This piece is based off of a close reading of Thomas Gold’s 1995 essay, The Deep, Hot Biosphere.

“What is life” sounds like a stupid question, one that the reader immediately feels like they have an answer for. It is clear what is living and what is not living! The difference between something animated and inanimate is so simple that the smallest child can tell. However, it is possible that our definition of “life” is highly limited, bordered by the scope of our perspectives, hemmed in by the scale of our thinking.

If we were planetary creatures, bound and tied to the surface, perhaps this would be a lower-tier concern. But because we have set out into the solar system, and beyond, with the hope of encountering other life, it is necessary to ask ourselves if we will be able to see if when we find it. 

Currently, the best answer we have to that question is an operational definition, a list of behaviors that one can pull from Wikipedia or Google. Homeostasis, structural organization, metabolism, growth, adaptation, response to stimuli, and reproduction. But there are exceptions to each case. Viruses look alive, but they don’t seem more like software than living entity. Mules can’t reproduce, though they’re alive by any reasonable interpretation. Spores don’t grow or metabolize but can be reactivated under the right conditions, etc. 

In addition to there being an entire collection of exceptions to every rule on the list, there are other considerations. It may be that life is the same everywhere else as it is on Earth because it is a singular process. There’s a narrow window in which it occurs, and every time it occurs, it will look exactly as it does in every other place. That suggests a universe filled with planets on which people that look a whole lot like us are making mistakes that look a whole lot like ours, and an entire world full of plants, insects, bacteria, and animals are throwing their hands up and going what is wrong with these people?

It’s possible! Most things are possible. On some days, you can even convince me of the fact that it seems possible since things have been SO constrained on Earth. We share metabolic frameworks with bacteria so intimately that it seems that only a single solution was possible. We’ve never managed to come up with an alternate solution in a laboratory, say a cell that runs on a heat gradient or an electromagnetic gradient - and so it may be that the system that evolved is simply the only one that’s possible, given the physical limitations of the universe.

There might be something really special about chemistry in that reactions can be made to proceed spontaneously under the right conditions, and so it might be the only possible mechanism by which cells can harvest energy since the other forms mentioned above don’t seem to exist as a potential - they’re just physical gradients that dissipate passively over time.

The other option is that the might actually be organisms on other planets, maybe even on our own planet, that stretch our very understanding of what “life” actually means. Given that our best minds have only managed to come up with working definitions that none of them can actually agree on, it starts to stretch the imagination that our conception of what it means to be alive is anywhere close to accurate. A friend of mine recently exclaimed, frustrated at being pushed about what it means for something to be alive, “what does it matter the definition! I can see it with my own eyes!” Fundamentally, that comes down to being the primary issue. What if we can’t actually see it with our own eyes, and fail to recognize the different scales and forms that life comes in?

It’s easiest for me to consider this as a question of scale. Start from your own perspective, standing in the living room, looking out the window. In front of you in a couch that hasn’t been dusted for a while, and in the window is a meadow, at the edge of which starts a forest that stretches, unbroken, for the next hundred miles.

Head down in scale, to the level of the cushions, and you would probably never sit on that couch again, crawling as it is with everything from dust mites to tardigrades. Head out of the window and into the forest, and there’s a sudden jump in scale. The lifespan of the tree is measured in centuries rather than decades, and the number of organisms sheltered in its corona above as below number in the thousands. At the microscopic scale, it’s still hard to see how, exactly, these creatures can be considered as a single organism - though they live together, consume each other’s waste, act as food for each other, and communicate with one another. We don’t have the language for discussing how the microscopic might be interdependent. The scale of the tree, though, is a different question altogether.

Living in the Pacific Northwest has driven home the realization that here, the trees are individual pieces of what we traditionally call an ecosystem - but what might more accurately be perceived as an organism in and of itself. It isn’t one that looks like us, but the forest has a shape that is dictated by the landscape that surrounds us - and is no less real than the shape of the body we inhabit. The greatest difference is the fact that we look at the forest and we don’t think “organism,” but it is a body, one with edges, which has nutrient cycles, responds to its environment, reproduces, adapts. The problem to me, it seems, is that thinking of it in this way would create a sort of personal crisis, that would force many of us to confront for the first time what it means to destabilize an environment so completely that we would destroy giants. 

Luckily, these ideas, though slow to catch on, have been making some progress. In 1995, Thomas Gold published a small paper - The Deep, Hot Biosphere. In this work, he spoke of how recent revelations about the existence of life in hydrothermal vents at the bottom of the ocean, showed that “there, the energy for life was derived from chemical processes, combining fluids-liquids and gasses that came up continuously from cracks in the ocean floor with substances available in the local rocks and in the ocean water. Such sources of chemical energy still exist on the Earth, because the materials here have never been able to reach the condition of the lowest chemical energy.” What he means, is that down below the surface of the earth, there are still the chemicals that we consider to be the starting materials for the very origin of life - and that gradually cooling temperatures have prevented these reactions from proceeding spontaneously.

But a cell, that has proteins, DNA, RNA, membranes, all of the pieces that we consider make something “alive,” is capable of causing these reactions to happen, simply by organizing things properly. Enzymes are well-known catalysts - sometimes because they require ATP, and active process - but sometimes because they’re able to put atoms in the right positions and let chemistry do the work. The intriguing ability that cells developed was the way to harness this process to do useful work, rather than just remaining a chemical oddity. In areas where chemical disequilibrium remains, “there will often be circumstances where chemical reactions with surrounding materials might be possible and would release energy, but where the temperature is too low for the activation of the reactions. This is just the circumstance where biology can successfully draw on chemical energy.” In the oceans, these conditions are found in the deep-sea vents, where bacteria can survive, feeding on the metal hydrides, methane, hydrogen, and sulfides that emanate from the shattered rocks beneath the surface. 

When Thomas Gold first wrote this piece, we had only just discovered the fact that the vents at the bottom of the ocean, the Lost City located just off the Mid Atlantic Ridge, hosted a profusion of bacterial life - which in turn supported a vibrant community of larger vertebrates. The discovery fractured what we knew about life on Earth. Our conviction that the sun was vitally important to the biosphere bent, wavered and then shattered. Life could look far different from what we had imagined. 

In a prescient turn, Gold points out that, even though we don’t exactly know if there is life elsewhere in the crust - it may be there. This is especially true because the life at the vents was only discovered in the first place because other, larger life was drawn to it. Bacteria deep in the rocks, on the other hand, couldn’t be identified in the same way. He points out first, that “the pore spaces in the rocks are quite sufficient to accommodate bacterial life, and the rocks themselves may contain many of the chemicals that can be nutrients together with the ascending fluids.” Thus, setting up the realization that inside the rocks, away from animal life, bacteria might be thriving. But, “just as bacterial life in the ocean vents would not have been discovered had the secondary larger life forms not drawn attention to it, so any active bacterial life deep in the solid crust could have gone largely unnoticed.” The limitation of our methods of detection, in this version of events, is what is preventing us from seeing what’s as plain as the nose on ann’s face.

It would take another decade or so before molecular techniques progressed sufficiently to identify the bacteria of the deep biosphere, but eventually, Gold was vindicated. What is interesting about this paper is not that he comes up with some kind of off-the-cuff prediction, like an institutional Nostradamus - the most interesting thing is how he traces his argument. The leap between ocean bacteria in the deep-sea vents and bacteria locked in the rocky depths away from the surface of the Earth comes down to the fact that there are hydrocarbons absolutely everywhere in the crust of the earth - so many of them that we’ve had a boom century, exploiting them for transportation, heating, and manufacturing.

When Gold was writing this piece, these hydrocarbon reserves were predominantly referred to as fossil fuels - left over from the breakdown of ancient dinosaur bones under great temperature and pressure. But Gold, exposed to the microbiological data, and the sudden expansion of the domain that was capable of supporting life suggested that perhaps the hydrocarbon reserves on the planet are microbial in origin, rather than a resource produced 65 million years ago that we can run out of. 

While Gold later came to think that the origin of hydrocarbons was a passive abiotic process, he was still fascinated with the role that biology could play in remodeling the surface of the Earth. Looking at the distribution of minerals and ores in the crust, which tends to congregate into veins, he asks the reader a rhetorical question. “How can processes in the crust lead to the production of a nugget of gold or a crystal of galena when the refining process had to concentrate these materials by a factor of more than 10^11 from the original elemental mix?” Not one to wait for someone else to provide him with an answer, he suggests that life can increase order at the expense of chaos elsewhere that could have caused this distribution of metals through the crust. And if that is possible, “If there exists this deep, hot biosphere, it will become a central item in the discussion of many, or indeed most, branches of the Earth sciences.”

Beyond the relationship with the Terran biosphere, Gold attempted to speculate on the search for life beyond our own planet. Taking what he had pieced together about the Earth’s deep biosphere from the recent observations that had been made at various borehole projects, he suggests that perhaps, there will be life on other planets, similarly hidden far away from the surface, where it is protected against the harsh conditions of the exposed surface. Down there, “underneath the type of biosphere that we have discussed here, there will generally lie a large domain that is too hot for the bacterial life we know, but that is nevertheless capable of supporting other systematic chemical processing systems that can mediate those energy reactions. Could there be such higher temperature systems that act in a way similar to life, even if we may not identify them as life?”

It is this last question that is at the heart of why it is worth asking, what is life? Many of Gold’s other predictions - about the extent of the deep hot biosphere, and the possibility that life originated from the depths of the Earth towards the surface, have become popular in recent years. His perspective is sufficiently widespread that the new Perseverance rover will have a tool for taking core samples of the Martian surface, and the landing site in the Jezero crater was selected specifically because of the exposed carbonate located there, which researchers hope will divulge some of the red planet’s secrets.

In some ways, this has left the domain of science - mechanistic explanation for apparent phenomena - and has entered into the realm of mythmaking, and is attempting to tell the story of the origin. The idea that there might be something life-like that is far larger than life is tantalizing and would fit neatly into the story that we are starting to tell ourselves lately. As we leave the era of certainty and enter the era of possibility, it suddenly becomes worthwhile to question how we limit our understanding and definition of life.

As Gold himself acknowledges, “there is a lot of distance between plain crystallography and life.” However, “it is the bridging of this distance that forms the central piece of the theories of the origin of life.” Though he does not come right out and say it, perhaps the presumption of an origin is worth revisiting - for it may be that we are but one stage in the slow cooling of material - and are simply the physical organization that is best suited for it at these temperatures.