Millions of years ago, there were two prokaryotic friends living harmoniously together. Jerry was photosynthetic, which means he could make his own food with energy from the sun. His best friend Bobby could use oxygen from the air and take it in to make ATP. They functioned perfectly well alongside one another until the day when Tommy came into town. Not only was he a much larger prokaryote, but he was a bully. In an attempt to gain their special powers, he absorbed Jerry and Bobby, leading to a period of uncontrollable indigestion. Instead of breaking them down, he had merely engulfed them in his cytoplasm where they remain to this day. (Inspired by Amoeba Sisters)
When we think of a eukaryotic cell, we typically picture it as a living organism that consists of several non-living organelles like a nucleus, endoplasmic reticulum, or Golgi apparatus. These organelles can work together within the cell to aid in its day-to-day functions, but they could never survive on their own, right?
Wrong! Although most organelles aren’t suspected to have any external origins, there are a few exceptions. As I have mentioned, the mitochondria (Bobby) and chloroplasts (Jerry) were fully-functioning prokaryotic cells that boasted their own nuclei — or at least this story is true according to the endosymbiotic theory.
This similarity among with many others suggests that they might have once been independently-functioning, which is unique among other organelles.
The first of these organelles, the mitochondria, likely evolved from purple nonsulfur bacteria that were absorbed by other cells and happened to survive the ordeal, functioning perfectly well in a cytoplasmic environment.
The endosymbiotic hypothesis may just be a guess, but it is well-informed and takes into account several comparisons between the mitochondria and overall cell. They both contain the following characteristics:
- Possession of DNA
- Translational machinery
- Double membrane
DNA from the mitochondria (mtDNA) is even used to trace inheritance from mother to child throughout an organism’s history. It changes very slowly over time, which makes it an ideal choice to measure the similarity between several different species.
As for Bobby, his story is similar. After engulfing a mitochondria, the ancestor prokaryote turned to cyanobacterium — an ancestor of the modern-day chloroplast. It took around 200 million years for chloroplasts and this ancestor to enter a symbiotic relationship, but after they supposedly did, there was significant evidence in favor of their separate origins.
The fact that chloroplasts also have their own DNA makes scientists believe they are linked with mitochondria under the endosymbiotic theory.
Long story short, it’s not the worst thing that Tommy swallowed Bobby and Jerry. Modern-day eukaryotes would not have the ATP they need, and plants would not be able to utilize the process of photosynthesis. Although the endosymbiotic theory was never completely proven, it provides an interesting perspective on the evolution of eukaryotic cells from prokaryotic cells. There is no telling how cells will evolve in the future, and if they undergo endocytosis with other smaller prokaryotes, their function may change significantly.