Is the origin of life an intrinsically planetary-scale process?

The View from Oregon — 372: Friday 19 December 2025

The past many newsletters on astrobiological themes have been leading up to a question and a thought experiment. Today I’m going to consider the question, and I’ll leave the thought experiment for later (if it still seems relevant after having gone through these concepts in the requisite degree of detail). The question that’s been on my mind for a few years is this: is the origin of life an intrinsically planetary-scale process? In other words, does life only arise on planets? This question is implicit in the space/time biotope matrix (discussed last week), in which we map min/max scale in space and in time for origins of life (and we can also do this for habitability, but I will leave that aside for present discussion), and we locate Earth in the center of this matrix as exemplifying the principle of mediocrity. If Earth is “just right” in terms of spatial scale and temporal duration for the origins of life (or close enough to being just right), that means that the boundary conditions for origins of life are an approximately planetary-sized object, and that in turn means that origins of life is intrinsically a planetary-scale process.

If life originated elsewhere and was brought to Earth, then the size of Earth or even its being a planet are not relevant to urability, though it remains relevant to habitability. If terrestrial life isn’t really terrestrial in origin but was seeded from elsewhere, then we can affirm only that Earth is habitable over cosmological scales of time (with current estimates having life on Earth about 3.8 billion years), but this doesn’t necessarily demonstrate anything about Earth’s urability. We could postulate another concept here, that of the development of life to further forms of complexity, as distinguished from the origins of life or bare habitability. Life on Earth might have endured here for 3.8 billion years and yet remained a single-cell biota dominated by horizontal gene transfer, rather than developing into an elaborate biosphere with eukaryotic cells and multi-cellular life characterized by vertical gene transfer, biodiversity, and a trophic pyramid.

The simpler alternative of these two would still demonstrate habitability, but not further development, so we could postulate a kind of environment in which life not only endures, but develops, and Earth would constitute such an environment, which we could call (following the tradition of using Greek nomenclature) metable or metabolable, adapting Aristotle’s word for change, μεταβολή. Life that originates off Earth could come from another planet, in which case planetary urability is still on the table, or it could originate on an asteroid, comet, or even in a cloud of dust and gas, in which case origins of life is not an intrinsically planetary-scale process. We cannot yet rule out the possibility that only non-planetary environments are urable, so that life must arise off a planetary surface and then be delivered to a planetary surface for long term habitability and metabolity.

While asteroids and comets may have distinctive chemical processes taking place on them, they wouldn’t have water cycles or rock cycles as we know them on Earth. Since comets partially melt as they approach the sun, we know that they have a water cycle of melting and freezing, but whether or not this water cycle would be sufficient for the chemical processes required for the origins of life remains unknown to us. Subsurface ocean worlds, like many of the moons in the outer solar system, will again have distinctive chemical processes taking place in them, but these will be different again both from the chemospheres of surface ocean planets on the one hand, and, on the other hand, the chemistry of asteroids and comets. It is possible that life can originate both on surface ocean worlds and subsurface ocean worlds, each being a planetary-scale origins of life event, even while these two origins of life events involve distinct planetary-scale mechanisms.

It is worth considering that these distinctive chemical processes of surface ocean worlds, subsurface ocean worlds, and asteroids and comets could well result in the production of distinctive biochemistries, so that the kind of life that originates in these different environments (if it does in fact arise) would be different in each case. If this were to prove to be the case, it would be a boon to astrobiologists since they would be able to work backward from any biota to discover the circumstances in which it originated. We could also speculate that the gravity of a given planet (taking the geophysical definition of a planet — on which cf. newsletter 114 — as a body that has become rounded through its own gravitation) could ultimately be expressed in any biochemistry that emerges on the planet in question, assuming some finite number of gravitational thresholds that result in a finite number of distinct biochemistries, each of which has a distinctive gravitational boundary condition. Again, this would be a boon to astrobiologists attempting to reconstruct the steps that life takes within a planetary system once it appears.

There are planetary-scale processes that intuitively seem to be exactly the kind of processes that would be implicated in the origins of life, in particular, the water cycle and the rock cycle, and the rock cycle could be a rock cycle on a stagnant lid planet (Mars, for example, had geological activity in its distant past, but doesn’t seem to have had plate tectonic movement) or plate tectonics rearranging the entire surface of the planet over geological time scales. A water cycle will involve wet/dry cycles as rain falls and then evaporates, or as bodies of water rise and fall, which can occur through precipitation or again through the mechanism of tidal forces. It is often suggested that the moon played an important role in the origins and development of life on Earth, and tidal forces could be one of these forces that makes life’s origins and development dependent upon the existence of the moon.

A rock cycle will minimally involve igneous and metamorphic rocks. If a water cycle is found together with a rock cycle (again, a planetary-scale phenomenon), then there will also be sedimentary rocks, and the rock cycle as we know it on Earth of sedimentary rocks being transformed into metamorphic rocks as they are subjected to heat and pressure, and then spewed out of volcanoes they become igneous rocks that can be worn down by weather to again result in sedimentary rock. These combined processes of a water cycle and a rock cycle result in more complex mineral species than those minerals that appear in the absence of these mechanisms. Some origins of life scenarios involve thin layers of clay as a superstructure for simple organic molecules to form a macromolecule, and for clay to form we need these complex geophysical processes of the water and rock cycles and their interaction.

But the origins of life is not yet a planetary biosphere. If life begins in a single, particular place on a planet’s surface, for this origins of life event to result in a biosphere, this local instance of life must distribute itself on a planetary scale, and this is in turn another process that must intervene between origins and biosphere for a biosphere to come into being. How long would it take for a lump of biota, reproducing itself in one place, to become a biosphere? Certainly there’s plenty of time in the history of Earth for the biospheric distribution of life to take place. Suppose we round off the estimate of life on Earth being about 3.8 billion years old to 4 billion years — that would give 200 million years for life that originated in one location to distribute itself on a planetary scale and so constitute a biosphere (traces of which biosphere that could, in principle, be detected at any point on Earth’s surface). With 200 million years to play with, a significant amount of distribution could be accomplished through plate tectonics, so that the growing lump of biota would be moved around the planet, shedding viable specimens of itself as it moved and so allowing life to take root wherever it had passed. This is effectively distribution by the rock cycle.

Still, this would be a slow process, and we can imagine more rapid processes. For example, we can imagine distribution by the water cycle. The lump of biota, once large enough, could be distributed by streams of water from rain, by tributaries, by rivers, and ultimately by oceans. If living bits of the blob of biota were small enough, they could be wafted away by the wind, being distributed inland against the flow of rivers draining downhill into basins. I was once on a beach on a very windy day, and I noticed that the wind not only stirred up the waves hammering the beach in a way to make a lot of foam, but the strong winds would tear off small bits of foam which were then blown inland, and with the strong wind it was possible to see these bits of sea foam following the contours of the landscape and passing over hills rather than being stopped by them. Any biota that found itself in a body of water similarly agitated by the wind could be distributed by the same mechanism.

I haven’t answered the question with which I began, namely, is the origin of life an intrinsically planetary-scale process? I don’t think that we possess either the empirical evidence or the conceptual framework to settle this question at present, but by exploring the question we get a sense of the astrobiological possibilities of the origins, development and long term habitability for life, and, beyond the specifically astrobiological possibilities, these biochemical possibilities could also be expressed in alternative emergent complexity regimes — life peers, but not life itself. With a sense of the possibilities, we could suggest any number of experiments that could be made to test these possibilities, though, as I have discussed elsewhere, experimentation in the origins of life will ultimately require “big science” on an unprecedented scale in both space and time. Whether or not we can ever undertake science at this scale is a question that is somewhat like the question of whether we could ever engage in an interstellar dialogue by way of radio telescope exchanges with some other civilization. What is the historical threshold — ten years, a hundred years, a thousand years? — beyond which messages can be sent but any real sense of dialogue is excluded by the time lag? And what is the historical threshold beyond which the continuity of a scientific research program would be lost?