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  • Part of what draws us to the study of natural history is our desire -- our drive -- to understand

  • our origins.

  • The more fossils that we can find and study, the more branches and twigs we can add to

  • the tree of life -- that web of relationships that connects all organisms, living and extinct.

  • But fossils can only get us so far.

  • As any astrophysicist will tell you, the story goes back way farther than any fossil.

  • The story of life actually begins with, well, the beginning of everything.

  • In the aftermath of the Big Bang the universe was an energetic, structureless mess.

  • But that mess pulled itself together into atoms and stars and galaxies and planets and,

  • finally, into life.

  • How did these little eddies of order form in a universe otherwise prone to increasing

  • disorder and chaos?

  • I’m Matt O’Dowd from PBS Space Time, and you can learn all about the physics of the

  • origin of life in a video over on our channel

  • Just follow the links.

  • But for us here on Eons, the search for our origins go back to a single common ancestor

  • -- one that remains shrouded in mystery.

  • If you trace all of the branches on the tree of life backward, you realize they all come

  • from the same trunk.

  • That initial point, before anything branches, is a single species.

  • The first species.

  • Darwin himself theorized that such a thing once lived.

  • He called it a “primordial form into which life was first breathed.”

  • Today, scientists call it the last universal common ancestor, or LUCA.

  • LUCA isn’t the first thing to have lived.

  • Instead, it’s the common ancestor of everything that’s alive today.

  • It’s the ancestor of everything we know.

  • Today, Eons is teaming up with our friends at Space Time and It’s Okay to Be Smart

  • to explore the origins of life.

  • And our journey begins with LUCA.

  • Even though Darwin suggested that there was a universal common ancestor, we couldn’t

  • even guess what it might have been -- until we started to master genomics, the science

  • of mapping and studying the genetics of all living things.

  • Today we know that all life uses the same molecules of RNA, DNA, and protein.

  • And the genetic code that’s responsible for making that stuff is basically universal,

  • from bacteria to humans.

  • That’s one of the best arguments to support the notion that everything came from the same

  • place.

  • It’s also why most of the research that’s gone into learning what LUCA was has involved

  • comparing the genomes of all kinds of living things, to see what else they have in common.

  • One of the first to take this approach was American biologist Carl Woese

  • In 1977, he discovered the existence of organisms that would make up a whole new domain of life,

  • and a key to the search for LUCA.

  • He discovered archaea.

  • Theyre a group of prokaryotes -- simple, single-celled organisms that are vaguely similar

  • to bacteria.

  • But they turned out to be so diverse, and so different from any other living thing,

  • that Woese proposed a new tree of life -- one that divided life into three domains: archaea,

  • bacteria, and eukaryotes.

  • And, he said, where those three main branches converged, there was LUCA.

  • But he saw the three domains arising not from a single cell, but from a chaotic environment,

  • more than 4 billion years ago, when cells didn’t quite exist yet.

  • In this scenario, he proposed, there were extremely simple things that were even more

  • basic than cells.

  • He called them progenotes and envisioned them as tiny scraps of genetic information, surrounded

  • by a membrane.

  • These progenotes wouldn’t have been complex enough to create true offspring.

  • They might have been able to copy their genetic material, but not accurately.

  • So instead, they may have just floated about at random, constantly swapping little snippets

  • of genetic code among themselves.

  • Sometimes, that genetic info might have worked well for a progenote, and could be copied.

  • Other times, not.

  • But out of this basically random transfer of information, Woese thought, some of the

  • key elements of life could have arisen.

  • For instance, the genetic code that makes up genes as we know them could have come about

  • pretty early.

  • But that information might have been stored in RNA, not DNA as it is today.

  • To explain how that might have worked, here’s a biologist I know who specializes in RNA:

  • Dr. Joe Hanson, host of It’s Okay to Be Smart:

  • Now, RNA, in addition to storing information, can do stuffbiochemical reactionslike

  • we see in the ribosome, where RNA is used to not only code for, but also build, proteins.

  • Early on, RNA machines like the ribosome would have evolved alongside genetics version 0.1.

  • So, it’s likely first life arose and evolved in this so-called RNA world, and only began

  • storing genetic information in DNA later on.

  • For more on how this happened -- and other theories about how life first arose on Earth -- check

  • out our video over on It’s Okay to Be Smart.

  • So, instead of being a specific organism, or even a group of things.

  • Woese thought that LUCA was the whole process by which progenotes acquired the genes to

  • make these essential molecules.

  • And from them would have come three lineages that evolved into modern bacteria, archaea,

  • and us eukaryotes.

  • Now, Woese’s view of LUCA hasn’t been abandoned, but many scientists have moved

  • away from it.

  • That’s partly because, when Woese was publishing these ideas, genetic sequencing was just coming

  • of age.

  • Back then, only a handful of genomes had been sequenced.

  • And now weve mapped the genes of thousands of living things.

  • And that means we can try looking for LUCA in new ways -- like, by lining up genomes

  • from across all of the domains of life and comparing them to see what they have in common.

  • If a gene appears in basically every living thing -- so it’s considered universal -- then

  • it must have come from LUCA ... or so the thinking goes.

  • So, many researchers who study LUCA are trying to reconstruct its genome.

  • One way to do that is by finding what they call the minimal genome -- the smallest amount

  • of genes that a cell can have and still survive.

  • Since these most basic, essential genes are thought to have come from LUCA, if we could

  • identify them, that could tell us how LUCA looked and lived.

  • This work has been spearheaded mainly by researchers in Maryland who have studied the genomes of

  • bacteria like Haemophilus and Mycoplasma.

  • And based on what those organisms had in common, the team proposed back in 2003 that LUCA probably

  • had 5 or 6 hundred genes.

  • Those genes would have provided for a simple metabolism and a genome based on RNA -- but

  • not for making and copying DNA.

  • Now, as genomes go, that is tiny.

  • A few modern organisms do have about 500 genes, but they are parasites that steal what they

  • need from their hosts instead of using genes to make stuff.

  • Meanwhile, the bacterium E. coli has around 5 thousand genes, and we humans get about

  • 25,000.

  • Could LUCA survive on its own with so few genes?

  • Well, another study done in 2006 found that LUCA would likely have had more like 1000

  • genes, maybe around 15 or 1600.

  • According to this slightly more recent take, our common ancestor might have been a little

  • more complex and may have seemed more familiar -- to microbiologists at least.

  • That means a genome based on DNA, ribosomes to translate the genetic code, and a metabolism

  • that could break down sugar for energy.

  • So, the basics of biochemistry as we know it.

  • And Joe talks a lot more about that, too, on It’s Okay to Be Smart.

  • Now, a lot of this minimal-genome research took place at the turn of the millennium -- around

  • the same time Woese was thinking about LUCA.

  • But in the past 20 years, the genetic revolution has redrawn the tree of life.

  • Recently, many scientists have begun to argue that the tree of life should have two main branches,

  • with bacteria on one branch, and archaea plus all of the eukaryotes on the other.

  • That’s because, the more we learn about genomics, the more it seems that all modern

  • eukaryotes are genetically more similar to archaea than to bacteria.

  • In fact, many researchers now believe that archaea are our ancestors.

  • So of course, this has enormous implications for what LUCA was, too.

  • In this case, LUCA would sit just below where those two main branches separate, before eukaryotes

  • even formed.

  • And based on this new line of thinking, a surprisingly complete picture of LUCA was

  • published in 2016, in the journal Nature Microbiology.

  • Here, evolutionary biologists based in Germany compared the genomes of more than 130 archaea

  • and over 1800 bacteria in an attempt to reconstruct LUCA’s genome.

  • Keep in mind here, in the two-branch model, archaea and bacteria are the most distantly related

  • forms of life on Earth.

  • So, if you can find any gene in both archaea and bacteria, then there are two possibilities:

  • Either the two groups traded genes at some point, which prokaryotes sometimes do, or

  • they both inherited that gene from LUCA.

  • The researchers looked for genes that appear in two different groups of archaea, and two

  • different groups of bacteria, reasoning that if a gene shows up in all four of those places,

  • it must go back pretty far.

  • Now, this is different from the minimal genome approach, because it tries to identify the

  • oldest genes, not the ones that everyone has.

  • And the genes that were recovered in this research suggest that LUCA lived inhydrothermal

  • vents.

  • How do they know?

  • Well for one thing, they recovered a set of genes that we know are used by extremely ancient

  • groups of archaea and bacteria that live in oxygen-free environments, where they metabolize

  • hydrogen gas and carbon dioxide into methane.

  • Hydrogen gas is hard to find on Earth, but it can come from deep sea vents; so that’s

  • one clue.

  • Other genes they found use metals like iron, nickel, and molybdenum in order to function.

  • And these are all found in the same kind of environment as hydrothermal vents.

  • Scalding-hot vents full of metals and sulfur might seem pretty hostile.

  • But our earliest ancestor might have called these places home.

  • Now, how could that be?

  • Like, we don’t metabolize hydrogen and CO2 to methane.

  • And neither do most of the organisms we know.

  • So how could the ancestor of everything have been so different from us?

  • Well, it’s at least possible, because genes are often lost over time.

  • As creatures evolve and adapt to new environments, lots of old genes aren’t always needed.

  • That’s basically why cats have genes to make fur, and not scales.

  • So by the same token, long after molecular oxygen became available on Earth, about 2.5

  • billion years ago, many of LUCA’s descendants were able to lose the genes for metabolizing

  • hydrogen and CO2, and still live comfortably.

  • Now, as awesome as this picture of LUCA is, it’s only one possible picture.

  • Itll be a long time before we know enough to agree on one single model.

  • Both the progenote model and the minimal genome idea have proven useful to guide our thinking,

  • but they probably don’t represent the true LUCA.

  • And not everyone agrees that LUCA lived in hydrothermal vents either.

  • There’s plenty more research to be done.

  • Because the search for LUCA just might be the one quest that defines the purpose of

  • natural history -- to reveal to us where we came from.

  • Now, to explore other facets about the origin of life, I encourage you to check out the

  • companion videos to this one, on PBS Space Time and It’s Okay to Be Smart.

  • Links are in the description.

  • And as always, I want to know what you want to learn more about!

  • So leave me a comment below, and don’t forget to go to youtube.com/eons and subscribe

  • And also, tell people about us. Please. It's so good, right?

Part of what draws us to the study of natural history is our desire -- our drive -- to understand

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什么是万物之祖?(What Was the Ancestor of Everything? (feat. PBS Space Time and It’s Okay To Be Smart))

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