Prelude: Life in an Infinite Universe
If the Universe is truly infinite, then anything that is possible, becomes probable.
The multiverse - String Theory, Bubble Universes, the Many-Worlds Interpretation, and even the simulation hypothesis all conceptualise a multiverse using vastly different mechanisms. Science Fiction frequently portrays the multiverse as a navigable construct, with parallel-consciousnesses seemingly linked through higher dimensions. Whilst this might bring comfort when visualising the sheer scale of existence, it is beyond the scope of this project, Instead, only one assumption is made here - that, like Planets, like Stars, like Galaxies, the universe is not special. Using this paradigm, parallel Earth's can be explained using probability alone. Importantly, our current observations suggest that an infinite universe is actually somewhat likely.
Part I: Origin of the Universe:
What preceded the big bang? What lies outside the observable universe? The answer to these questions is actually quite simple; possibly even underwhelming - we don’t know. And we may never know. It’s beyond our means of testing. Not very satisfying is it?
So let’s dig a little deeper.
The universe came into being some 13.8 billion years ago, in an event we call “The Big Bang”. Most of us are familiar with this story. Given that the speed of light is constant, you would be forgiven for assuming that the size of the universe must be close to this figure. You'd be wrong. But you'd be forgiven. Measuring the Doppler effect of distant galaxies revealed something interesting - they seemed to be moving away. Not only that. The further away they are, the faster they seemed to be moving away from us. This is known as Hubble's Law. And it's a large factor in dating the origin of the universe.
So how big is the universe?
The Cosmic Microwave Background is the oldest light we can see, dated at around 400,000 after the Big Bang, close to the 13.8 billion year old mark. While the light itself is close to 14 billion years old, factoring in the uneven expansion of the universe reveals it's current distance to be ~46 Light Years away (giving the observable universe a total diameter of ~92 billion light years). This appears to be the case regardless of which direction you’re looking in. No matter where you point your telescope, the universe appears mostly uniform. So why does Earth appear to be in the centre of the Universe? This all comes back to the rate of expansion. Because the rate of expansion increases with distance, there comes a point where the space between the most distant galaxies increases at a faster rate than light can travel. This means that no matter how long we wait; and no matter how advanced our tools become, we will never be able to detect light sources beyond this distance. If we could tunnel through higher dimensions to warp to the edge of the observable universe, we would probably find that the universe would look mostly the same, retaining a near-uniform distribution in all directions. The size of our observable universe then is only a mere slice of its actual size. Recent studies suggest the actual universe may be at least 250x larger than what we can observe. But many higher estimates have also been made. In fact, general relativity allows the universe to take three forms: Positively curved, Negatively curved, or flat. This is because matter warps spacetime. A negatively curved universe would be finite, like a sphere. Travelling in a straight line would eventually loop you back to where you started. Luckily then, our observations so far indicate that the universe is flat, at least as far as we can tell. Which means the universe would not be limited by its own geometry and could, theoretically, expand indefinitely.
There are an estimated 100-200 billion galaxies in the universe and 100 million-100 billion stars in the average galaxy - equating to 10-20,000 trillion stars in the universe. Other estimates even place the total number of stars in the billions of trillions. But this only includes stars that lie in our current snapshot of the universe. The sun, for instance, is a third generation star. The formation of new stars isn’t estimated to end until the heat-death of the universe, some 1-100 trillion years ahead. If we took 10 billion years as the average lifespan for a star, that theoretically allows for 100-10,000 generations of stars on average. Since the rate of star formation would slow down as entropy increases, we will use the lower estimate.
20 trillion stars in the observable universe x 250 (size of actual universe) x 100 generations = 500,000 trillion (5*10^17) stars over the universe’s existence. Using the oft-cited “200 billion trillion” estimate, yields a total of 25 septillion (2.5*10^25), or 5 million billion trillion stars That’s a lot of chances for an Earth-like planet to form.
Part II: Origins of life:
Unfortunately for us, we only know of one planet to give rise to life, and that's Earth. It may be possible for life, or life-like forms to emerge using completely different biochemistry. In fact hundreds of amino acids have been identified as have many alternative Nucleic Acids, or XNAs, such as Threose Nucleic Acid. Some of these have even been shown to undergo Darwinian evolution. Whilst this shines a light on the potential of exotic life forms, the viability of these biochemistries for complex life remains unverified. Speculation on how these life forms may arise and evolve and what properties their home planet may require remains completely hypothetical. Therefore, any extrapolation is ultimate futile.
Fortunately for us, however, the origin of life on Earth is a topic pursued ferociously by researchers, and our understanding of biology has been blown wide open with the discovery of genetics. Dating the origin of life on Earth is tricky. But not impossible. Stromatolites, fossil remnants of early microbial life, have been found across the globe with several clocking in at a whopping 3.5 billion years old. The number of rock samples older than this are few and far between, and yet, chemical signatures of potentially biological origin have been identified in rocks as old as 3.8-4.1 billion years old. The Nuvvuagittuq Greenstone Belt contains rocks dated to 3.7 billion years ago, or more controversially 4.3 billion years ago, depending on who you ask. Recently, Microfossils of early bacteria have been found in these rocks, analogous to the structures and signatures of microbes that form around hydrothermal vents. Considering Earth formed just 4.5 billion years ago, and the oceans 100 million years after, it is reasonably safe to infer that it did not take life long to become widespread on the planet. To quote Andrew Knoll - "We run out of rocks to examine before we run out of evidence for life".
Prerequisites of life:
While competing theories of life's origins are still around, the hydrothermal vent theory is the most substantiated. Lab experiments have shown that the most foundational components of life can be synthesised fairly easily from inorganic matter under the conditions of a primitive earth. The warm, alkaline, and mineral-rich updrafts from hydrothermal vents provide all the necessary components, and even convenient mechanisms for life to arise. Lab results, field research and the fossil record all indicate that life, or at least the precursor to it, must have appeared fairly early in the planet's history. One may even conclude that wherever a tectonically active oceanic planet exists, life would almost certainly follow. The only issue here is that plate tectonics seem to be quite rare.
For the most part plate tectonics have been explained by the convection currents within Earth's mantle. However, this has so far been unable to produce meaningful predictions about the precise movements of plates. Something seems to be missing. Or at least, that's what was suggested by one recent study. The somewhat controversial model found that the orbital harmonics between the Earth, Moon and Sun, are sufficient to periodically shift the location of the Earth-Moon barycenter within the Earth's mantle. The study suggests that this gravitational influence is likely the predominant factor in driving plate tectonics. Interestingly, this model also appears to corroborate the long-held belief that plate tectonics did not start on Earth until 3-4 billion years or so, demonstrating that this period coincides with the Earth-Moon barycenter migrating outside of the Earth's core. Whilst these findings are yet to stand the test of time, they should be taken into consideration when exploring alien life.
Part III: The Search for Life:
While life itself seems to have started very early in Earth's history, Eukaryotes took over 2 billion years to evolve [2.1-1.7Gya], with multicellular life appearing a few hundred million years later [~1.6Gya]. Bacteria today is estimated to have a total biomass of 70 gigatonnes. A single bacterium has a typical mass of just 1 picogram. This equates to 7*10^28 individual bacteria. A reproduction rate of 30 minutes results in approximately 3.5*10^13 generations of bacteria or 2.45*10^42 individual organisms over 2 billion years. Where life can, life tends to do. So the apparent difficulty in the evolution of multicellular life must be assumed to be true for life on other planets. What does this tell us then? Quite a lot, actually. It tells us that while life itself may be common in the universe, advanced life would almost certainly only be possible around stars smaller than ~1.5 solar masses. Why? Because any star larger than this could not sustain itself for more than 3.5 billion years. One potential counter to this is that the rise of oxygen in the atmosphere from cyanobacteria is thought to have paved the way for complex life to exist, and that oxygen may depend on Earth's day length. A larger moon might have slowed Earth's rotation much sooner, triggering the great oxygenation event much sooner in Earth's history.
Additionally if we look at earth we see, logically, photosynthesis is the primary source of energy. Wavelengths below 380nm are when ionization becomes apparent, which would be problematic for earth-like organic matter. Thus, wavelengths shorter than this are unlikely to be used for photosynthesis. One recent study computed the photosynthetically active radiation (PAR), and associated energies, against stellar habitability and found that stars below 2600K (or 0.1 Solar masses) would not provide enough energy to sustain oxygenic photosynthesis. As of July 2022, there are 5100 known exoplanets in 3,779 star systems, with 826 systems known to harbor more than one planet (~22%). 59 known planets lie within their habitable zones (1.5%). But only a single identified exoplanet, Kepler-442b, was found to meet the parameters to sustain Oxygenic Photosynthesis. As it stands our sample is simply too small to make useful inferences. But we could tentatively call this a ratio of 2/5000 (or 0.04%) if we count Earth.
All-in-all, we can conclude that for Earth-like life to exist, we probably need a planet within a suitable PAR range, that has a large moon, and orbits a stable star. Since singular planets would be notably vulnerable, we can also assume it lies within a multi-planet system. N-body simulations of early solar systems resulted in 1 in 12 terrestrial planets hosting a large moon (with a given range between 1 in 4 to 1 in 45). If we take our earlier estimates for the number of stars systems over the span of the universe then multiply it together we get: (5*10^17) or (2.5*10^25) [systems]*0.0004 [PAR]*0.02 [large moon]*0.02 [multi-planet] = 80 billion to 4 million billion planets that could host earth-like complex life, or around 3.2 million planets among the currently visible light. There's reason to suspect that Earth's chemical composition plays some role in the suitability for life on Earth but we do not have the data samples to factor this in. But what I can say is this: Even if earth's composition was one-in-a-million, you would still expect to find at least 80,000 extremely earth-like planets, with lifespans ranging from 5 billion to upwards of 20 billion years.
Part IV: Convergence:
Why are aliens always depicted as animal analogues? And why are the plants on their homeworld always, well, plants?
Now that we've established parameters for alien life, lets explore the implications of them. Again, our sample size of one means that we can only infer rules from Earth. This does not rule out the possibility of life existing outside of these parameters.
First and foremost - for life as we know it to exist, we must assume the planet has a chemical composition sufficient to provide the necessary component parts. Second to this, for life as we know it to evolve, we need liquid oceans. If hydrothermal vents were the first source of nutrients, chemotrophy would be the basal condition. The next most logical energy source is sunlight. Phototrophy, incidentally, also appeared very early in Earth's history - some 3.5 billion years ago. Suddenly (okay maybe not that suddenly), our primitive life is no longer bound to the vents for sustenance... And we now have concentrated clusters of organic matter. The first organism to evolve that can feed on these life forms would no longer need to synthesise it's own food. Now, we have our earliest detritivores, and potentially, the ancestors of our animal or amorphea-analogues.
Like all natural processes, Evolution is ultimately driven by math. This is why ancient stromatolites have distinct spacing between their structures, why sauropods have long necks, and why most fish aren't cube-shaped. If life as we know it requires oceans to evolve, then life on other planets will be subject to the same laws of hydrodynamics.
Air flow study of various shapes
Body plans of various marine organisms (and a submarine)
Evolution is driven by self-replication. The better things are at reproducing, the better they are at, err... reproducing. It's not a profound concept. Mutations that decrease genetic fitness tend to get weeded out fairly quickly. Mutations with direct benefits, likewise tend to be selected for. For example: the CCR5-delta 32 mutation in humans provides immunity to the HIV virus. Most mutations will have no tangible benefits on an individual's reproductive success - but these mutations start to become apparent when they proliferate throughout populations. This can lead to what is known as a phenotype. Over generations, and over successive mutations, these phenotypes can become pronounced enough for natural selection to become a factor. As a mutation spreads through the population with subsequent generations, even the slightest benefits would still offer a statistical advantage. When modelled over a large enough population, these marginal advantages, these statistics, start to add up. At the extremes, a miniscule number of fish that would have died from, say, lack of resources or predation may just have the edge it needs to survive. This would lead to a marginal increase of the frequency of this genotype within the total gene pool. Which makes the chance of this gene passing on more likely... Each iteration would increase the survivorship bias.
Likewise, A fish with a form that produces marginally less drag might see no reproductive benefits but a distant ancestor might inherit another phenotype that has a marginally more efficient fin shape, etc. As the effects of individual variations compound, so to does the selection pressure. Eventually this new phenotype might allow these fish to swim a little faster, or it might allow them to grow a little larger from the energy they have conserved. This would then increase the selection pressure on their prey, which in turn, increases the selection pressure on the fish, and so forth. Meanwhile phenotypes that led to a previously average swimming efficiency start to become detrimental as the baseline shifts, causing them to become selected against. This relationship would be much more pronounced in periods of stiff competition, such as during times of resource scarcity, or during evolutionary radiation events. Radiation events like the Cambrian Explosion.
Going one step further, the advantages an internal skeleton offers are enormous. Vertebrates have dominated the highest trophic levels for some 400 million years, and quickly became the dominant group on both land and in the skies. Having a solid anchor point for muscles allows them to contract much more effectively. Having a stiff backbone allows the front end of the body to remain stable, confining the muscle movement to the rear end. This minimises drag and maximises thrust. Eels, which use simple unguilliform locomotion have great mobility but fare much worse in open waters than fish with more derived swimming methods. Being an effective hunter in open waters would quickly broaden an organisms geographic range compared to bottom dwellers or suspension feeders.
Any free-swimming organism that is not neutrally buoyant would be required to generate its own thrust to stay afloat. Because of this, we can assume that almost all marine life will have a near-neutral buoyancy. If we look at animals on Earth, we find this holds true. Fish and some cephalopods evolved swim bladders, sharks have oil-filled livers, some squid have stores of ammonium to compensate for their heavier tissues while nautiloids have gas chambers to counteract their dense shells. The interesting thing about liquids is that they are functionally incompressible. Water has a density of around 1g/cm3 regardless of the ambient temperature, pressure, or surface gravity. These rules apply on any oceanic planet. This makes estimating the mass of any water-based life pretty simple.
What about terrestrial environments?
Since water is so fundamental to metabolisms, cells across all phyla are 60-80% water. Any form of life that evolves on an ocean world would probably also use water, the most abundant chemical in its environment, in its biochemistry. So soft tissues, again, are going to be fairly close to the density of water. Life on lower gravity worlds might have increased pneumaticity, or reduced bones and muscle tissues for their size but the density of the tissues themselves would remain the same. This is extremely useful for determining mass. Mass can in turn be used to estimate energetics. A sauropod on a low-gravity world would expend less energy with each footstep than a sauropod on Earth, meaning that energy could be put to growth. Likewise, it's neck would experience proportionally less load, allowing it to grow even longer. Having established numbers for mass, gravity and (to a lesser extent) atmospheric conditions allows you to make all kinds of inferences about the animal in question. The math itself can be used to define the creatures morphology. Even ball-park estimates would be useful to visualising organisms. Contrastingly, even if movement was near-effortless, it would still need to eat to gain mass. So the limiting factor would not be weight, but how much it can simply eat over the course of a day. Similarly on Earth, the bio-mechanical limit on skeletons is far greater than what any animal can achieve, because there's only so much an animal can eat. Blue whales for example are limited by dive frequency and prey density, which effectively caps their maximum size at around 33m.
Part V: To Infinity:
This is the part where I tell you to disregard everything you've just read. If the universe is truly infinite, and matter goes on forever, then none of the above really matters. Heck, if the universe is truly infinite then there's an infinite number of you floating throughout space, formed by sheer coincidence of the random arrangement of particles. Such a life would be rather disappointing though. Instead, consider some infinities are bigger than others. And existence is nothing more than a manifestation of probability. The size of our universe means that simple life is probably extremely common. Complex life would then be somewhat rare. Over the billions of years this complex life would exist, something analogous to an animal would be extremely likely. Something functionally indistinguishable from a jellyfish might crop of quite frequently on alien oceans, as would worm-like organisms. Likewise, Arthropod-like and Fish-like forms would be rarer but would not be uncommon among earth-like alien oceans. The laws of probability dictate that as the number of earth-like planets increases, so to does the chance of things evolving twice. If big bangs are as special as supernovae, these probabilities begin to stack up quite rapidly.
In a closed system, all things tend toward disorder. That's arguably the most fundamental truth of reality. Entropy dictates that without external energy, particles tend towards the most evenly distributed arrangements. Open a sealed can of tuna and the particles carrying the smell will immediately begin to diffuse around the room. Why does life appear to increase in complexity, in order, over time then? Schrödinger was the first to postulate that life may exist because of entropy - as two opposing substances, such as the Earth's hot mantle and the cool surface oceans, fight to reach equilibrium thermodynamic "free" energy is provided. In essence: life increases in order at the expense of its environment. Earth went from pre-life, to life, to multicellular life, and eventually to more and more complex nervous systems that gave rise to a multitude of intelligent species. While "sapient" species like humans are sometimes considered to be a fluke of nature, if we zoom out we find an increasing trend in complexity - from the complex brains of mammals, birds and cephalopods, to eusociality in insects. Humans in turn built increasingly ordered tools. Tools that are used to increase the entropy of our environment. Underground reservoirs of oil, gas and coal that sat untouched for hundreds of millions of years are now being diffused into the atmosphere. Minerals are mined from deep within earth's crust and flung out into space. Even the soils of mars are not free from our service of entropy. If humans had not existed, the increasingly complex brains of Earth's fauna over the next 5 billion years would only increase the chance of such a species arising. With each mass extinction paving the way for more advanced forms to radiate. Now consider that some stars can host habitable planets for up to 30 billion years; 30 billion years of increasing complexity; perhaps human-like intelligence isn't so rare after all.
The takeaway here is that probability is not the same as luck. Life is governed by very precise rules, we know these rules as mathematics. The body plans life on Earth took; the materials we construct our skeletons, hair and skin out of; the emergence of complicated nervous systems may have arisen by chance but the mechanics that led to them are bound by logic. These five sections then, should not be taken as gospel, there are all kinds of nuances and exceptions to these rules. What this is, is simply a visual aid to demonstrate that even in the incomprehensible convergence of infinitesimally small probabilities, there's still a reason why things are the way the are.