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What if you could absorb
another organism
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and take on its abilities?
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Imagine you swallowed a small bird
and suddenly gained the ability to fly.
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Or if you engulfed a cobra
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and were then able to spit poisonous venom
from your teeth.
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Throughout the history of life,
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specifically during the evolution
of complex eukaryotic cells,
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things like this happened all the time.
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One organism absorbed another,
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and they united to become a new organism
with the combined abilities of both.
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We think that around 2 billion years ago,
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the only living organisms on Earth
were procaryotes,
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single-celled organisms
lacking membrane-bound organelles.
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Let's look closely at just three of them.
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One was a big, simple blob-like cell
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with the ability to absorb things
by wrapping its cell membrane around them.
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Another was a bacterial cell
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that converted solar energy into sugar
molecules through photosynthesis.
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A third used oxygen gas to break down
materials like sugar,
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and release its energy into a form useful
for life activities.
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The blob cells would occasionally absorb
the little photosynthetic bacteria.
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These bacteria then lived inside the blob,
and divided like they always had,
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but their existence became linked.
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If you stumbled upon
this living arrangement,
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you might just think that the whole thing
was one organism,
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that the green photosynthetic bacteria
were just a part of the blob
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that performed one of its life functions,
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just like your heart is a part of you
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that performs the function
of pumping your blood.
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This process of cells living together
is called endosymbiosis,
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one organism living inside another.
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But the endosymbiosis didn't stop there.
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What would happen
if the other bacteria moved in too?
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Now the cells of this species started
becoming highly complex.
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They were big and full
of intricate structures
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that we call chloroplasts
and mitochondria.
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These structures work together
to harness sunlight,
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make sugar,
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and break down that sugar using the oxygen
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that right around this time started
to appear in the Earth's atmostphere.
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Organisms absorbing other organisms
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was one way species adapted
to the changing environmental conditions
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of their surroundings.
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This little story highlights what
biologists call the endosymbiotic theory,
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the current best explanation
of how complex cells evolved.
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There's a lot of evidence
that supports this theory,
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but let's look at three main pieces.
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First, the chloroplasts and mitochondria
in our cells multiply the very same way
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as those ancient bacteria,
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which are still around, by the way.
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In fact, if you destroy these structures
in a cell, no new ones will appear.
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The cell can't make them.
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They can only make more of themselves.
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Second piece of evidence.
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Chloroplasts and mitochondria both contain
their own DNA and ribosomes.
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Their DNA has a circular structure
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that is strikingly similar to the DNA
of the ancient bacteria,
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and it also contains many similar genes.
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The ribosomes, or protein assembly
machines of chloroplasts and mitochondria,
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also have the same structure as ribosomes
of ancient bacteria,
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but are different from the ribosomes
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hanging around
the rest of eukaryotic cell.
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Lastly, think about the membranes involved
in the engulfing process.
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Chloroplasts and mitochondria
both have two membranes surrounding them,
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and inner and outer membrane.
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Their inner membrane contains
some particular lipids and proteins
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that are not present
in the outer membrane.
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Why is that significant?
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Because their outer membrane
use to belong to the blob cell.
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When they were engulfed
in the endosymbiosis process,
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they got wrapped up in that membrane,
and kept their own as their inner one.
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Surely enough, those same lipids
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and proteins are found on the membranes
of the ancient bacteria.
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Biologists now use this theory
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to explain the origin of the vast
variety of eukaryotic organisms.
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Take the green algae that grow on
the walls of swimming pools.
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A larger eukaryotic cell with spinning
tail structures, or flagella,
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at some point absorbed algae like these
to form what we now call euglena.
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Euglena can perform photosynthesis,
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break down sugar using oxygen,
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and swim around pond water.
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And as the theory would predict,
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the chloroplasts in these euglena
have three membranes
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since they had two before being engulfed.
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The absorbing process
of endosymbiotic theory
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allowed organisms to combine
powerful abilities
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to become better adapted to life on Earth.
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The results were species
capable of much more
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than when they were separate organisms,
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and this was an evolutionary leap
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that lead to the microorganisms, plants,
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and animals we observe
on the planet today.