The Threshold of Land Invasion
Thought Experiments in Waterworld Biospheres
Waterworld biospheres include both planets with liquid surface water and planets (or, in the case of our solar system, moons) with subsurface global oceans under a permanent ice surface. In this post I will only consider the former subset of waterworlds, those with liquid water on the surface, and, within this subset, I will consider only the subset of waterworld biospheres with extremely small areas of dry land. (I have previously formulated thought experiments concerning subsurface waterworld biospheres in Subsurface Ocean Worlds and Not Terraforming, but Something Else…)
I am interested in waterworld biospheres with very small regions of exposed land surface because I want to explore a particular question: What is the threshold of land invasion? We will explore this question in relation to counterfactual thought experiments. If there is a threshold, a lower bound of exposed land surface, for land invasion by life from the sea, i.e., for aquatic-to-terrestrial transitions, then this threshold influenced the development of life on Earth no less than it would bear upon the development of life on other worlds with much less land surface area.
Because Earth is presumably well above the threshold of land invasion, we can postulate that this macro-selection pressure on biospheric evolution was not strongly selective in the history of life on Earth, but in the cases of examples that we will consider below, such a threshold might be strongly selective, not for some one, particular species, but for the overall life history of the biosphere.
Let us begin with a thought experiment in which the Earth is covered by so much water that everything is submerged except for the peak of Mount Everest. On a planet with a single, small area of dry land, like the peak of a mountain, and the rest of the world given over to a global ocean, what would life and evolution look like? What I am thinking here is whether any organisms would evolve to live on land if the land available were small enough that it was essentially just a rock sticking up out of the ocean.
There are some rocky reefs in our terrestrial oceans that are so difficult to see that sometimes a sailboat will come to grief on them (the big ships are more carefully navigated). I once read an account by a man who spent many weeks on such a rocky, barren reef with his damaged sailboat. Eventually he was rescued, but he had to collect rainwater in his sail to keep himself alive. If this is any kind of guide as to what lives on a little piece of rock more-or-less stranded in an ocean, then a planet with a single mountain peak sticking out of a global ocean would probably be barren.
Suppose this is the case: how large would the area of land have to be before it presented an opportunity for evolution such that plants or animals would evolve a land-dwelling way of life because that opportunity was available? Would a square kilometer be enough? Would a square mile be enough? Ten square miles? A hundred square miles? At what point does an evolutionary opportunity pass over a threshold that it becomes a region that can be exploited for a distinctive way of life?
The way I have phrased this so far is pretty abstract and artificial, but considering the ways in which this thought experiment is overly abstract suggests ways in which we might make it more realistic. For example, there would be a very big difference between an isolated peak like the tip of Mount Everest poking out of a global ocean that was many kilometers deep on the one hand, and, on the other hand, and a planet that had a very shallow global ocean with a single peak sticking up from this shallow ocean.
Even if the whole ocean was not shallow, if a single island were surrounded by a large littoral zone of shallow waters, those waters would be inhabited by different kinds of life than would be found in the deep waters of a global ocean. A shallow sea or a littoral zone would mean sunlight penetrating to the bottom, and this usually means a crowded ecosystem of organisms on the bottom using photosynthesis, and lots of other organisms darting through the vegetation filling the waters. This paints a little different picture than that of an isolated rock that large fish might swim past, but which would not interact with a barren rock.
Another artificiality in the thought experiment is the assumption of a static peak sticking up out of a static ocean. Any peak that stuck up above an ocean would eventually be worn away by a combination of erosion from the waves and gravitational wasting. The only way a peak would remain above the waves for geologically significant periods of time would be if the planet was geologically active and plate tectonics was building up a peak even as it was being worn away by other forces. And in regard to the static ocean, if the planet had a moon the ocean would have tides, and tides would mean that a small amount of bare land would either be completely underwater on a regular basis, or that a small area always exposed above the water would be surrounded by a much larger area that was periodically exposed with the tides. Just this would change the nature of any life evolving in such a small biome. Any sea grasses or tidepool life living in the shallows on a planet with tides will be repeatedly submerged and then exposed, so that such plants are already partially adapted to the land.
It now seems there is evidence that Mars once had shallow oceans some billions of years ago. If Mars had had enough water that it covered everything on the surface except Olympus Mons, and given that Mars is essentially geologically dead, it would be reasonable enough to suppose that a structure as large as Olympus Mons could remain for geologically significant periods of time. However, Olympus Mons is so large that, if it stuck up among a global Martian ocean, it would almost certainly pass the threshold of land invasion I have posited of being enough room for life to evolve land dwelling organisms, so that doesn’t quite get at the point I wanted to make, though it’s still an interesting idea.
As I noted previously, the point I wanted to make was whether there would be a spatial area that could pass a certain threshold that it would become a clement biome for life to specifically evolve to live on this area. On a geologically active planet that could throw up mountain peaks above a global ocean, over geological time this area would expand and contract according to the geological conditions of the moment. There might be a time when a hundred square miles were exposed above the water level, and there might be a time when this dwindled to just a few square miles. Such a variation in land surface could serve as a strong speciation pump.
While considering thresholds, we can add a couple of more thresholds beyond mere spatial extent, and these would also add more realism to the thought experiment. Geology that could result in mountain peaks being pushed above a global ocean would more likely produce archipelagos than single, isolated islands. So in relation to an archipelago, and the biome that would come to be associated with an archipelago, the other thresholds would be how many islands were available as a potential evolutionary opportunity, and how widely spaced these islands were.
Intuitively, I don’t think anyone would object to the assumption that, the more islands there were, the more likely it would be that these islands would host life specifically evolved to live on these islands. Also intuitively, I don’t think anyone would object to the assumption that, the closer these islands were to each other, the better habitat they would present to life. Perhaps slightly more of a reach would be the assumption that a larger number of islands more closely spaced together would result in increasing biodiversity as the number of islands grew and the geographical distance between them shrank.
Again, the ocean itself, and indeed the crust that is pushing up these peaks in underwater orogeny events, would bear importantly on any land dwelling life that evolved. An archipelago in shallow water would be a different environment from isolated islands surrounded by very deep waters. Islands on opposite sides of a waterworld would probably be much too isolated to exchange life, so that, if life evolved on both widely separated islands or archipelagos, that life would consist of distinct evolutionary pathways to land invasion.
With all these variations on a theme of isolated islands in a global ocean you can probably by now get a feel for what I am getting at. There are a lot more variables that could be thrown into the equation to yield different results in a world-building thought experiment. But while world-building is fun, my motivation isn’t fun per se, but rather getting at unformulated, and perhaps even unsuspected, laws of evolution, as well as relationships and correlations that we do not know about because life and Earth has not developed in such a way as to manifest these relationships and correlations.
In any and all such unsuspected aspects of evolution, natural selection is still doing the heavy lifting, with occasional help from genetic drift, sexual selection, and some other forces in the biosphere, and forces which emerge from the nature of life itself. So I’m not trying to set up this thought experiment as one of those tiresome, “What Darwin Didn’t Know” articles that we see so often; I simply want to explore aspects of life that Earth does not bring out because of the kind of world that it is. Other worlds will have other life (if there is any other life to be found), and this other life will be different not because evolution will be different, but because the conditions under which evolution unfolds will be different. This is an important distinction to make, and I’m working on a more systematic exposition of this idea.