How Did We Get Here? (Part 2)
After millions of years, Earth finally has Organic Compounds. What’s Next?
Read Part 1 first
3.8 billion years ago was a boring time to be on Earth. Rugged, rocky terrain stretched out as far as the eye could as large, soupy oceans lapped at the banks. And… that was about it. No flora, no fauna, not even any oxygen in the atmosphere. Aside from the terrain, there were only some basic molecules and compounds. Compared to Earth today, Earth back then might as well have been any other asteroid.
So what changed?
Proto-Cells
Before 3.8 billion years ago, Earth was a sweltering hellscape; with an atmosphere consistently over 100 degrees Celsius, all water on the planet had evaporated into water vapor. But over time, Earth slowly cooled down and blue splotches started to appear on the planet’s rocky canvas. And in those blue splotches was where the world’s first proto-cell formed.
As its name suggests, protocells are primitive cells, ancestors of the ones we know today. They were just self-organized spherical balls of lipids¹, a structure that forms very easily.
If you’ve been in any high school biology class before, you’ll likely know how certain lipids have both a water-loving (hydrophilic) head and a water-fearing (hydrophobic) tail. Because of this unique structure, when many of these lipids group together, they tend to form balls where their hydrophilic heads face water while the hydrophobic tails hide behind the heads in a water-tight pocket. (This is why phospholipids are so adept at creating membranes².)
The oceans discussed previously contained many fatty acids, a type of lipid with the partially hydrophobic, partially hydrophilic properties discussed above. Thus, as soon as it was possible, multiple fatty acids coalesced in the water, forming spherical balls (AKA fatty acid micelles). Earth had met its first protocell.
As time went on, these fatty acid micelles would take it a step further, fusing with each other to create protocells bordered by two layers of fatty acids (called double membranes as opposed to the single membranes of the earlier protocells).
With this advancement of the double membrane came what is now widely regarded as one of the most important parts of the cell³. The creation of a solid, reliable membrane allowed cells to start filling up with more cell components, putting cells on the fast track toward advancement.
Endosymbiosis
Over generations of cells, new components (such as nuclei and ribosomes) started to trickle in, becoming mainstays within no time. These components, called organelles, greatly upgraded the abilities of the cells, whether it was by allowing cells to “clone” or make proteins. However, two of the most important organelles were never supposed to even be cell organelles.
Despite their significance in cell function, the precursors to mitochondria and chloroplasts were originally independent cells, complete with their own DNA, ribosomes (protein makers), membranes, and more. However, despite these various features, these two precursors were lacking one crucial feature: protection against predators.
Even in an environment of only cells, there was still a predator-prey dynamic; predator cells were constantly looking to engulf and break down weaker neighboring cells. At one point, a eukaryotic predator cell engulfed an oxygen-using prokaryotic cell (as any predator cell would do at the time). However, instead of just digesting its meal, the predator cell accidentally kept its prey, creating a hybrid organism with the prey cell in the predator cell: the first endosymbiont.
This botched fusion allowed the eukaryotic cell to have a second, working component inside of it. Over time, as this hybrid cell replicated, the two cells (prey and predator) would grow to cooperate better, contributing energy to each other in exchange for better shelter or more safety.
How did scientists come up with this theory? Today’s mitochondria and chloroplasts gave it away. Unlike the images you might see in textbooks, mitochondria and chloroplasts are dynamic structures, constantly moving around and sometimes dividing. What’s more, the two organelles have their own DNA, ribosomes, and double membrane, components found in no other organelles.
Multicellularity
Fast forward a couple of million years, and Earth finally hosts hoards of functional cells. Yet, every single cell is a loner, minding its own business and doing its own thing. Unlike the plants and animals you see today, none of the cells would work together to create something bigger than themselves⁴. How did life go from this unicellular (one-celled) life to multicellular (multiple-celled) life⁵?
Anticlimactically, according to current hypotheses, multicellular life was the result of either just simple cell division or attraction between cells (which caused them to clump together).
Though its creation was nothing special, the introduction of multicellular life would usher in a new era of development for all life, opening the door to plants, animals, fungi, and possibly more in the future.
So there you have it. Within a couple of billions of years, a smattering of organic compounds transformed into protocells, which became endosymbionts, and finally turned into multicellular organisms. With the introduction of multicellular life, the capabilities have grown exponentially, something which will be explored in Part 3.
¹Lipids are a category of substances that do not mix with water (e.g. all fats)
²By sealing against water, these lipid membranes can form a barrier between water on one side and another fluid on the other side. They’re so good at this that every cell in the world has at least one phospholipid membrane.
³ Today, lipid membranes are found around every type of cell. They regulate what goes in and what comes out of a cell, which is a crucial function for any cell (so cells don’t accidentally take in viruses or expel crucial cell parts)
⁴To give a point of reference, an ant is made of around 20 million cells
⁵If you didn’t already know, multicellular life (organisms made of multiple cells, not just one) was one of the greatest leaps (for life) ever made. The effectiveness of multicellularity allows multicellular organisms to do great things, a fact backed by how every plant and animal is multicellular. When you don’t have to rely on one cell to do all the life-maintaining tasks, you can start having specialized cells that concentrate more on specific fields.
