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  • 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.

What if you could absorb another organism

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TED-ED】複雑な細胞はどのように進化したと考えるか - アダム・ジェイコブソン (【TED-Ed】How we think complex cells evolved - Adam Jacobson)

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    Mike に公開 2021 年 01 月 14 日
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