Basic Science

How we think complex cells evolved

What if you could absorb another organism and take on its abilities? Imagine you swallowed a small bird and suddenly gained the ability to fly. Or if you engulfed a cobra and were then able to spit poisonous venom from your teeth. Throughout the history of life, specifically during the evolution of complex eukaryotic cells, things like this happened all the time. One organism absorbed another, and they united to become a new organism with the combined abilities of both. We think that around 2 billion years ago, the only living organisms on Earth were prokaryotes, single-celled organisms lacking membrane-bound organelles.

Let’s look closely at just three of them. One was a big, simple blob-like cell with the ability to absorb things by wrapping its cell membrane around them. Another was a bacterial cell that converted solar energy into sugar molecules through photosynthesis. A third used oxygen gas to break down materials like sugar and release its energy into a form useful for life activities. The blob cells would occasionally absorb the little photosynthetic bacteria. These bacteria then lived inside the blob and divided like they always had, but their existence became linked. If you stumbled upon this living arrangement, you might just think that the whole thing was one organism, that the green photosynthetic bacteria were just a part of the blob that performed one of its life functions, just like your heart is a part of you that performs the function of pumping your blood.

This process of cells living together is called endosymbiosis, one organism living inside another. But the endosymbiosis didn’t stop there. What would happen if the other bacteria moved in, too? Now the cells of this species started becoming highly complex. They were big and full of intricate structures that we call chloroplasts and mitochondria. These structures work together to harness sunlight, make sugar, and break down that sugar using the oxygen that right around this time started to appear in the Earth’s atmosphere.

Organisms absorbing other organisms was one way species adapted to the changing environmental conditions of their surroundings. This little story highlights what biologists call the endosymbiotic theory, the current best explanation of how complex cells evolved. There’s a lot of evidence that supports this theory, but let’s look at three main pieces. First, the chloroplasts and mitochondria in our cells multiply the very same way as those ancient bacteria, which are still around, by the way. In fact, if you destroy these structures in a cell, no new ones will appear. The cell can’t make them. They can only make more of themselves.

Second piece of evidence. Chloroplasts and mitochondria both contain their own DNA and ribosomes. Their DNA has a circular structure that is strikingly similar to the DNA of the ancient bacteria, and it also contains many similar genes. The ribosomes, or protein assembly machines of chloroplasts and mitochondria, also have the same structure as ribosomes of ancient bacteria, but are different from the ribosomes hanging around the rest of eukaryotic cell. Lastly, think about the membranes involved in the engulfing process.

Chloroplasts and mitochondria both have two membranes surrounding them, an inner and outer membrane. Their inner membrane contains some particular lipids and proteins that are not present in the outer membrane. Why is that significant? Because their outer membrane used to belong to the blob cell. When they were engulfed in the endosymbiosis process, they got wrapped up in that membrane and kept their own as their inner one. Surely enough, those same lipids and proteins are found on the membranes of the ancient bacteria.

Biologists now use this theory to explain the origin of the vast variety of eukaryotic organisms. Take the green algae that grow on the walls of swimming pools. A larger eukaryotic cell with spinning tail structures, or flagella, at some point absorbed algae like these to form what we now call euglena. Euglena can perform photosynthesis, break down sugar using oxygen, and swim around pond water.

And as the theory would predict, the chloroplasts in these euglena have three membranes since they had two before being engulfed. The absorbing process of endosymbiotic theory allowed organisms to combine powerful abilities to become better adapted to life on Earth. The results were species capable of much more than when they were separate organisms, and this was an evolutionary leap that lead to the microorganisms, plants, and animals we observe on the planet today.

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