The Speculative Evolution of Project Hail Mary

Imagine that a microscopic single-celled life form had evolved to survive in the vacuum of space and to propel itself at velocities approaching the speed of light. This algae-like organism travels between solar systems, feeding on radiation from stars and using carbon dioxide-rich planetary atmospheres to reproduce. Such organisms, once they had reproduced enough, would act like an infection to a solar system, especially one supporting life on an orbiting body. These alien algae, by feeding on the star's energy, would cause the amount of solar radiation emitted from the star to decrease. A solar dimming event would follow, and any rocky planet supporting life would eventually be facing a catastrophic and very sudden ice age.

Now imagine this happens to our star. What would happen to life on Earth if the sun became infected with this alien astrophage? The novel *Project Hail Mary* explores this absolutely fascinating concept and features alien species with some of the most brilliant, wholly original speculative biology written by Andy Weir, who is best known for writing *The Martian*. *Project Hail Mary* was published in 2021. News of a film adaptation starring Ryan Gosling soon followed. And now, nearly 5 years later, here it is. And what a fantastic film it is. As a massive fan of speculative evolution and an enjoyer of Weir's other works, the book very quickly became one of my all-time favorites, and I would highly, highly recommend it to any fan of science fiction or speculative evolution. And it's especially great as an audiobook.

So, what is this space algae and how does it travel between stars to infect solar systems? Named Astrophage by the book's protagonist, which is Greek for "star eater." They're single-celled organisms with some incredible biology that evolved on a planet orbiting Taeti. This planet, Taeti E, or Adrien as it comes to be known in the novel, is a large rocky planet with a radius almost double that of Earth's and a mass almost four times as large. The atmosphere of Adrien supports a remarkable diversity of microscopic life forms, including a lineage that inhabited so far into the upper atmosphere that they eventually evolved the ability to travel through the vacuum of space and hop between celestial bodies. This is the lineage that gave rise to the astrophage.

The astrophage cells propel themselves by emitting light at a certain infrared wavelength, utilizing the momentum of light to produce thrust, which in turn pushes them through space. Considering the tiny size of the astrophage cells at just 10 microns in diameter, the minuscule amount of thrust from the light emission is in fact a very effective mode of propulsion for these organisms, and they're capable of reaching up to 92% of the speed of light. Astrophage cells don't look like much at first, appearing to just be moving black dots under a microscope. In fact, the cells appear completely black while alive, as they're entirely opaque to all forms of radiation, having what's termed "super cross-sectionality." This is stated to mean that nothing can quantum tunnel through the cells, going against all known laws of particle physics. However, this super cross-sectionality is lost once a cell dies, immediately making the internal structures visible as light is able to pass through them.

The internal anatomy of an astrophage cell is somewhat similar to the cell of an Earth organism, specifically eukaryotic organisms, in that they also contain DNA, mitochondria, and other equivalents of organelles like those found in our own cells. Plus, they're filled with water. But in addition to these organelles, it's revealed later on in the novel that astrophage cells contain a lot of free hydrogen ions, so protons, within the cellular membranes. Astrophage cells maintain a constant internal temperature of 96.415° C. It's explained that at this temperature, the protons collide with each other inside the cells with enough kinetic energy to create a pair of neutrinos. These neutrinos act as the astrophage's energy storage mechanism. Since when two of the stored neutrinos then collide, they annihilate one another and produce two photons of a particular wavelength of light. The photons are in turn used to propel the cells through space. And the stored energy can also be used to heat the cells back up to the critical temperature of 96.415°. Conversely, if the cells are heated to more than this critical temperature, such as they would be on the surface of a star, this extra heat energy just makes the protons within the cells collide harder to produce neutrino pairs. And then the protons still have leftover kinetic energy. This means the protons can continue to collide, in turn, making more and more neutrinos that are stored within the cell. So the ultimate result is that the astrophage cells quickly build up a neutrino store while the temperature of the cell remains constant. Hence why these alien organisms feed on stars. Importantly, this makes astrophage an incredibly efficient energy storage medium, as the cells are capable of converting energy to mass and back to energy again with no loss.

Now, I'll admit here that I'm no particle physicist, so this is all beyond my level of understanding. But from what I've seen of some actual physicists explaining the scientific accuracy of this hypothetical energy storage mechanism, it requires some suspension of disbelief. As protons don't quite work like this, but hey, this is why it's speculative evolution and it's clearly in a sci-fi setting, so it certainly doesn't take away from the story. In fact, it's pretty damn imaginative. Anyway, since neutrinos regularly pass through our entire planet and even our bodies without interacting with any other particles, they shouldn't be expected to be contained within astrophage cells. However, it's explained that they can be due to that super cross-sectionality property, meaning that the cells collide with all matter that tries to pass through despite how unlikely a collision should be. So, astrophage is a brilliantly fascinating, if terrifying, speculative alien life form.

Moving on to the life cycle of these alien algae. Astrophage cells require carbon and oxygen to reproduce and metabolize in addition to heat energy from stars for propulsion. So, their life cycle involves a migration through space between the star and a suitable orbiting planet with a carbon dioxide-rich atmosphere. In the case of our own solar system, the planet that astrophage migrates to in order to reproduce is Venus. So upon reaching a solar system, astrophage will first propel themselves towards the star, living on the surface where they feed on heat energy and store it within their cells through mass conversion, generating the neutrinos. Once the cells are fully enriched, storing a maximum of 1.5 megajoules, they follow the magnetic field lines of the star, traveling away from the north pole of the sun until they are far enough away that they can detect wavelengths of light consistent with those which reflect off a planet with a CO2-rich atmosphere and propel themselves straight towards this planet. Then upon slowing down in the atmosphere, they have a source of elements necessary to grow and reproduce, which they do via mitosis. Once a parent astrophage cell has divided into two daughter cells, they both head back to the star where they feed and continue the cycle of reproduction. So, you can see how a rapidly growing population of astrophage that infects a new solar system and remains unchecked can spell disaster for any planets in the system supporting life. The exponential growth of these organisms can have a significant impact on the overall luminosity of the star as more and more cells continue to appear on the surface to feed and so more radiation is blocked by the super cross-sectional nature of the life forms. As a result, the infection of astrophage in our own solar system as they travel from the sun to Venus and back again threatens to plunge the Earth into a very sudden ice age as the luminosity of our star begins to decrease.

This is where the project named *Hail Mary* comes into play. A last-ditch effort to send a team of scientists to investigate why the star Taeti is not decreasing in luminosity despite the fact that multiple other stars in the surrounding vicinity are. As we learn in the book, Taeti remains unaffected because it is the astrophage's home system and alongside this organism are astrophage predators that keep the populations regulated. Another key part of the astrophage lifestyle is its sporing stage. This occurs when the cells travel between solar systems and it appears they have a maximum range of about 8 light-years that they can travel before they die without the energy of a star to support them. During this spore stage, the cells are capable of coasting along in space until they make it to a new system and reactivate. In the novel, it's stated that our solar system was infected by astrophage that arrived from WISE 0855-0714, a sub-brown dwarf star about 7.4 light-years from the sun, which was in turn infected by Sirius.

The remarkable biology of astrophage means that it also has many uses for humans. And this is another aspect that's explored a great deal in the novel. In fact, astrophage is used to power the vessel that takes the scientists from Earth to Taeti, a vessel called the *Hail Mary*. Astrophage is utilized as a mode of propulsion for this vessel using the same principles that the individual cells employ to move themselves through the vacuum of space. The *Hail Mary* features over a thousand individual spin drives, devices that take advantage of the natural life cycles of the astrophage by exposing them to the wavelengths of light they have evolved to propel themselves toward, pushing them against the ship and therefore exerting thrust. With millions of individual astrophage cells pushing against the vessel, this proves to be a remarkably efficient propulsion method. Astrophage is also utilized as a miracle radiation shield. Thanks to its super cross-sectional properties, living astrophage cells will block radiation from passing through them. And so, a thin layer of the hole in which astrophage is injected is constructed around the crew areas of the *Hail Mary*, protecting the humans inside from radiation exposure in space.

So, how exactly could such an extraordinary hypothetical organism evolve? Adrien is a large rocky planet with a surface gravity of approximately 1.4 g's. So that's 1.4 times Earth's surface gravity. Its atmospheric composition is dominated by carbon dioxide at 91%, followed by 7% methane, 1% argon, and then various trace gases. The atmosphere is described as being very thick. So thick that from orbit, you can't see down to the ground at all. It's obscured by green, wispy clouds. Within the thick atmosphere, there are distinct concentrations of astrophage. But alongside these organisms are a whole host of other diverse single-celled aliens, including the primary astrophage predator called Taliba by the novel's protagonist. There are also other Adrien organisms living alongside the astrophage cells as they migrate to and from the star through space. Meaning that astrophage is not the only species from this planet that evolved the ability to locomote in a vacuum. There's actually an entire ecosystem living within the so-called "petroaline" between Adrien and Taeti. And astrophage is just one species that managed to escape the solar system and infect neighboring stars.

Presumably, the evolution of life on Adrien went through some similar stages to the evolution of life on Earth in real life. Perhaps Adrien life began in the planet's oceans or in warm shallow pools, but then rapidly dispersed throughout the strata of the atmosphere due to the greater density, enabling these single-celled organisms to locomote through the air more easily, as there would be more resistance for them to push against. It would be really interesting to know if any multicellular life evolved on Adrien, maybe down on the surface, but as far as the novel mentions, there's only the diverse high-atmosphere unicellular species. A hypothetical astrophage ancestor presumably lived extremely high in Adrien's atmosphere and over millions of years underwent selective pressures that drove it to further and further extremes and closer to the vacuum of space. Perhaps the ancestral astrophage had initially been utilizing a process similar to photosynthesis to convert light energy to chemical energy. This is maybe supported by the green-tinged appearance of Adrien. Perhaps most of the other life forms swirling about in the atmosphere are still photosynthetic. However, the astrophage lineage took things a step further. Perhaps driven in tandem by selective pressures to avoid radiation damage at the outskirts of Adrien's atmosphere and for more efficient energy conversion and storage, their cell membranes developed super cross-sectional properties. This made it possible to contain neutrinos and in turn permitted an entirely new way of locomoting in the extremely thin upper atmosphere and eventually in the vacuum of space.

The novel also strongly suggests that panspermia explains the similarities between Earth-based life and life on Adrien. Panspermia is a hypothesis that proposes life on Earth was seeded from an extraterrestrial origin and that life throughout the galaxy may have a single shared origin, becoming distributed across various planets. In the *Project Hail Mary* universe, the fact that terrestrial and Adrien life share the same DNA structures, presumably even down to including the same nucleotide bases, is a strong indicator that the life on these planets had a common origin. Since Adrien is home to single-celled organisms with a known capability to travel between star systems under their own propulsion, it would appear that Adrien might in fact be the source of all known life in the galaxy, or at least in our part of the galaxy. This would therefore presumably mean that some species that lived around 3.8 billion years ago on Adrien had also evolved the capability to travel between stars. This species was again presumably quite different from astrophage, considering that our solar system was not already inhabited by organisms migrating from Venus to the sun and back again. Instead, this hypothetical Adrien life must have found its way to the atmosphere of the primordial Earth and begun colonizing this new world, leaving behind its interstellar travels and undergoing an adaptive radiation on another rocky body.

This, however, raises an intriguing question. Why does the book note that astrophage contains mitochondria? As pointed out in a fascinating Reddit post by a microbiologist who read the novel, which I'll link below, the presence of mitochondria in these alien cells raises a bit of a mystery about their origins. This is the issue of endosymbiosis. The earliest cellular life on Earth would not have had mitochondria because mitochondria are derived from single-celled organisms that were incorporated into other cells, a phenomenon called endosymbiosis. So astrophage possessing mitochondria-like organelles infers that at some point in their evolutionary history, they also incorporated another life form into their metabolic cycles. But if we're to assume that something like astrophage originally seeded life on Earth, then how come there are so many kinds of organisms on our planet that lack mitochondria? Now, many details of the other mechanisms within the astrophage cells are left pretty vague in the book. So there's plenty of room for speculation. I don't think that an independent occurrence of endosymbiosis in astrophage is particularly improbable, considering how many times it's occurred in the history of life on our planet, but also because the astrophage species itself would not be the ancestral lineage of all life on Earth. Astrophage is a much more recent arrival in our neck of the woods, an invasive species without natural predators in this solar system. Whatever Adrien species seeded the ancient Earth would have been a long extinct lineage that I think may have independently evolved the capability to travel between star systems. I'm hypothesizing that Adrien has long been a hotbed for the evolution of high-altitude single-cellular life. And every few million or maybe even billions of years, a species encounters selective pressures that drive it further and further out of the atmosphere until eventually it must locomote through a vacuum. This is what happened with the species responsible for the panspermia event billions of years ago. And then it also happened independently with the astrophage lineage, an example of convergent evolution. Perhaps the other single-celled Adrien species living alongside the migrating astrophage in the Taeti system are close relatives of the astrophage lineage, descended from a common ancestor that evolved space travel. Alternatively, maybe they all independently evolved this ability. The novel doesn't expand much more on the life within the "provoline." So, it's up for interpretation, but it does further add to the idea that all sorts of varieties of Adrien life are regularly traveling through space. So, perhaps the prehistoric Adrien life that traveled between stars did not yet possess mitochondria. And maybe it wasn't until they reached Earth and seeded our world that certain lineages of the explosive radiation resulting from the founding species underwent endosymbiosis, incorporating other single-celled organisms into their biology.

But astrophage and Taliba are not the only alien species featured in *Project Hail Mary*. A brilliant part of the novel is the revelation that our human protagonist, Ryland Grace, is not alone on his mission to the Taeti system. Another species of sentient organism has been affected by the astrophage, since their star is also being colonized. As it turns out, they also sent a vessel to Taeti. But like Ryland, only one individual has survived the journey. This is where we meet Rocky, my absolute favorite alien in all of fiction. Rocky is from a planet in the 40 Eridani star system and originated on the planet closest to the star 40 Eridani A. This planet is therefore given the name Erid, and Rocky is an Eridian. Of course, this is just the English name given to the planet and species because Eridians communicate in a very different language. There are a few key differences between the Rocky we meet in the film and how his species is described in the book. However, much of the overall biology still remains consistent between both versions. Many of the details of what I'm about to delve into are included in a guide to Eridian biology that author Andy Weir wrote to maintain internal consistency with how Eridians work, and he very kindly sent it to me in an email exchange. The guide goes into loads of detail not featured in the book or the film. So, let's explore these amazing concepts.

Eridians are pentaradially symmetrical organisms with a tough exoskeleton composed of oxidized minerals. So essentially, their outer bodies are made of rock, hence Grace's name for Rocky. Their five identical limbs radiate out from evenly spaced points on their thorax, connected by ball-and-socket joints. The limbs have hinge joints midway down and they end in trifurcated claws which can open or close depending on what action they want to complete. Each limb can be used as an arm or a leg, and Eridians have no sense of a forward-facing direction due to their radial symmetry. An average Eridian stands about half a meter tall. The film depiction of an Eridian seems to be a little more asymmetrical than described in the book. Rocky has limbs that don't appear to be equally positioned around the thorax, and the carapace is shaped more unevenly. The thorax is also deeper, but overall, it's pretty similar to how Andy Weir describes them. If anything, he looks a bit more rock-like in the film.

Eridians also do not have any sense of sight, nor any organs capable of detecting electromagnetic radiation. Instead, they have extremely well-developed hearing. The carapace has hundreds of thousands of small openings, with each oracle tuned to specific frequencies. In a way, the entire carapace is like a microphone. Piezoelectric flashes send information directly to their fiber-optic nervous systems, allowing their brains to build a mental picture of their surroundings from ambient noise and the sound absorption qualities of nearby objects. If there's no ambient noise, then they will tap the ground, essentially echolocating. This reliance on sound and lack of any sight organs is a result of the conditions on the Eridian home world. Erid is a rocky planet with a very thick atmosphere. It has a surface gravity of 2.09 g's, an atmosphere mostly composed of ammonia with a few other trace gases, a surface pressure of 28 atmospheres, and an average surface temperature of 210° C. The magnetic field is about 25 times the strength of Earth's, and days on Erid are about 5.1 hours long. Erid's ecosystem spans the layers of its thick atmosphere, with photosynthetic microbes inhabiting the uppermost parts, which in turn support predatory airborne microbes that feed on them and various small flying animal equivalent life forms that are even higher up the food chain. Because Erid is so close to its star, an extremely large amount of energy enters the biosphere. However, I must mention that in the film, it seems Erid might not be as close to its star as it's described in the novel, as it possesses a ring system. Critically, there's a boundary life layer on Erid, composed of flying organisms that hunt in the skies but also spend a lot of time on the surface, fueling the land ecology. The Eridians evolved as the apex predators of the terrestrial ecosystem. But because of the thick atmosphere and the density of life forms inhabiting it, almost no light from their star reaches the ground. As such, the land-based organisms of Erid have no light-sensing organs and instead rely on sound to navigate, communicate, and hunt.

The very different conditions of Erid compared to Earth mean that Eridians are drastically different in their biology from the terrestrial life forms we're used to. The best way to think about the Eridian organism is that it is essentially what would happen if bees could construct a beehive that moved, then build a brain to coordinate it. Most of the body is made up of inorganic material. And really, it's all there to house and maintain the worker cells, single-celled organisms that inhabit an internal colony and travel out from there to support and repair the rest of the body. To illustrate just how different the Eridian body is from a human's, it's stated that while the entire individual can weigh something like 400 kg, less than a kilogram of that is organic mass. The worker cells have structures similar to those of Earth life with cell membranes, nuclei containing DNA, and mitochondria. Again, it's hypothesized by Grace and Rocky in the novel that Erid was also seeded by the panspermia event originating from Adrien, explaining these basic biological similarities. Each worker cell type has its own distinct genetics, with thousands of specialized types found in the body that perform different functions. The colony at the center of the thorax is a breeding area and safe zone for the worker cells and also houses the brain. Some 20% of the worker cells are specialized for servicing the brain, while only around 8% travel around the circulatory system to maintain the rest of the body. The other 72% of the worker cells function as part of a self-balancing biosphere that allows the colony to exist, maintaining this closed ecosphere. This ecosphere is described in the book as containing plant-like cells and animal-like cells. The rest of the Eridian body is practically just a support system to keep this internal world going.

The Eridian brain is not actually biological or made of cells. Instead, it's crystalline in structure, made up of a complex set of refractive materials. Impulses of light are generated in the "hindbrain," and it's the different areas of refractive capability that function like neurons, directing the flow of light, reinforcing it, or canceling it. Tendrils of crystals extend around the body and work like fiber optics, conveying nerve impulses to other regions. The crystalline brain requires the worker cells to maintain it, with the biological components of the Eridian servicing it to remove impurities in the crystal that might alter how light passes through it. When there's a short-term refractory change to the neural network, the worker cells will also add or remove certain elements in the crystalline brain to make the changes permanent, essentially solidifying long-term information storage. The extremely large amount of energy that enters Erid's biosphere also means that Eridians actually possess specialized sacks around the body that just store ATP, the compound that functions as the energy currency of living cells. The blood equivalent of Eridians is mostly composed of liquid mercury, which serves both to regulate heat and to transport worker cells throughout the body. Various other metals dissolved within the mercury are utilized by worker cells at different locations to construct and repair the inorganic body. Two unconnected bloodstreams run through their bodies: the ambient circulatory system (ACS), which is kept at the outside temperature, so about 210° C, and the hot circulatory system (HCS), which is kept at about 305°. The ACS is larger in volume and composed of vessels made from various types of sodium silicate. Radiator organs on the top of the carapace maintain the ambient temperature in the ACS. This is the system that transports worker cells throughout the body, as the HCS is too hot for them to survive. The HCS blood vessels are lined with a very efficient thermal insulator and it's maintained at this higher temperature thanks to a reservoir within the thorax that's heated using energy from the digestive system. Eridians possess five hearts, one associated with each leg, which allows the heavy liquid mercury blood to be pumped up and down the limbs. The temperature difference between the two circulatory systems is key to how Eridians move their exoskeletons. Their major skeletal muscles are spongy and cylindrical and are full of small openings that contain liquid water within them. The muscles are surrounded and interspersed with capillaries from the HCS and ACS. And when the muscle is instructed to move, the ACS capillaries constrict while the HCS vessels increase the flow to the muscle. This increased flow of the hotter system causes the water in the muscle vesicles to become gas and expands the pockets, making the muscle in turn expand. To relax the muscle, the process reverses and the water returns to liquid form. Essentially, Eridian muscles are steam-powered. The internal supports or bone equivalents upon which these muscles act are honeycombed metallic alloys. Eridians also possess micro-muscles within their circulatory systems, which control the flow of blood via valves and flaps. Micro-muscles are crystalline structures and are powered by a piezoelectric effect. A constant microvoltage potential is sustained between the mercury blood and the surrounding parts of the body. When a signal is received from the brain, a chemical change will occur to the blood vessel wall near the micro-muscle, and the current flow through the muscle therefore deforms the crystalline structure, curling a flexible flap and altering the flow of blood between regions of the circulatory system. The constant heating and cooling of the major skeletal muscles require Eridians to continuously regulate the temperature of their ambient circulatory system, which serves as the heat sink for cooling the muscles. That's the purpose of the radiator organ, which takes in ambient air and passes it over capillaries through slits in the carapace. The radiator organ comprises a rigid set of capillaries positioned just under a cap segment on the top of the body and it's protected by baffles, as damage to this region of densely packed capillary bundles would be devastating. Air moves through five in-vents around the perimeter and exits through five out-vents at the top. A diaphragm beneath the capillary bundle moves up and down to pump the air. So although Eridians do take air into the carapace, they don't actually breathe like Earth animals do, since nothing from the atmosphere directly enters the bloodstream.

Sleeping is a crucial part of maintaining an Eridian's function, but it's very different to the way humans sleep. Eridians will enter into a period of dormancy at irregular intervals, and the length of dormancy is highly variable. During the dormant state, the brain is semi-active, but only to maintain basic life support. Eridians don't dream and they have no sense of the passage of time in this state. During sleep, the hot circulatory system cools to the same temperature as the ambient, which means that specialized worker cells are then able to enter the HCS to repair anything that's required, as it's no longer too hot for them to survive. However, because the systems cool to the same temperature, the Eridian is literally paralyzed while sleeping, as there's no temperature gradient for the movement of their muscles. The hearts also stop beating and blood circulation completely stops. Most worker cells, other than the specialized ones that service the HCS, retreat into the colony to conserve energy. So, sleep is how an Eridian maintains its hot circulatory system. But sleep is also highly significant to Eridians for another reason. Because of the paralyzed, helpless state they enter during this time. This drove ancient Eridians to develop social instincts. The desire to have others around to watch over and protect the helpless dormant body led to Eridian sociality and ultimately the emergence of their culture.

Eridians also need to feed to power their enclosed biological colony and obtain the necessary elements and compounds to repair and grow their inorganic components. In the book, feeding is described as a very personal, quite disgusting, and socially frowned-upon process in Eridian culture, so it's not done in public. Nevertheless, we do get a rather graphic description of it in the novel, which differs from its portrayal in the film, in which it's a much more socially acceptable biological process, but still rather disgusting. Feeding only occurs about once a week, and it begins with the Eridian manually tearing their food up into small pieces with their claws, sorting out the useful parts of their meal and grinding it together. The Eridian mouth is on the underside of the carapace. Before consuming their processed food, the carapace cracks open along a weakened plane on the underside, from which the waste of their previous meal is ejected. They then flip onto their backs, place their new meal into their cracked carapace, and quickly seal up the opening with worker cells able to completely heal over the crack within just a few hours. This quick resealing is needed to minimize exposure of the internal worker cells to the highly aggressive pathogens that inhabit Erid's atmosphere. The first stage of internal digestion is also a sterilization process, as the food is first brought to the temperature of the hot circulatory system to kill any harmful organisms. Eridians don't have anything like an intestinal tract, and after the food is allowed to cool back down again, often during a period of dormancy (aka a food coma), worker cells collect whatever is needed and leave the rest.

Since Eridians evolved in a world without light and rely on sound to image their surroundings, it makes sense that they also communicate using sound. Five sets of gas bladders within their bodies comprise their communication system. Gas is pushed from one bladder to another through finely controlled apertures. The five sets of vocal systems allow Eridians to speak in chords, and they are able to perfectly distinguish between different pitches, meaning that in general, any given Eridian language is about six times as information-dense as human speech. Eridian language is described as sounding a bit like whale song, though not quite, because there are several songs at once. So like "whale chords," as Grace describes it.

The differences between Eridian and human intelligence are interesting as well. For example, humans have better spatial memory than an Eridian because Eridians can sense it in all directions at all times, meaning they don't have to remember what a particular room is like. Conversely, Eridians are far better multitaskers because their omnidirectional nature makes them well-suited to performing multiple objectives with different limbs simultaneously. This does mean they can find it hard to focus on a single task. However, for a human, focusing is generally easier than multitasking. Eridians are extremely good at memorizing things precisely, too, which means they're far better than most humans at a lot of math simply because they've memorized the answers and stored them in their crystalline brains. So, Eridian biology is an absolutely extraordinary example of creative speculative alien life. And it's seriously impressive just how much detail and thought went into these organisms.

But how could such an amazing alien species evolve? Well, to explore the speculative evolution of the Eridians, it's important to first understand how they reproduce. Andy Weir also details this in his notes on the species. Eridians are hermaphrodites, as are most organisms on Erid. The two parents will both lay clutches of eggs together and cover them over with soil or another medium while they gestate. The eggs contain nutrients and raw materials, as well as worker cells representing each of the thousands of specialized subtypes contained within the adult Eridians. The eggs have permeable membranes that merge with one another, allowing the worker cells to flow between the eggs laid by each parent. Using the materials in the egg, the worker cells from both parents rapidly reproduce, and an arms race occurs between each subtype to try to overwhelm and essentially win dominance over the subtype equivalent from the other egg until only one lineage is left for each worker cell type. This is how Eridians exchange genetic information and introduce variation to their offspring. Functionally, the worker cells act like the genes from each parent, and the offspring ends up with a combination of cells from both parents. After this battle for dominance over the egg is finished, this new combination of worker cells begins constructing the colony and enclosed biosphere and builds up the inorganic body around it. And that's how a baby Eridian is made.

The evolution of multicellular life on Erid must therefore have been very different to how it evolved on Earth. In a way, Eridian life never evolved true multicellularity in the sense that these are not organisms that all originate from a single cell that contains all the genetic information and divides over and over again to form a fully integrated multicellular organism. They are actually made up of thousands of distinct lineages of single-celled organisms that formed colonies and began maintaining larger bodies to house these colonies and eventually created a thinking brain to coordinate them, thereby increasing their chances of survival. So if Erid was like Earth, seeded by an ancient organism from Adrien that evolved a way to travel between star systems, then this could explain its diverse atmospheric ecology. It would seem that unlike on Earth, where the life we descend from originated in water down on the surface, Eridian life probably began in the upper atmosphere where light actually reached. I'm speculating that an organism from Adrien reached Erid and underwent an adaptive radiation in the extreme reaches of its dense atmosphere. Here, some descendants of this Adrien cedar began to photosynthesize, and then others evolved to predate on the photosynthesizers. Eventually, over billions of years, some lineages of these different single-celled organisms would have begun to work symbiotically. There must have been a tipping point in the evolution of Eridian life when accumulations of single-celled lineages started to coordinate at a level where they began to construct external exoskeletons from the available elements and compounds in the atmosphere and grow crystalline structures that could process and store information to better instruct the overall function of the colony. It's stated that the predominant metal in an Eridian body is iron, and silicates are also very common. So since Erid is also said to be tectonically and volcanically active, perhaps these materials were made available to the early symbiotic lineages thanks to millions of years of eruptions, ejecting large quantities of them into the atmosphere where the density of Erid's atmosphere would mean that the particles lingered here for long spans of time and hence could be utilized by the colonial cells. This watershed moment in Eridian evolution when colonies began encasing themselves in protective inorganic exoskeletons was likely the result of intense selection pressures imposed by highly aggressive predatory single-celled organisms thriving in the atmosphere. There's a whole division of Eridian life that remained single-celled throughout the evolution of the biosphere, much as on our planet, and these would give rise to numerous highly aggressive pathogens that affected the multicellular colonies, triggering an evolutionary arms race.

It's interesting to think that presumably the earliest ancestors of the modern Eridians must have been flying organisms. Considering that Eridian biology utilizes sets of gas bladders for communication, perhaps these ancient flying Eridian aliens initially propelled their encased colonies around in the air using gases. The thick atmosphere of Erid means that moving through the air on this world would in effect be a lot more like swimming in water on Earth rather than flying through our comparatively thin atmosphere. In my hypothetical Eridian evolution, then I'm imagining that the earliest multicellular exoskeleton-clad colonial organisms would have moved about with jet propulsion, perhaps intaking air through vents not too dissimilar from those on the cap segments of later Eridians and shooting it out to locomote inorganic materials and also radially symmetrical. The omnidirectional nature of modern Eridians likely had its roots in these ancient jet-propelled creatures, which may have had five outlet vents to quickly maneuver and change direction in the thick atmosphere of prehistoric Erid. At some point, the ancestors of the Eridians must have started to become facultative flyers rather than obligate ones, meaning that they could still jet through the air but began coming down to the surface more and more instead of only flying. These facultative flyers may have had small, basic jointed limbs that acted as landing struts on the dark surface and could be used to quickly launch the individual back into the air if a threat was detected. Presumably, surface-dwelling habits arose multiple times across many branches of Eridian life at various points in the world's prehistory. At some stage, many millions of years ago, a land-based ecology became established thanks to flying organisms that began conveying energy to the ground as they spent more time in this new environment, creating that boundary life layer we mentioned before. The rich trophic levels of the skies were starting to expand down to the dark depths of Erid's surface, building an increasingly more complex food web across the planet. Eventually, the Eridian ancestors would have fully committed to life on the dark surface. A whole radiation of fully terrestrial organisms presumably would have sprung up when these lineages began exploiting all the newly available niches. The lineage that would one day give rise to the Eridians were perhaps initially rather small in overall body size and with proportionately longer limbs that enabled them to scamper quickly over the terrain, sensing for larger predators and for meals. This lineage was presumably quite innately social from these early days. The commitment to life on the ground opens the way for these organisms to utilize liquid mercury as a medium for circulating worker cells around the body and for steam-powering their muscles. Liquid mercury circulating around the body would have proved a detriment to their flying ancestors, as the added weight would have limited their maneuverability in the air. So presumably their bodies were circulating with something else. But the advent of dual circulatory systems flowing with liquid mercury at different temperatures would have meant that even these early Eridians were becoming paralyzed and helpless at irregular sleeping times when the hot circulatory system needed servicing. So I think anyway, they had a long ancestry of being social creatures as they watched over each other sleeping. This innate need to be social organisms, coupled with the co-opting of their ancestral jet propulsion systems into gas bladders for complex communication, would therefore have made these creatures formidable predators of the terrestrial ecosystems, as they were capable of coordinating hunts and safeguarding their accumulated food. This in turn would have driven the development of numerous cultures across different regions of Erid's surface as their populations migrated across the world. Perhaps, as with humans, numerous species of almost modern Eridians once coexisted across different parts of the world, and the complex ebb and flow of interbreeding populations contributed to the eventual emergence of the modern-day Eridian as a united, technologically advanced species. In fact, interbreeding between ancient Eridian populations was probably a far more interesting and flexible aspect of their history than even human evolution, since I assume genetic exchanges between Eridian species would have involved the transfer of entire specialized worker cell lineages rather than the direct exchange of DNA sequences. Lots of very interesting ideas about how that could have worked and how it could have influenced modern Eridians.

Ultimately, modern Eridian culture was established. And though these extraordinary people had never even seen the stars, they nevertheless created a space program made possible by the amazing space elevator they constructed. We learn a great deal about Eridian culture throughout the novel. And one especially interesting and biological part of this culture is their ability to "thrum." This is described as like a song and discussion in one. When many Eridians gather and all vocalize at the same time, the thrum leads to decisions being made, and the thrum is much smarter than any single Eridian involved in it. They turn into a larger hive mind for a while. So, in a way, it's like these colonial organisms join an even larger colony and function as a superorganism temporarily. What a sound that must be. As bizarre as the anatomy of an Eridian might seem to us, being largely inorganic, this is the perfect example of a super creative hypothetical alien life form. Eridians are so different in their basic biology, but it's all the result of the selective pressures on their very different home world, which is what a realistic alien life form would be like.

Well, I hope you enjoyed this exploration of the speculative evolution and biology of some truly imaginative aliens. I hope you'll read or listen to the book yourself if you enjoyed this video, and hopefully you'll see the film as well. I'd also like to thank Andy Weir for creating such a phenomenal, hopeful story of science and friendship and for making such interesting hypothetical species. They've been a lot of fun to explore. And thank you also for sending me your notes on Eridian biology. They were immensely useful for this video. In addition, thank you so much to the two artists I commissioned to produce the fantastic artwork you've been seeing throughout the video, Cororax Lara and Wyatt Andrews Workshop. I will link to both of their Instagram pages below. Go check them out. I hope you've all enjoyed watching and do let me know if there are any other fictional alien species you'd like me to explain the speculative evolution of in the future.