Hey guys, Nick here, and welcome back to another Mini Update.
Today we’ve got a longer update as it’s a post from one of our recent discussions on the Microbe Stage that we felt would be interesting to share. Instead of talking about any technical aspects of the development this time, we’re instead going to focus on some of the science behind the game’s design.
What’s the topic? Planetary evolution. Particularly, the evolution of the climate and geochemical cycles of a planet that lead to the origins of life, also known as abiogenesis. Currently the player spawns into a static world, with fixed biomes that do not change. However, the early history of the planet should be a turbulent world experiencing many drastic environmental changes. The transition of planets from molten masses of rock during their inception to worlds with oceans and continents and the first life involves several major natural phenomena. These natural phenomena should shape the environment of the Microbe Stage, affecting biomes, compounds, sunlight, temperature, and many other factors.
First we need to understand how an Earth-like planet would evolve in its early history in accordance with how Earth evolved (Let’s call it the Model Planet). This will be the default starting planet for players in Thrive for now. Then, using the blueprint of the Model Planet, we can later extrapolate to alternate courses our planet could have taken that would have also yielded life, and then even more exotic planets and how life could begin there.
Alright so, how will the planet evolve leading up to and during the Microbe Stage?
Well, glad you asked because I’ve been doing a Belgium-load of reading on this topic these past few days so I want to show something for it. Bear in mind this is my understanding of science’s current understanding of how it happened.
The Model Planet starts as a collection of rocks, minerals, and gases that have freshly aggregated into a planet in its star’s habitable zone. When it first forms, the planet is loaded with heat flow and unstable geothermal gradients remaining from the energy of the collisions that led to its creation. This leads to severe volcanic activity and an extremely hot surface to the planet, with literal oceans of magma 200km deep. There is also intense bombardment from meteors and comets. A scene from this period would look like hell on Earth, seriously!
This period gives us the starting compounds on the planet (note that “compound” is Thrive’s general term for all resources, elements, and substances in the game). As the planet forms, the denser compounds sink to the core and the lighter compounds emerge on the surface. This leads to nickel and iron forming the core, magnesium silicates forming the mantle, and silicates of aluminum, sodium, calcium, and potassium forming the crust.
Many gasses get trapped underneath the rocky surface when the planet forms. Fissures and vents in the surface lead to “outgassing” of gasses being released into the atmosphere from these subterranean pockets. From this we get nitrogen, hydrogen, ammonia (would get destroyed by the sun’s UV radiation though), methane, hydrogen sulfide, carbon monoxide, carbon dioxide, and water vapour. From among these, carbon dioxide is the majority and leads to the creation of a carbon dioxide dense atmosphere. Water vapour stays as a gas at this point because the surface is so hot liquid water immediately evaporates. Keep an eye on these compounds, they will come up again later.
Volcanoes spew forth many of these compounds also, as well as gaseous compounds containing sulfur and phosphorous and nitrogen. Meteors and comets and small planetoids also introduce many of these gaseous compounds. Some of these foreign cosmic objects also carry some strange other compounds on them: simple sugars (for our purposes, glucose), amino acids, nucleic acids, fatty acids, and other building blocks of life. Most of these get destroyed by the harsh conditions of the planet’s surface, but later some will survive when there is liquid water to catch and protect them.
Oceans of Water
At some point very early in the history of the Model Planet, the growing carbon dioxide atmosphere does an amazing thing. It gets so dense, that it actually puts pressure onto the water vapour in the air and liquefies it. Even though the surface is still very hot (~230°C), the pressure from the heavy carbon dioxide atmosphere leads to the creation of the first liquid water oceans (above 27 atmospheres water is still liquid). Many of the earlier mentioned atmospheric gasses dissolve into the oceans as they form. Areas where vents in the rocky surface were submerged by these new oceans became underwater (or hydrothermal) vents. Alongside the gasses that these vents eject, they also release plenty of iron and manganese which dissolves into the oceans. Keep an eye on these vents.
The Model Planet cools as the years go by. The seas of magma dwindle and disappear and are replaced by seas of water, and the volcanism gets less intense. The rate of meteor and comet impacts also reduces greatly. Interesting fact, the sun is younger and thus less bright, which would normally mean the planet would eventually cool until it is covered by ice. However, the planet does not reach this point because the carbon dioxide atmosphere performs a greenhouse effect and traps the heat inside the atmosphere.
Building Blocks of Life
At this point the Model Planet has continents, oceans, and an atmosphere with lots of carbon dioxide. It’s not long before the building blocks of life start being formed.
Recall that the Model Planet has slowly been accumulating methane and ammonia and hydrogen sulfide in its atmosphere and oceans. It turns out that lightning strikes and radiation from the sun converts these compounds into amino acids, nucleic acids, fatty acids, sugars and other building blocks of life. So why did this not matter before? Because before the harsh conditions of the planet, including the very radiation that created them, would just destroy them immediately after they were created. However, the depths of the ocean now provide a place where these organic molecules can build up in safety. Now when radiation or lightning hits methane and ammonia on the ocean’s surface, a fraction of the amino acids and others that are formed are carried by swells of water to the deep ocean where they stay intact and accumulate. High pressures at the bottom of the ocean also leads to the creation of some organic molecules (like fatty acids).
Another site of organic molecule synthesis are the hydrothermal vents! It turns out that high heat is another catalyst for the formation of these compounds, and so there we start to see amino acids and the rest getting created as well.
Interesting side note, many of these simple organic molecules, when they interact with hot solid surfaces like grains of dust or sand, will then polymerize and form more complex structures (like full on proteins). A moon orbiting the planet would also introduce high tides to the ocean, causing these molecules to mix around a lot more and help speed up their creation.
The Dawn of Life
These conditions are what we refer to as the primordial soup. Some estimates say that the model planet’s oceans would have about 1% concentration or more of organic molecules, which is the same concentration as chicken broth. We’ve got sugars, amino acids, fatty acids, and nucleic acids, the perfect starting materials for life to form.
And form it does! It’s at this point that the first cells start to appear in the deep ocean and around the hydrothermal vents of the model planet (first protocells and then cells). The first cells are prokaryotes, and soon after we get eukaryotes. Thrive currently skips the prokaryote stage so that the player gets to start with a nucleus and endoplasmic reticulum and golgi apparatus (though we will later return and add that stage in). These initial cells are swimming in a motherlode of free floating nutrients that is being naturally replenished. Alongside the glucose and amino acids and fatty acids, there are also high amounts of dissolved iron, hydrogen sulfide, and manganese in the ocean that have been released by the hydrothermal vents. Cells produce energy by metabolizing the glucose, iron, manganese, and hydrogen sulfide, meaning the initial life are predominantly chemosynthesizers (some also possibly evolve to scavenge or kill other cells). Because of this, hydrothermal vents are the hub of cellular activity, especially because the original cell species lack pigments to protect against the UV radiation of the surface, as well as the tolerance for low pressures. Evolution takes place and these cells diversify into a variety of species.
Photosynthesis and Oxygenation
Then, a big thing happens. The first species evolves photosynthesis. Instead of metabolizing the iron or manganese or hydrogen sulfide dissolved in the ocean, they metabolize the carbon dioxide, they use light to do it, and they produce oxygen. Up until this point, oxygen was VERY rare in the model planet. But these species gradually grow over a long time, leading to huge amounts of oxygen ultimately being produced.
Oxygen is a very reactive compound, so it immediately has a domino effect on the entire planet’s environment. It begins by reacting with all the sulfur and iron dissolved in the oceans to form sulfur oxides (solid sulfur) and iron oxides (iron ore) which sink to the seafloor as solids. Carbon in the atmosphere also is oxidized as oxygen starts to replace carbon as a major gas in the atmosphere. All of the unicellular species that have evolved thus far are anoxic, meaning they don’t use oxygen to survive, and that oxygen is toxic to them. Many used fermentation to produce energy, and it can only take place in an anaerobic environment. This leads to a huge extinction event, in which most species die off except for the ones that adapt to the new oxygen filled environment. The only anoxic species that survive are the ones that reside in remote areas that do not fill with oxygen, such as deep caves.
Another interesting side effect of the oxygenation of the planet? Once oxygen has reacted with everything it could react with, it starts to build up as free gaseous oxygen in the atmosphere. This accumulation eventually leads to the creation of an ozone (O3) layer outside the atmosphere by UV radiation hitting atmospheric oxygen. Ozone blocks much of the harmful radiation of the sun, but also the same radiation that was synthesizing all the amino acids and glucose and other organic molecules. Yes lightning and heat and high pressure still exist, and some radiation is still getting through, but also a lot of the methane and hydrogen sulfide and other precursors to these organic molecules have been converted to different molecules by the oxygen. Additionally, the anoxic cells that produced methane as waste have been killed off. Over time, the free floating clouds of glucose and amino acids and these other building blocks of life disappear, and species now need to evolve to use sunlight and carbon dioxide to produce these nutrients themselves, or scavenge and kill other cells and steal their nutrients. It’s also possible that the oxygen that filled up the atmosphere was less effective at trapping heat on the planet as a greenhouse gas than methane and carbon dioxide, leading to several “Snowball Earth” periods.
On the plus side, the ocean surface has much less severe levels of UV radiation. Evolving some pigments (and low pressure tolerance) is all that’s necessary to start migrating upwards, where there is no competition for sunlight. Additionally, oxygen allows a new chemical process to take place, aerobic respiration. This new process is far more efficient at producing energy than earlier methods (such as the mitochondrion organelle, which is a powerhouse of energy to cells that evolve it), and reciprocates many of its ingredients with the photosynthesis carried out by plant cells. This huge new source of energy allows cells to start developing and becoming more complex, and ultimately bonding together to form multicellular colonies.
Eventually, these multicellular colonies began to have their cells work in concert and specialize their cells for particular functions, forming full-fledged multicellular organisms, and the rest is history…