TLDR ; All throughout the universe, many worlds are likely to take on evolutionary routes different from our own, remaining permanently microbial. However, while there is good evidence to imagine this will be the case, it's much harder to imagine what life on these worlds would be like. However, by studying the ecosystems that once flourished in Earth's distant past and those that exist in the modern day, we might find our answer.
In my book Microcosmos, I plan to discuss how life will develop elsewhere in the universe. However, as opposed focusing on planets similar to our own, the worlds I describe have gone down a different evolutionary path. Instead, their biospheres have never achieved primary endosymbiosis (or at the very least, a form of endosymbiosis in which the endosymbiont gives up ATP or some other cofactor, as mitochondria do with their eukaryotic hosts). As a result, the organisms on these planets had never developed the necessary adaptations that would aid them in scaling up their energy production and supporting larger genomes.
This also means that they wouldn't have had the chance to develop analogues of animals and plants, let alone protists. Instead, what we're left with are essentially "worlds of bacteria". In which, organisms continue to rapidly diversify, but are held back by the same evolutionary pressures that have limited the development of bacteria and archaea here on Earth.
In order to explore the potential for evolution on these planets, I've made a theoretical model, dubbed "Leeuwenhoek" (named after the French microbiologist Antonie Van Leeuwenhoek). This current model has a mass slightly greater than that of Earth, a vast global ocean, and a moon that is smaller and orbits a bit further out than our own, resulting in weaker tides.
I addition to simulating how a planet would develop if complex life had never evolved, I also wanted to use this opportunity to see what might happen if the minerology of the planet were altered as well. For this reason, I decided to make Leeuwenhoek a "low-metallicity planet".
For in-universe context, Leeuwenhoek orbits Tau Ceti, a real star in the constellation Cetus. However, Tau Ceti has a relatively low metallicity percentage, at around 25-31% that of our sun. From this, we can extrapolate that Leeuwenhoek and other hypothetical planets orbiting Tau Ceti would start out with similar elemental abundances.
Following there formation, these planets would have also undergone differentiation, with denser materials sinking deeper into the mantle and lighter elements drifting upwards towards the surface. Since Leeuwenhoek is a bit larger than our planet, at around 1.7 Earth masses, this effect would be more significant. As a result, an even greater fraction of the iron and other heavy elements would have become incorporated into the core and mantle. This would have left the crust starved of many of the ingredients required for life (Leeuwenhoek has a global productivity rate ~15% that of modern levels observed on Earth).
*Note : Productivity is the rate at which carbon is removed from the atmosphere by living organisms and turned into biomass. This is done by plants, algae, cyanobacteria, and other primary producers.
Despite this, microorganisms would still flourish and go on to carve out a number of unique ecosystems in and along the planet's surface. These include coastal microbial mats dominated by photosynthesizers akin to cyanobacteria, terrestrial microbial mats that form vast plains along the margins of continents, as well as a number of more exotic cases, such as those inhabiting desert varnish and snow mounts.
For context, here's a complete list of the kinds of microbial ecosystems one might find on Leeuwenhoek:
*Note that while many of these ecosystems have proxies on Earth, they may not behave the same or have the same biogeographical distributions as their terrestrial counterparts. This can be due to a number of different reasons, ranging from minimal interactions with protists and fungi to differences in global nutrient cycling (all of which will be described in the book).
In order to create this list, I pulled inspiration from ecosystems that persist on Earth today that have little interaction with plants and animals. I also took a number of my ideas from various paleontological sites that were preserved prior to the Avalon explosion (the initial diversification of animal life that took place during the late Neoproterozoic).
It's also important to note that throughout much of the Archean and Proterozoic eons, the crust was thinner and less malleable, leading to less subduction and uplifting. As a result, large mountain ranges wouldn't have formed and less erosion would have taken place, greatly reducing the rate at which essential nutrients would have been cycled. In addition, low oxygen levels would have led to reduced oxidative weathering along the surfaces of continents. This in turn would have prevented "redox sensitive metals" (such as nickel, vanadium, molybdenum, etc.) from becoming freely accessible for organisms to use.
Both of these factors are now thought to have made the early Earth a low-productivity world, with some estimates suggesting productivity levels as low as 1-10% that of modern levels. As a result, the early Earth served as a good stand in for Leeuwenhoek, given that both planets would have imposed similar pressures.
At first, I assumed that both planets would have had relatively nutrient-poor surfaces, with areas of slightly higher productivity scattered throughout (these include environments with active erosion, such as coastlines, rivers, streams, hydrothermal vents, springs, and calderas.) However, in doing some research, I came to realize that some other environments on the early Earth may have been especially good at concentrating organic carbon and stimulating microbial ecosystems.
For example, Roper Seaway in Northern Australia dates back to 1.38 billion years ago and represents a deep-water marine habitat in which carbon from the photosynthetic mat-dominated coastlines was later deposited. Within this environment, organic carbon would be degraded by a host of microorganisms, causing the water to become temporarily euxinic (rich in hydrogen sulfide).
Meanwhile, Borden Basin of Northern Canada dates back to 1.2 billion years ago and seems to bear similarly high depositional rates. However, unlike the Roper Seaway, Borden Basin was once a massive inland lake that was totally cut off from the ocean. Instead, this body of water was connected to a braided delta.
As within all flowing rivers, the continuous movement of water erodes source rock and liberates key nutrients, allowing for significant amounts of productivity. This in turn would have generated large amounts of organic matter which would have flowed into Borden Basin where it would undergo deposition and serve as food for microorganisms. Over time, this community of microbes became so robust that they were able to change the water chemistry, causing intermittent periods of euxinia in the lower-water column (much like what was observed in Roper Seaway).
A 200-meter-tall rock face near Borden Basin, containing high concentrations of organic
carbon (Those dots to the left of the rock formation are people!) The alkalinity of the Borden Basin had also played a role in favoring interactions between organic carbon and sulfide, creating bonds which would have made much of this organic detritus difficult for microbes to break down (Hence why so much of it survived to the present and underwent fossilization.)
In both cases, productive environments were able to generate high levels of organic material which then fed into massive depositional environments, creating novel microbial habitats. As a result, it would only be reasonable to assume that environments on nutrient-starved, microbially-dominated worlds would likely have similar features, paving the way for all sorts of interesting microbial adaptations.
For instance, stratified lakes on Leeuwenhoek with permanently anoxic bottom layers (similar to Lake Cadagno in Sweden) could contain euxinic zones that reach high enough into the water column to come into contact with sunlight. Within this unique habitat, anoxygenic photosynthesizers would form massive swarms, turning the water layer they inhabit a deep shade of emerald green or purple. In some cases, photosynthetic microorganisms might actively swim in order to maintain their ideal depth in the water column. In other cases, they could control their buoyancy with the aid of gas vacuoles, hovering in the water like trillions of hot air balloons. Additionally, some may even form intricate partnerships with their motile neighbors, exchanging nutrients for transportation. In doing so, these small phototrophs might develop sophisticated modes of chemical communication, "puppeteering" their hosts and encouraging to chase after the ideal levels of sunlight (as observed in green sulfur consortia).
(Top) A diver emerging from a meromictic lake with a collection tube containing purple sulfur bacteria. (Bottom) A green sulfur consortium (Check out the video about them that I linked below in the citations if your interested!)
* Note : Meromictic lakes receive water from precipitation as well as ground water. Since this groundwater is more saline due to interacting with minerals in the subsurface (such as dolomite), it is far more dense that the water in the uppermost portion of the lake. This prevents the two layers from mixing. As a result, if the bottom layer is rich in organics and all of the oxygen within it gets depleted by scavenging microbes, it can become permanently euxinic / anoxic due to a lack of aeration. This creates a stable environment for anoxygenic phytoplankton (Environments with similar conditions include Lake Cadagno as well as The Black Sea.)
Meanwhile, along the bottoms of Leeuwenhoek's euxinic lakebeds and depositional coastal habitats, we might also find massive microbial mats, similar to those mentioned in Entry #2. In some cases, these mats might even span hundreds of miles along the seabed, harboring a host of strange and unique symbiotic relationships and microorganisms.
This is where we'll leave off for now, but be sure to look out for more posts on Leeuwenhoek's ecology moving forward.
Citations :
https://www.sciencedirect.com/science/article/abs/pii/S0012821X20303289
https://www.sciencedirect.com/science/article/pii/S0009254116303254
https://www.sciencedirect.com/science/article/abs/pii/S0301926812000812
https://www.youtube.com/watch?v=KF6rClLH4n4&t=88s
https://www.youtube.com/watch?v=GvQrH8qas0Q&t=9s
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