, according to Stephen Benner. Yet Cronin emphasizes that their work still needs to be discussed in more depth, especially whether it is truly applicable to exotic environments. Cronin has been challenged by Benner to test the approach in simulated laboratory conditions as if they were in the atmosphere of Venus with samples of “semi-complex” material synthesized from simple carbon precursors by Benner’s group. The environment here is real, says Benner, one that will soon be explored on a new mission to deep space. Venusian life would follow a very different chemistry from life on Earth if it exists in the clouds above Venus”, says Benner. This, he says, makes Venus the best test site for the molecular-complexity metric in the near term.
Cronin responded by noting how challenging Benner’s samples are because they are exposed to sulfuric acid-which breaks down organic molecules and lowers their detectable organic complexity. In spite of this, Cronin is optimistic that even if a molecule is intact, measurements can be taken, even in the most difficult samples.
At the same time, Green and colleagues at NASA have wondered if assembly theory can be used to facilitate the analysis of data from mass spectrometers that have visited other planets during the agency’s various missions to other planets. In his first argument, Green focused on the Cassini mass spectrometer. Saturn’s icy moon Enceladus had plumes of water vapor venting from its surface. Cassini’s instrument only registered masses up to 100 atomic mass units (amu), and assembly theory can be applied only to molecules weighing at least 150 amu.
The instrument on the Curiosity and Perseverance Mars rovers, although they reached 150 amu and beyond, also fell short, lacking the specificity to study single molecular species. As Green says, future missions should use mass spectrometers that measure masses with greater specificity and able to register higher masses. As a nuclear-powered quadcopter, NASA’s Dragonfly mission is set to set off in the mid-2030s to explore Titan’s atmosphere and surface. As Graham points out, Dragonfly’s mass spectrometer will have the ability to detect complex molecules, despite lacking some of the capabilities of lab spectrometers.
Further planned missions may explore astrobiological hotspots across the solar system for signs of life’s molecular complexity. At some point, Cronin speculates, assembly theory could be used to evaluate the presence of biosignatures in the atmospheres of potentially habitable exoplanets that are detected remotely by large telescopes.
For now, the approach has supplied theorists and experimenters alike with new ways to comprehend life’s cosmic complexity.