The Boring Billion
Hi, Joe here. Life on this planet has been around for at least 3.7 billion years, but for most of that time, it was really, really, really boring. It was all just simple little cells squirming around in water. No cute furry things, no weird bugs, no trees. Just microscopic blobs blobbing.
The Explosion of Interestingness
Then, around half a billion years ago, something amazing happened. Life exploded into all kinds of interestingness. Worms, trilobites, a five-eyed sea creature called Opabinia, and countless other complex lifeforms. Out of nowhere, life on Earth began a new chapter.
If the history of life were a single day and life began at midnight, life didn’t get interesting until around 10:30 PM. And that explosion of interestingness took place in less than an hour on our history of life clock.
What Changed?
But why didn’t life just stay boring? What changed? When we typically think of natural selection, the process by which organisms evolve, change and adapt, we think of this gradual thing that happens over huge time periods. But that’s not what happened here. The ancestors of all life’s complex creatures showed up and the variety of life on Earth exploded like that. And that’s because, right before that explosion, a new kind of chemistry entered the equation. A chemistry that, in excess, could spell certain death, but in the right doses was about to set the stage for life as we know it today.
And the volume of a cell is what determines how much energy it needs to survive. So as cells get bigger, they need more and more energy just to stay alive., Thats where oxygen comes in. Oxygen is a very reactive molecule, and when it reacts with other molecules, it releases a lot of energy. So when oxygen started to fill the atmosphere, it enabled cells to get much more energy than before.
Early Life on Earth
When life first showed up on Earth the air was most likely made of nitrogen, plus some CO2 water, vapor and small amounts of other gases. There was hardly any oxygen, so if you went back in time, 3.7 billion years, you’d immediately suffocate and die. Early life was anaerobic, it didn’t breathe oxygen. Instead, it got all its energy from minerals dissolved into the ocean. Cells just sat there while seawater passed through their membranes absorbing dissolved compounds.
ATP
These compounds interacted with other molecules in the cells and set off chain reactions. Electrons hopping from one molecule to the other molecules joining together or split apart and in the end creating a famous molecule known as ATP. ATP exists in every single cell in every single living thing. It’s life’s universal way of storing energy cells need to function.
Early Lifeforms
But this lazy anaerobic way to get energy only produced enough ATP to take care of basic housekeeping. Early, boring lifeforms didn’t have spare energy they could use to grow bigger. Now, since these early cells were basically nutrient traps, it might seem like they should work like fishing nets, the bigger the net, the bigger the catch. But for cells it actually doesn’t work out that way. As cells get bigger its true that they have more surface area to let in nutrients, but they also have much much more volume. And the volume of a cell is what determines how much energy it needs to survive. So as cells get bigger, they need more and more energy just to stay alive.
Oxygen
That’s where oxygen comes in. Oxygen is a very reactive molecule, and when it reacts with other molecules, it releases a lot of energy. So when oxygen started to fill the atmosphere, it enabled cells to get much more energy than before. Understanding why this happened and what this new chemistry enabled might even tell us something important about what life might look like elsewhere.
The Evolution of Oxygen
Early anaerobic lifeforms were limited in size due to their inability to consume enough to feed their volume. This meant that they were doomed to stay small and boring. Around 2.4 billion years ago, something happened that changed the course of life. Some of these simple cells learned a new trick – photosynthesis. This allowed them to eat light and air to power their cells, and create a waste product – oxygen gas.
Over the next few hundred million years, oxygen levels went from almost nothing to about 10 – the amount of oxygen in our air today. According to one leading theory, oxygen reacted with greenhouse gasses, changing the makeup of the atmosphere so that it stopped trapping as much heat, causing Earth’s temperature to plunge 1.9 billion years ago.
The Benefits of Oxygen
At first, oxygen was catastrophic, but it is also the reason that we are around today. By using oxygen, our cells are able to metabolize food completely differently than those early lifeforms. We get way more ATP for every bit of organic matter that we consume – it is about 10 times more efficient than those original anaerobic ways of creating ATP.
The evolution of oxygen has had a profound effect on the planet and on life as we know it. Without oxygen, we would not be here today.
The Great Oxygenation
The Great Oxygenation is often credited for paving the way for complex life, creating air that could be breathed and enabling life to get complex. But this is not the full story. Without a couple of freak accidents and a crucial give and take between biology and some rocks, we would never have gotten the incredible array of lifeforms living on every inch of this planet.
Earth Freezes Over
After oxygen showed up, Earth froze over. Volcanoes pumped some greenhouse gases back into the atmosphere and things gradually warmed up enough to thaw.
A Crucial Leap of Evolution
Something happened between two microscopic cells that changed everything. At least 1.7 billion years ago, one cell just gulped up another one. The swallowed cell started living inside the other one and evolved into what we call the mitochondrion, an organelle whose main job is to create ATP for its host cell. Up until this point, all cells were prokaryotes, simple bags of stuff without any division of labor going on inside. But suddenly, in this leap of evolution, that changed. Some biologists think evolution would have stalled if this had not happened, because every single animal and plant on Earth is made of eukaryotic cells, complex cells, whose ancestors were born in that moment when one cell swallowed another and took it hostage and squeezed all the energy out of it.
The General Blueprint of Single-Celled Organisms
The general blueprint of single celled organisms has hardly changed in 3 billion years. Prokaryotes didn’t gradually morph into eukaryotes through a bunch of gradual evolutionary steps. This one evolutionary leap was basically a freak accident that changed the entire course of life’s history.
The Freak Event That Happened Twice
This freak event happened twice. Around 1.25 billion years ago, a eukaryotic cell cannibalized another cell. This time it was a photosynthesizing bacterium that got gulped up. Over time, this evolved into a chloroplast, the organelle that does photosynthesis. Today, every single plant has cells like this full of chloroplasts and mitochondria.
Forget Gradual Change
Without sudden huge leaps in evolution, plants and animals just don’t exist. But what’s weird is even though 1.25 billion years ago, life already had the building blocks it needed to create all sorts of complex plants and animals. We didn’t get this immediate burst of interestingness at all. In fact, some biologists call this time period from 1.8 billion years ago to 800 million years ago, the “Boring Billion”. Evolution was happening, but really slowly.
The Oxygen Problem
The problem was, the amount of oxygen bacteria could make likely depended on something else: phosphorus. Cells use phosphorus to make membranes, proteins, and DNA. Basically, no phosphorus, no cells. And at the time, Earth had lots of phosphorus, but it was mostly locked up in the crust, not dissolved in the ocean where the bacteria were.
Prokaryotes
Prokaryotes were starved for phosphorus, so their population likely stalled for hundreds of millions of years. But over time, a series of ice ages created glaciers that scraped up the ground, freeing up some of the locked up phosphorus. Wind and rain wore down the continents, washing a bunch of that phosphorus into the oceans.
Eukaryotic Cells
Life finally had all the ingredients that it needed to get interesting. It had eukaryotic cells, which would become the building blocks of complex organisms. It had photosynthesizing cells pumping out oxygen. And it had enough phosphorus to sustain all of this.
Explosion of Life
So after around 3 billion years of life on Earth, evolution was no longer on hold. Natural selection could do its thing and life exploded into zillions of different forms in just hundreds of millions of years. I know that sounds like a long time, but as major evolutionary changes go, it’s basically overnight.
Getting Complex
So how does life get complex? Well, first, cells start living together and they split up tasks, some digesting the food, others providing structure, some just for mating. Eventually, some of these cell communes evolved into simple animals. And over time, animals kept getting bigger and weirder. Some were shaped like ribbons and tubes. They stopped staying in one place and started taking advantage of those big stores of ATP to seek out other organisms for fuel. Harvesting and hunting brought in more fuel for their bodies, which let them get even bigger. Organisms also got more complex as they evolved new tactics for finding dinner and avoiding becoming dinner.
Oxygen and Life
Natural selection went wild when oxygen was introduced to our atmosphere. This allowed for the growth of heads, tails, armors and spines. While oxygen was beneficial to life, it also came with a few risks. One of the first risks was freezing the planet into an Earth-sized snowball. On the flip side, oxygen also gave us fire, as nothing would burn on Earth without oxygen in the air.
Toxicity of Oxygen
The big catch with oxygen and life is that oxygen is toxic. The air we breathe is around 21% oxygen, but if people are exposed to higher concentrations of oxygen, they can be poisoned. This is because oxygen is super reactive and has two unpaired electrons which want to form chemical bonds. This process of oxygen stealing electrons from molecules is known as oxidizing and is how we get rust. When oxygen steals electrons from molecules in our bodies, it creates unstable molecules called free radicals which can damage cells.
Defenses Against Oxygen
Our bodies have some natural defenses against oxygen toxicity. Antioxidants are molecules found in many fruits and vegetables that can give up electrons to free radicals and keep them from reacting with cells and doing damage. However, these defenses can be overwhelmed if the oxygen concentration is too high.
The Great Oxygenation
The Great Oxygenation was a major event in Earth’s history that allowed for the development of more complex life forms. However, oxygen is not an elixir of life, but a double-edged sword. It can both give and take away life, and it took much more than a burst of oxygen to give us life as we know it.
The Story of Life on Earth
The story of life on Earth is not just a story about biology, but one that involves geology and chemistry as well. For billions of years, Earth had to be able to support simple life forms, and some of these life forms had to fill the air with oxygen. Additionally, some well-timed freak accidents had to occur to create the building blocks of complex life.
The Conditions for Complex Life
The conditions for complex life to exist had to be just right. This suggests that even if we do one day find another planet with simple life forms, life may never get interesting or intelligent. Our planet is covered in incredible plants and animals, and the fact that we are here at all might be even more remarkable than we thought.
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