Captions are on! Click CC to turn off. There are a lot of amazing theories in science. General theory of Relativity …Atomic theory…The cell theory. And one thing we want to point out about a scientific theory is that it’s a lot different from how you might use the word ‘theory’ in your daily life. A scientific theory is not an educated guess. A scientific theory is an explanation of a scientific event supported by scientific evidence---it must be testable and tested over and over and over again. And while theories can be changed or even disproven, you do want to realize there are a lot of facts behind them. One of our favorite theories of all time---is the endosymbiotic theory. We love it, because we just find the events in this theory to be amazing. The theory gives an explanation for how eukaryote cells could have evolved from prokaryotic cells that lived in symbiosis. Prokaryote symbiosis means these organisms lived together. So just a recap from our intro to cells video: when you think of prokaryote, think of ‘pro’ rhyming with NO. No nucleus. No membrane-bound organelles. Eukaryotes on the other hand---remember “eu” rhymes with “do” and they DO have a nucleus and other membrane-bound organelles. Both prokaryotes and eukaryotes ARE cells though---and therefore they do have a few things in common that cells have including a cell membrane, cytoplasm, ribosomes, and genetic material. But just look at these two cell types and you will see that one is WAY more simple. So what explanation does the endosymbiotic theory provide for how eukaryote cells could have evolved from prokaryote cells? A long, long time ago…seriously, a long time---over 2 billion years ago there were prokaryotes. Much like there are today. They had been around for a long time. Just as there are in many organisms, there was variety in prokaryotes. Some of these prokaryotes had photosynthetic abilities, which means, they could make their food using sunlight energy. These were photosynthetic bacteria. Some of these prokaryotes were bacteria that had the ability to use oxygen to produce ATP energy. And some prokaryotes were larger and could consume others. Engulfing them. So obviously, they had to be larger. The endosymbiotic theory is that some of these large cells engulfed some of these small bacteria---but instead of those small bacteria getting digested, some of them remained intact and they actually began to live as symbionts. That means, they lived together! This is believed to be the ancestor heterotroph eukaryote cell. Eventually SOME of these cells engulfed the small bacteria that could do photosynthesis in addition and there was more living as symbionts. This is believed to be the ancestor autotroph eukaryote. Now it’s time to do a little reveal. We consider the bacteria that used oxygen to produce their own energy to have evolved into what is now mitochondria. And the bacteria that use sunlight energy to produce their own food? Well we consider those to have evolved into what are now chloroplasts. It is likely that these ancient eukaryotes had an advantage in their endosymbiosis. It is also likely that the mitochondria developed first before chloroplasts, because if you remember from our intro to cells video, nearly all eukaryote cells have mitochondria. It’s just that eukaryote cells that can do photosynthesis can have both mitochondria and chloroplasts. Now if you remember our beginning about theories, you may be wondering, where are the facts for this? We’ll give you a few of the facts. First, mitochondria and chloroplasts have their OWN DNA! Yes! Separate DNA from what is found in the nucleus. Not to mention that their DNA is arranged in a similar way to prokaryote DNA---specifically, bacterial DNA. The size of mitochondria and chloroplasts tends to be similar to the size of bacteria and when mitochondria and chloroplasts divide---which they can divide all on their own, independently---they divide in a way that is similar to how bacteria divide. So the endosymbiotic theory provides an explanation for how modern eukaryotes evolved from prokaryotes. There’s obviously more questions you may want to explore---for example, what about some of the other structures and organelles in eukaryotes? Well keep exploring---secondary endosymbiosis is a great place to start. One last thing we want to make sure to emphasize. Endosymbiosis isn’t just reserved for a theory that explains a past event in ancient history. No because endosymybiosis is actually happening today with many other kinds of organisms. One of our favorite examples? The termite! Yes, termites can have prokaryotes that live in their gut and help them digest wood. And without them? Well let’s just say that digesting wood won’t happen so well… Well that’s it for the Amoeba Sisters, and we remind you to stay curious!