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  • Hey guys, Joe here.

  • Some people would argue the most important year in the history of soup was 1962, when

  • Andy Warhol released his soup-er soupy pop art.

  • But I think soup's best year came a decade earlier, in 1952, when a scientist named Stanley

  • Miller first cooked up primordial soup.

  • Miller's experiment took some simple chemicals, like those found on early Earth, bubbled them

  • up through a tube, zapped them with electricity, and after a few days, floating in this soup,

  • he found amino acidsthe building blocks of proteins, and one of the essential ingredients

  • for life.

  • This ideathat life's origins could be found in a puddle of chemicalsis an old

  • one.

  • In the 1920s, two different scientists theorized about life arising from what they called a

  • prebiotic soup”.

  • And this soupy speculation even goes back (unsurprisingly) to Charles Darwin, who in

  • 1871 wondered if life may have formed from chemicals “…in some warm little pond…”

  • What made Miller's experiment so special was it gave us proof: regular non-life stuff

  • could become cool life stuff super-easily.

  • Buteverythinglivingwe see today, even the most basic bacteria, is so complex,

  • built of such intricate machinery, it's impossible to imagine they just popped into

  • existence out of some soup.

  • That's because they didn't.

  • We're going to go on a journey in search of the origin of life, and along the way there

  • will be a few forks in the road, maybe a couple speedbumps, and we're going to need help

  • from a couple friends.

  • We'll come to see that Miller's primordial soup isn't exactly how this story began.

  • But the FIRST question we should ask isn't how life started, it's when.

  • Life on Earth couldn't exist before Earth existed, and it formed around four and a half

  • billion years ago, at the dawn of the Hadean Era/Eon.

  • Soon after that, another planet collided with the young Earth, melted the entire crust,

  • and created the moon in the process.

  • After the crust cooled, there was even some liquid waterat least for a little while.

  • Because for the next couple hundred million years, Earth was showered with hundreds of

  • massive space rocks.

  • The oceans boiled away, the crust melted again, and Earth was basically no place for life

  • until things settled down about 4 billion years ago, at the dawn of the Archean Eon.

  • This is the earliest possible time that life could have started on Earth, the beginning

  • of what we call the habitability boundary.

  • And fossil and chemical evidence tell us that early microbes existed by 3.7 billion years

  • ago, what's known as the biosignature boundary.

  • At some moment in here, non-life became life: we call this abiogenesis.

  • Now, I don't have a time machine.

  • As far as I know, no one does.

  • Therefore we can't go back and find that exact moment.

  • But if we could, what would we look for?

  • This brings us to the next big question on this journeywhat is life?

  • You'd think biology would have a good definition for life, the thing it studies.

  • But as a biologist I can tell you this is much harder than it sounds.

  • In one chapter of biologist JBS Haldane's 1949 book What Is Life? he literally writes

  • “I am not going to answer this question.”

  • Life is a board game, a delicious breakfast cereal, and a highway?

  • According to the dictionary, it's the time between birth and death.

  • But none of these definitions really help us.

  • I think we might be asking the wrong question, because life isn't a thing that things have,

  • life is what living things do.

  • In school, many people learn a checklist for the characteristics a thing must have in order

  • to bealive”: MRS GREN.

  • But this list came from looking at life as we know it today.

  • Life at the very beginning was probably much simpler.

  • A physicist, Erwin Schrödinger, looked at all these things that life does and saw something

  • only a physicist would see:

  • According to the second law of thermodynamics,  . But inside of living cells there's a

  • huge amount of order and complexity.

  • In 1944, Schrödinger defined life as a struggle against entropythe persistent resistance

  • of decay, the preservation of DISequilibrium.

  • Since then we're learned a lot more about entropy, and it may be that the rise of complexity

  • is as inevitable as its decay.

  • That sounds pretty good.

  • Life creates these little closed systems where it works to keep things nice and ordered.

  • But this definition still leaves out one important thing: Living things evolve.

  • Inside the very first living things must have been moleculeschains of atomsthat carried

  • informationinstructions for building things or codes for doing stuff.

  • Those molecules must have copied and made more of themselves, some a little different

  • than the others.

  • And a few of those codes and instructions must have been better at doing whatever they

  • did, so they made even more of themselves.

  • What we're describing is evolution by natural selection, Darwin's famous idea, and for

  • life to move forward, it must have been there from the beginning.

  • Life is a product of evolution.

  • With all this in mind, maybe we're finally able to come up with a better definition:

  • Life began the moment that molecules of information started to reproduce and evolve by natural

  • selection.

  • And now that we have a definition we can make some rules for what something

  • has to do to bealive”.

  • 1.

  • A living thing must work to avoid decay and disorder

  • 2.

  • To do that, a living thing has to create a closed system, or be made of cells

  • 3.

  • They have some molecule that can carry information about how to build cell machinery

  • 4.

  • This information must evolve by natural selection Sounds pretty good, but rules are one thing.

  • The ultimate question is how would this actually happen?

  • Let's take these rules one by one.

  • What would it require for these things to arise?

  • Andmost importantlyhow likely are each of these steps based on what we know from

  • good 'ol real, actual, hard science?!

  • Today, no matter where we look on the tree of life, most cell machinery is made of protein

  • chains of folded amino acids.

  • When modern cells make proteins, they copy genes from DNA into RNA and then use that

  • RNA as a blueprint for making the proteins.

  • We call this universal pathway the central dogma of biology,

  • because it sounds really cool, and because it's something that all life shares.

  • But there's a paradox hidden in here–a puzzle.

  • It's a chicken and egg problem!

  • DNA needs proteins to make more of itself.

  • And cells need DNA and the instructions it holds to make proteins.

  • So which came first?

  • We can solve this paradox in a pretty simple way.

  • Just get rid of DNA and protein in the earliest days of life, and let RNA do everything.

  • RNA is the molecular cousin of DNA.

  • It contains the same four-letter alphabet code as DNA, only T is replaced by a similar

  • molecule, U.

  • And instead of two strings in a helix, RNA is usually found in just one string.

  • RNA is special, because in addition to carrying information in that 4-letter code, it can

  • fold up into interesting shapes and actually do stuff.

  • The same way that protein enzymes can do all kinds of chemical reactions, RNA enzymescalled

  • ribozymescan work life's machinery too.

  • It's now thought that life began in an RNA world.

  • Before DNA became a more permanent form of storage, different RNA chains could have carried

  • information and been the machines for all of life's important chemistry.

  • Unfortunately, the RNA-only world went extinct more than 3 billion years ago, but we can

  • make these RNA enzymes today.

  • Scientists have constructed ribozymes that can copy themselves, just like DNA gets copied.

  • And those copies occasionally have errors or changes, so RNA can evolve too.

  • If you need more proof you can find it right inside your cells.

  • The ribosome, the massive structure that stitches amino acids into protein, is mostly RNA.

  • We also find nucleotides, the single molecular units of RNA, inside a bunch of other molecules

  • our cells need for metabolism.

  • This all makes sense only if the earliest days of living chemistry were dominated by

  • RNA.

  • And it solves our chicken and egg problem.

  • The RNA world takes care of two of our four rules: A molecule that can carry information

  • (3), and that can evolve (4).

  • To find answers for the other two, we need to ask one more question: Where did life begin?

  • There's been a lot of theories about where life came from, but they boil down to these:

  • Either life arose on Earth, or life arose somewhere else and was brought here.

  • It's well-known that space is full of the chemical building blocks of life, from amino

  • acids to DNA and RNA letters...

  • ...buried inside meteorites like this one that fell on Australia in 1969.

  • It shows the chemistry that makes biological molecules can happen pretty much anywhere.

  • But the idea that life was delivered to Earth on space rocks, which goes by the awesome

  • name panspermiawell there's just no proof it ever happened, and it doesn't really

  • explain the origin of life anyway.

  • It just moves it somewhere else.

  • Life probably started here.

  • Nozoom out a little.

  • We know early Earth had plenty of chemical ingredients, but the problem with that old

  • idea of primordial soup is that soup can't do anything on its ownthose chemicals can't

  • react without outside energy.

  • We get a hint of where this primordial energy came from by looking (again) at our own cells.

  • Instead of lightning, or heat energy, our cells pile up a bunch of hydrogen ions (protons)

  • on one side of a wall, let 'em flow downhill, and use this like a water wheel to push on

  • cellular machinery (and make things like ATP in the mitochondria)

  • We burn food to keep our hydrogen pump going, but the first life forms wouldn't have been

  • able to do this, because tacos hadn't been invented yet.

  • Instead, they would have needed some natural source, and they could have found it at the

  • bottom of the ocean.

  • Deep-sea hydrothermal vents are covered in microscopic little pockets, which could have

  • served as molds for the first cells.

  • Molecules with one oily water-hating end and one water-loving end have a neat habit of

  • forming bubbles and sheets all on their own

  • and there were plenty of these in the chemical soup near deep sea vents, ready to give rise

  • to the first cell membranes.

  • These vents also create natural streams of hydrogen ions near those little pockets in

  • the rock.

  • Imagine an early life form sitting there, wrapped in its little membrane bubble,

  • with a free source of energy flowing by, powering all the work it takes to create ordered life

  • and resist entropy.

  • But this would have been the absolute simplest form that life could take.

  • For this life form to become life that looks like what we know today, a lot more stuff

  • had to happen: it had to switch from storing its genetic information in RNA and started

  • using DNA.

  • Instead of using RNA and ribozymes to run all its cellular machinery, it had to start

  • stitching amino acids into proteins.

  • This opened up new possibilities for making and storing energy that let early life become

  • free-living and more complex.

  • One of these complex life forms is the ancestor of everything alive today, the last universal

  • common ancestor, or LUCA.

  • This is the end of our journey, searching for the origin of life on Earth.

  • A lot has happened since.

  • This story is based on things we've actually seen, not just on what's possible.

  • We've figured out when life could have started.

  • We've come up with rules for what life is.

  • We've found clues inside our own cells that explain how the first life satisfied these

  • rules, and where that life might have started.

  • The only question we haven't answered is why, but that's not really a question for

  • science, is it?

  • There's still quite a few gaps to fill in this story, and if you're looking for a

  • nice, neat answer for how life started, you're probably not going to find it.

  • Life is just a thing that happens.

  • It's still happening today, and it will evolve and continue as long as there's a

  • place it can happen.

  • Darwin didn't know it when he wondered about that warm little pond, full of chemicals,

  • giving rise to life, but his theory of how things change and adapt turned out to be so

  • powerful it encompasses life not just in its endless forms, but also in its first ones.

  • Stay curious.

  • Wow.

  • That was a LOT.

  • This is probably the deepest story I've ever done on this channel, and it's one

  • that involves some of the science I actually used to do, so this was a lot of fun for me.

  • I hope you enjoyed it too.

  • But this is only part of the story of how life began.

  • What happened before, to made Earth a place where life could happen?

  • And what happened after chemistry became biology, what life form lives at the bottom of our

  • tree of life?

  • For those answers, go check out these videos from our friends at PBS Space Time and Eons.

Hey guys, Joe here.

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B1 US life rna soup dna earth machinery

Where Did Life Come From? (feat. PBS Space Time and Eons!)

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    joey joey posted on 2021/05/04
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