Where Do Cells Come From?
All cells come from other cells—so then where did the first cells come from? The answer lies in a hot primordial soup, at least 3.8 billion years ago.
There's no direct evidence of our biological kickstart. But there is much indirect evidence that leads to several possible explanations. Today we're looking at the popular RNA World hypothesis.
The Primordial Soup
Once upon a time, there was a primordial soup. It was a tad watery and flavourless, but it did contain some rather promising ingredients for life.
Then lightning struck. Or maybe it was a meteorite, or a lava flow, or a hydrothermal vent. We can't be sure until we invent those time-dilating portals to the past you keep talking about.
Regardless, the heat triggered chemical reactions between carbon, hydrogen, oxygen, nitrogen, and rock-based phosphorus (CHONP) to produce the first nucleic acids.
The First Nucleic Acids
Bingo! Exciting, new-fangled molecules out of a lifeless soup. And it was all down to the innate proclivity for atoms to join into molecules.
Atoms are not goal-oriented. They simply bond with their neighbours according to the laws of chemistry and physics. It's spontaneous, rule-bound—and inevitable.
Nucleic acid were so abundant that they accumulated, layer upon layer, as brownish crystals on rocks. When ribose sugars (carbon, hydrogen, and oxygen) joined the party, the first RNA was born.
All living organisms have RNA—including humans. It's the single-stranded sister molecule of DNA. And it has some extraordinary properties. For instance, spontaneous base-pairing means RNA can create negative blueprints of itself. This is self-replication.
Types of RNA
Linear RNA strands can loop and fold, forming internal bonds between the A, U, C, and G bases. Different base sequences produce RNA machines with different shapes and lengths.
The first cells had several RNA machines that performed structural, catalytic, and data storage roles. This idea of RNA coming first is known as RNA World.
Where Did The First Cells Come From?
Chemically speaking, RNA was the hard part. The first cells were easy—so long as we're thinking of cells as simple membranes or sacs, separating an interior environment from an exterior one.
How do you create a cell membrane? You need fats—or lipids—which are naturally insoluble in water.
Fats were abundant in the primordial soup. They're simply long chains of carbon, hydrogen, and oxygen atoms which, billions of years later, some savvy scientist would name fatty acids.
When these linear molecules hooked up with phosphorus and oxygen, they made phospholipids, which have some rather special properties.
The soup became a bubble bath. And just like bubbles, phospholipids assembled to form bi-layered membranes: tiny spheres with water trapped inside.
With membranes and RNA in abundance in the primordial soup, it wasn't long before the two coalesced. This is ultimately where cells come from.
We're now looking at the most primitive form of Bacteria. With the ability to retain and replicate a basic source code, these were the first living organisms to colonise Earth.
Where Did The First Amino Acids Come From?
Modern cells are much more than RNA and membranes. I'm looking at you, amino acids. These are the building blocks of proteins which do an awful lot of work in cells.
The simplest amino acids formed spontaneously in the chemical soup. But more complex types are only produced through biosynthesis—they're a product of life itself. In this chicken-or-egg scenario, where did the first complex amino acids come from?
Amino acids were routinely deposited on Earth via meteorite impacts. Such events were frequent during the Late Heavy Bombardment around 4 billion years ago.
These extra-terrestrial acid drops still occur today. For instance, in 1969, the Murchison meteorite crashed into rural Australia carrying more than 100 different amino acids. Take that thought to sleep with you tonight as you consider how exactly amino acids came to be on comets in the first place.
Back on Earth, the RNA machines got to work on the alien molecules. Long chains of amino acids were strung together to make polypeptides, which twist and fold to give rise to proteins. We see these same reactions take place in our own cells today.
As proteins took on new roles in cells—like catalysing reactions and creating internal structure—Bacteria gained better chances of surviving and replicating.
This trial-and-error process meant many early cells were doomed to failure. Cells didn't specify their end goals; nature simply pruned away configurations that didn't work.
Surviving cells propelled their winning RNA blueprints into the next generation. Today, we call these winning blueprints genes. They thrive hand-in-hand with the useful protein products they describe.
With increasing protein and genetic diversity, bacterial cells branched out into many different species. The oldest evidence of cellular life comes from a 3.5 billion-year-old fossil containing no less than 11 different bacteria species.
Now we know where cells come from, or at least, we have one very compelling hypothesis in RNA World. How did single-celled organisms evolve into the complex life we see today?
How Did Bacteria Evolve into Complex Life Forms?
Bacteria evolved on their lonesome for a while. Mistakes in RNA replication led to cellular changes which were either damaging, neutral, or advantageous to survival. This is evolution.
Cascades of helpful mutations created novel cell features and behaviours, like the ability for some Bacteria to go ahead and eat others. Diversity blossomed.
Eventually, some mutant Bacteria took to living in extreme environments like hot springs. They became so diverse they landed themselves a new name altogether: Archaea.
Bacteria and Archaea ruled the Earth for at least a billion years. Then something curious happened. It was a drizzly Tuesday afternoon when a single Archaeon engulfed a Bacterium. She just clean swallowed him whole.
But she did not digest him for energy. Oh, no. Instead, she decided to use him as an energy generator.
Individually, both cells found their newfound endosymbiosis rather agreeable. The Archaeon gained a second powerhouse of energy, while the Bacterium gained an extra layer of protection.
Together, they were unstoppable. This new organism marked the birth of Eukarya, the third and final domain of life that encompasses all the plants, fungi, and animals you've ever heard of.
Today, almost all your body cells contain Bacteria called mitochondria. They are the cells within your cells, complete with their own set of DNA and their own journey of evolution inside you.
But wait a second. The first eukaryotes were single-celled organisms too. How did they scale up to become multi-cellular organisms like you and me?
That's another story. Check it out in my article 16 Landmark Moments in Animal Evolution. It traces all the major steps in animal evolution that ultimately brought humans into being.