The Most Important Moment in the History of Life

In our last video, I talked about why and how life became multicellular… one of the
most important transitions in our evolutionary history, but still one that happened dozens
of times Today I’m going to tell you about a different
event, one so rare it only happened once in the nearly 4 billion years life has been on
Earth… what I would call the most important moment in history. If it hadn’t happened… [Wind] We got lucky. [Music] Can you pick out the human cell? Complex life looks pretty different on the
outside. But whether it’s a plant, a protist, or a peacock, at the most basic level, all
of Earth’s complex life, eukaryotes, are built from essentially the same stuff.
Because eukaryotes share so many things, it’s likely that we all have a common ancestor.
You and that oak tree you used to climb are cousins, just a few billion times removed. But for most of the history of life on Earth,
there was no such things as eukaryotes. Earth was home to only two domains of basic life,
bacteria and archaea. Known as “prokaryotes,” these single-celled organisms dominated the
Earth, but in all of life’s history they’ve never gotten big or complicated like eukaryotes
did. Because they couldn’t muster the energy. Our energy currency is ATP. Like a token that
gives you one play in the cellular arcade. Maybe you use a token in a protein that cuts
DNA, or maybe one that breaks down food. When you run out of tokens, game over. But prokaryotes can only manufacture those energy tokens on
their surface membrane, and expanding their surface area to create more energy tokens would exponentially
increase their volume, which then needs more energy to sustain. Say a bacteria wanted to increase
its radius by 25 times. Then it would have 625 times as much membrane to make ATP with!
All right! But you’d also need to manufacture 625 times more protein and membrane for those
new ATP factories. In the meantime, the volume of your cell has increased 15,000 times, and
you’ve gotta fill that with protein machinery too, so you’re gonna need 15,000 genomes
to keep up with demand. It was a catch-22 that prokaryotes could never
overcome. On their own. But then, something unimaginably improbable
happened. An event so rare it only happened once in all of life’s history. One day,
an archaean cell bumped into a bacterium, and slurped it up. And for some reason, it
didn’t digest it. We don’t exactly know why, but for the first time one organism was
living in symbiosis not with another organism, but inside it. ENDOsymbiosis. And every living
thing you can see with the naked eye is descended from that synthesis By cooperating with its new host, this bacterium
was able to ditch 99% of its genome, shut down most of its own machinery, and concentrate
on one task: making ATP. Thus the first mitochondria was born, and its power level was over 9000. That first eukaryote was rich! [Music] More ATP than
it knew what to do with! It was free to grow and specialize and experiment with new crazy
abilities. Later, some of them gulped up a second bacterium that had figured out photosynthesis,
and they eventually became plants. And before too long, eukaryotes morphed into
everything from jellyfish to redwood trees to pangolins and penguins… even us. A single one of your cells can use 10 million
ATP tokens every second. All together, you burn through your body weight in ATP every
day. Considering we only have 60 grams of ATP in our whole body at any one time, there’s
a lot of recycling going on. And that all happens in the mitochondria. To recycle all the ATP we need in a day, it
takes an incredible number of mitochondria, hundreds or thousands per cell, maybe a quadrillion
in our whole body. If we stretched their crinkled membranes out all together, your body would
hold four football fields worth of ATP-making real estate. We know the mighty mito started this way because
they still contain their own genome, a circle of DNA like a prokaryote’s, and we can trace
one third of our genes to either bacteria or archaea. Eukaryotes are a genetic mashup. In a world filled with bacteria and archaea
bumping into each other every minute of every day for the past couple billion years, this
eukaryotic origin story has only happened once. You and the oak tree’s common ancestor
tells us so. It’s all thanks to that single moment, when two branches from two divergent
trunks on the tree of life, joined back into one, freeing life from its energetic constraints,
and giving nature the freedom to build endless forms most beautiful. Have you ever done a science and wished you had a t-shirt to celebrate the occasion? Well [snap] now you do. This new It’s Okay to Be Smart Shirt is now available on our merch store If you’d like to pick one up for yourself You can also find the link down in the description and if you find yourself doing a science wearing this shirt then post a picture on our instragram or tweet us with the hashtag I did a Science alright, Stay Curious and stylish

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