This column was written for Many Worlds by Michael L. Wong and Stuart Bartlett. Wong is a postdoctoral research associate at the University of Washington’s Astronomy and Astrobiology program and is a member of NASA’s Nexus for Exoplanet System Science (NExSS) initiative as part of the university’s Virtual Planetary Laboratory team. Bartlett is a postdoctoral scholar in Geochemistry at the California Institute of Technology and has been a fellow at the Earth-Live Science Institute (ELSI) in Tokyo.
By Michael L. Wong and Stuart Bartlett
The search for extraterrestrial life is in its early phase still and, the truth is, we don’t yet know if life exists beyond our pale blue dot. Or, if it does, whether it will be easily recognizable or truly bizarre.
Predicting what might be out there, and how to find it, is a hypothesis-driven area of research at present — one that has given rise to hundreds of possible definitions for the “life” we are looking for.
But after grounding ourselves in scientific principles, it may be that our greatest tool is to let our imaginations fly. Science fiction often helps us embrace our ignorance of what might be out there.
In the Star Trek universe, our galaxy is teeming with life—both astonishingly familiar and incredibly different.
The familiar variety includes Mr. Spock, the U.S.S. Enterprise’s half-human, half-Vulcan science officer. He is the product of an extraordinary cosmic coincidence: the emergence of nearly identical biochemical machinery on two completely separate worlds. Vulcans—despite their pointy ears, upswept eyebrows, and a nearly religious devotion to bowl cuts—are incredibly similar to humans on the cellular, genetic, and metabolic level.
We can share meals, share air, and, yes, share intimacy. Even their green, copper-based blood is not as alien as it might seem; this trait is typical of most mollusks and crustaceans on Earth.
But Star Trek also depicts life forms that are incredibly dissimilar from you, me, or Mr. Spock.
Take the Horta, for example. This lumpy mass, like a misshapen meatball crossed with a child’s volcano science experiment, is a silicon-based life form composed of molten rock and acid.
Then there’s Q, a non-corporeal being that possesses god-like powers which, it seems, are directed solely upon harassing Captain Jean-Luc Picard. There are the purely photonic beings that refuse to entertain the crazy (to them) possibility of organic, solid/liquid life forms. And, of course, there’s the Borg: sinister cybernetic zombies that proliferate not by cell division or sexual reproduction, but by assimilating outside individuals into their collective consciousness.
In Star Trek, there are many instances of life as we know it (humans, Vulcans, Tribbles, etc.) and then there is…everything else. The Horta, the Q, and the Borg compel us to consider wider possibilities for life in our universe. Furthermore, they compel us to create a new vocabulary for defining such possibilities.
In a recently published paper, Bartlett and I introduced a new term, lyfe, to describe life in its most general sense. We outline four criteria—the four pillars of lyfe—that represent a necessary and sufficient description of the living state. These are: dissipation, autocatalysis, homeostasis, and learning. In our new classification scheme, the word “life” retains its familiar meaning, referring to life as we know it (in other words, humans and Vulcans but not Horta).
Let’s walk through the four pillars one by one.
1/ Dissipation is essentially harnessing usable energy and putting it to work. Imagine a waterfall crashing over a cliff. With a clever enough contraption, one can use the water’s kinetic energy to drive any number of useful tasks, including the generation of electricity. Eventually, the originally useful kinetic energy will be dissipated into heat, which is fundamentally difficult to harness. In biology, dissipation takes many forms, from chemical reactions to flows of ions to capturing rays of sunlight. Without the useful work supplied by dissipation of useful energy to less useful energy (useful is in fact a well-defined concept in thermodynamics), it’s impossible to imagine a system being able to perform any of the other pillars. That’s why dissipation comes first.
2/ Next is autocatalysis, the idea that living systems are self-promoting. This could be on the scale of molecules, where one product leads to another product which leads back to the original reactants—a chemical cycle. Or it could be on the scale of populations, as one bacterium begets two, which each beget two more. Life’s activities reinforce themselves in ways that produce positive feedback loops and ever-increasing activity under ideal circumstances.
3/ Third, homeostasis. This refers to life’s ability to regulate itself against the chaos of the outside world and from self-destruction. From single-celled organisms buffering their own acidity to human bodies maintaining steady core temperatures to nations creating governments with checks and balances, we see myriad examples of biology’s vigilant watch over its internal properties.
4/ Finally, learning is life’s ability to record information about its environment and use it to increase its chances of survival. Darwinian evolution is one form of learning in which life on Earth is engaged, operating through trial and error across generational time. Cognitive learning, exhibited by more complex organisms, is one way for life to gain and process information during their lifetimes. Modes of learning are varied, subtle, and not yet fully understood.
Any system that performs all of these pillars constitutes lyfe—even systems that don’t perform the pillars exactly as we do here on Earth.
Consider the Borg from Star Trek. Their cybernetic Collective does not learn in a vertical fashion, such as by bequeathing genes to one’s descendants or transferring knowledge to one’s students. Instead, the Borg learn horizontally, through assimilating other life forms into their hive mind. They do not generate their own novelty; they allow others to do so, then add those civilizations’ “biological and technological distinctiveness” to their own. Their vast library of knowledge allows them to adapt quickly to challenges and spread autocatalytically like a highly virulent disease across the galaxy.
The four pillars of lyfe are also indifferent to the material from which the system is made. So, the silicon-based Horta is very much alyve, as are the extra-dimensional Q. In real life, Saturn’s moon Titan offers a tantalizing location for the existence of lyfe, where hypothetical liquid-methane-based organisms might exist in a much different modality than Earth’s water-based life. If the Enterprise were to survey Titan and find methane-based organisms, Mr. Spock might indeed feel compelled to report, “It’s lyfe, Jim, but not as we know it.”
Because the pillars of lyfe are all abstract processes, rather than a list of essential components, they have the potential to reshape our approach to origins-of-life research.
At the outset, it seems obvious that any theory explaining the origin of life on Earth must square with two things: the nature of the early Earth and the nature of life today.
Unfortunately, there is a great deal of uncertainty regarding the nature of early Earth. Geologists read layers of rock like pages in a book to uncover stories from our planet’s past. But the constant churning of plate tectonics has essentially ripped all but a few of the oldest pages from their binding, replacing them with new pages that will, too, eventually become recycled.
At the same time, trying to infer the chemical nature of life’s origins by observing life today is confounded by a haze of horizontal gene transfer and the possibility of multiple origins. Our best technique for identifying the features of ancient life—molecular phylogenetics—can at best probe as far back as the Last Universal Common Ancestor, or LUCA (in fact, it may even be impossible to trace back this far). However, we are blind to developments that occurred between the origin of life and LUCA, and whether those stages of life bore any molecular similarities to us.
These uncertainties haven’t stopped scientists from trying to paint a picture of our own beginnings. But with little evidence to bound the problem, many different portraits fit the same spotty description. Unsurprisingly, there is great division among scientists regarding the way that life began.
Many explanations for the origin of life take the form of an ‘X-first’ theory, where X could be one of several features of life as we know it: template-based replication of nucleic-acid polymers, proton–phosphate coupling in metabolism, or compartmentalization in lipid membranes. To constrain the problem of the origin of life, scientists often craft narratives where some X (replication, metabolism, or compartmentalization) is the most fundamental aspect of life. They then focus their efforts on the creation of highly specific building blocks associated with that X. Examples include RNA in shallow ponds or ancient metabolic pathways in deep-sea hydrothermal vents.
However, the concept of lyfe offers us new targets beyond the narrow range of molecules that are assumed—but not proven—to have played a role in life’s origins. We suggest using the pillars of lyfe as signposts: look for systems that perform the four pillars, not just those that crank out a shopping list of prebiotic foodstuffs.
Aiming for the pillars of lyfe could lead us down some truly unexpected roads, be it combinations of chemicals that had never been considered before or a greater exchange of ideas between the origins-of-life and artificial-life communities. Let us not forget that the modern world was built from ideas that seemed crazy at first.
More importantly, the search for lyfe might teach us something deeper about who and why we are.
In the finale of Star Trek: The Next Generation, Q takes Captain Picard some three-and-a-half billion years into the past to witness the origin of life on Earth. At the base of a cliff where volcanic lava meets a roiling sea, they study a primordial pool of goo in which, Q states, “Amino acids are about to combine to form the first protein.”
While this is one possible conception of life’s origins, scientists may never truly know or agree upon exactly what occurred on our planet all those eons ago. But even without a time-bending intervention from Q, perhaps we will one day discover that there are unifying principles behind the living state—foundations of the physical world that allow all forms of lyfe to materialize in myriad environmental realms.
The concept of lyfe encourages us to dream wide. Not only should we ask, “How did life originate?” but also, “Why does lyfe emerge?”
In other words, let us explore the initial conditions, dynamics, and interactions that could give rise to the emergence of each of the four pillars in phenomena across the universe. With this mindset, all avenues of origins-of-life research can be valid and promising. Lyfe invites us to “yes, and” ideas for the emergence of living processes, rather than pooh poohing them simply because they are not our own pet favorites. By pursuing all geochemical environments with interest—along with a host of synthetic scenarios—we may come to understand that the underlying principles by which inanimate matter produces the four pillars of lyfe can operate in a variety of systems.
Perhaps life exactly as we know it is rare, but lyfe could be an emergent organizational state embedded in the physics of the universe.
While it is entirely possible that nothing remotely resembling the Horta, the Q, or the Borg actually awaits us among the stars, something even more exotic might. Indeed, nature has repeatedly proven more creative than human imagination (think how often we hear, “Scientists never anticipated…”). Hopefully, the four pillars of lyfe give us an even better chance of seeing a universe for what it is—a playground of possibilities, differences, and commonalities—and open our minds to an even richer version of the timeless question: “Are we alone?”