Origin of Eukaryotes
When life first got started on Earth during the Precambrian Era, there were just a bunch of simplified prokaryotes at the birthday soirée, all without organelles. Some simple eukaryotes eventually showed up, mostly single-celled eukaryotes resembling today's protists. It took almost another billion years, during the Cambrian Explosion (think new species popping up out of a tin can of eukaryotic popcorn kernels), for more complex eukaryotes to evolve. These multicellular organisms were ancestors of our modern day plant and animal counterparts.
Life started in the water, and so did the first eukaryotes. Plants were the first eukaryotes to "step" out of the sea and onto land, followed by animals, and then The Little Mermaid. It should be no surprise that insects were among the first animals to leave the water for the lush landscape, shelter, and food that plants provided on dry land. After all, insects are opportunists currently running amuck in your home for the very same reasons. Blech.
How exactly did we get the first eukaryotic cells from these early prokaryotes back in Earth's "younger" days? Were eukaryotic cells just a part of Earth's midlife crisis? Or were they a way to liven up the birthday party?
To understand how eukaryotes most likely evolved, we have to go back to a prokaryote that played a major role. About 2.7 billion years ago, Earth evolved the gift of cyanobacteria. These tiny bacteria, which still exist today, were capable of performing photosynthesis—the process that converts energy from the sunlight into food. Cyanobacteria didn't just cook their own meals; they themselves became a food source for other cells to feast on.
During photosynthesis, they also produced the world's first primitive oxygen bar, changing the world's atmosphere into something eukaryotes could use to perform cellular respiration—the process that converts food into usable energy for non-photosynthesizing organisms (like us).
We know what happens to us when we're surrounded by more food sources…like a room full of every flavor birthday cake imaginable. We tend to eat more cake. We also might get a little bigger. Maybe we'll eat so much cake that we become cake. After all, you are what you eat, right?
We're here today because, fortunately, there were some early cells that really liked to eat cake. Or cyanobacteria. The cyanobacteria were a brand new "food" source that looked appetizing.
Unicellular cyanobacteria are the little green rod-shaped prokaryotes in this picture. Delicious looking, are they not?
The Endosymbiotic Theory explains how eukaryotes got their fill of membrane-bound organelles while evolving from prokaryotes. Specifically, we can use this theory to talk about how we got photosynthesizing eukaryotes, and eventually plants, from cyanobacteria.
Once upon a time, some larger prokaryotes munched on some cyanobacteria by swallowing them whole. Since these prokaryotes didn't have mouths, they ate the cyanobacteria by wrapping their plasma membranes around them, internalizing the smaller cyanobacteria inside themselves. This resulted in a larger prokaryote with a smaller cyanobacterium inside it. The cyanobacterium retained its own membrane, but was now also surrounded by part of the host cell's plasma membrane too. This double membrane is important. We'll come back to it in a few paragraphs.
Now, rather than being digested like some red velvet deliciousness, these cyanobacteria duplicated within the host and were passed down to future host cells. The cyanobacteria stayed intact because they were a pretty (sweet) deal for their hosts—a perfect example of a symbiotic relationship, where both parties mutually benefit from the living arrangement. The larger prokaryote benefited from the food produced from cyanobacteria, while the cyanobacteria benefited from the resources and shelter of the larger cell.
Overtime, these cyanobacteria evolved into what we now know as chloroplasts, which are capable of performing photosynthesis within the host cell. This provides a food source for both the eukaryotic cell and the rest of us that snack on it. These cells also evolved into modernized, multicellular, photosynthesizing eukaryotes—like algae and palm trees and houseplants and spinach. Popeye is forever indebted to cyanobacteria.
What about that double-membrane feature of the swallowed-up cyanobacteria? Chloroplasts in today's eukaryotes have kept this second membrane that they picked up on the way into the cell, evidence that backs up the Endosymbiotic Theory. Further, transport occurs across this membrane just like it does across the plasma membrane. There are other membrane-bound organelles in cells, but they do not all have this characteristic two-membrane feature. Therefore, it may not be that all organelles used to be other prokaryotes at one time.
However, there's good evidence to support that mitochondria were also a tasty treat and also part of this evolutionary process. Indeed, the inner and outer membranes surrounding the mitochondria and the chloroplast have different compositions, as they originated from different places.
Mitochondria perform cellular respiration. They are able to use food to get energy for a cell. You can see how the cell would also find this an excellent symbiotic relationship, getting energy from mitochondria to perform more complex processes and do things faster. And eat more cake.
Both mitochondria and chloroplasts also have their own DNA, independent and different from the DNA in the nucleus. This is a further piece of evidence that they were once their own cells with their own genetic materials. In addition, mitochondria and chloroplasts have their own ribosomes. These ribosomes are smaller than eukaryotic ribosomes in the cytoplasm and more similar to prokaryotic ribosomes. New mitochondria and chloroplasts also replicate within a cell like how bacteria still replicate today, through a process similar to binary fission.
With all this evidence adding up, it's pretty cool to see how the endosymbiotic theory is supported. Prokaryotes ate other prokaryotes, decided to allow them to live inside them, and then used them to become the first eukaryotes. Not too long after that, the Cambrian Explosion, and boom! Here we are today. Now who wants some ice cream?
Eukaryotic plant cells that perform photosynthesis likely evolved from larger prokaryotes that engulfed smaller photosynthesizing cyanobacteria. This also explains why chloroplasts carry their own DNA.