WEBVTT

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Welcome back to the deep dive. Today, we're going

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to look at a picture. Just one picture. Just

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one. Yeah. But it's an image you have absolutely

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seen a thousand times. It's on the walls of biology

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classrooms. It's, you know, in textbooks, probably

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on a few t -shirts. We call it the tree of life.

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Oh, right. It's totally iconic. And usually when

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we see it, we just think, ah. OK, pretty picture.

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Roots at the bottom, branches, monkeys, and then

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us sitting right at the top. It feels very intuitive.

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Exactly. I mean, we could completely take it

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for granted. But the stack of sources we're diving

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into today, which spans from French priests in

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1801 all the way to modern genetic databases,

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it suggests that this image is actually one of

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the most controversial, difficult, and misunderstood

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models in the history of science. It really is.

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It functions as a metaphor, a conceptual model,

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and a research tool all at the same time. And

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the really fascinating thing we're going to uncover

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is that the tree isn't just a drawing of nature.

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It's actually a drawing of how humans think about

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nature. And that thinking has changed so radically

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over time. So our mission today, for everyone

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listening, is to trace that evolution. We're

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going to find out why the first tree of life

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had absolutely nothing to do with evolution,

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how Darwin turned a drawing into a mathematical

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calculation, and why modern genetics is currently

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threatening to just chop the whole tree down.

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Yeah, it's basically a journey from perfect order

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to absolute chaos. So let's start at the beginning,

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or I guess the pre -beginning, because before

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we had evolutionary trees, we had, well, regular

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family trees. Right, if you go back to medieval

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times, tree diagrams were everywhere, but they

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were strictly for human genealogy. Oh, like for

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royalty. Exactly. If you were a king or a duke,

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you had a family tree to show your pedigree.

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You showed how noble blood flowed down to you.

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So as a status symbol, look at my mighty roots.

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Yeah, pretty much. Yeah. But nobody applied that

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visual to nature. Nature was usually viewed as

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a ladder, the skull and atre. Right, the great

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chain of being. Mm -hmm. A fixed hierarchy with

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rocks at the bottom. than plants, animals, humans,

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angels, and God at the very top. You don't climb

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the ladder, you just stay exactly where you're

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put. It seems so obvious to us now to use a tree

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for species, but back then, the ladder was the

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dominant mental model. So when does the tree

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metaphor actually jump the fence from human families

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over to biology? The sources point us to 1801,

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and this is our first big surprise. The man who

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draws the first real botanical tree, isn't a

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radical scientist. He's a French Catholic priest.

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Augustin Agier. That's the one. Which is so ironic,

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right? You'd think a priest would stick to the

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latter idea, given the theology of the time.

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You really would. But Agier was looking at plants,

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and he saw relationships that a latter just couldn't

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capture. So he published this work called the

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arbre botanique, the botanical tree. And if you

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look at the image in the sources, it looks remarkably

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like a modern evolutionary tree. Yeah, it has

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a trunk, branches, leaves representing different

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plant families. And here's the key distinction

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we need to make. This wasn't about time or change.

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Not at all. No. Auggie wasn't saying this plant

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evolved from that plant. He was saying this is

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the perfect order of nature as instituted by

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God at the moment of creation. So it's basically

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a static map. Precisely. It's a timeless snapshot.

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He believed God created everything in perfect

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order, and the tree shape just happened to be

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the best way to draw that order on a piece of

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paper. It's so interesting that the visual came

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before the theory. He had the shape, but he didn't

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have the mechanism for evolution. Right. The

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mechanism. The idea that things actually do change.

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That starts creeping in a few years later with

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Jean -Baptiste Lamarck. Ah, Lamarck. I feel like

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he always gets a bad rap in high school biology.

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Oh, definitely. He's just the guy who thought

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giraffes have long necks because they stretch

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them. That is the caricature, yes. Yeah. But

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Lamarck was a heavyweight. In 1809, he published

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his tableau, and he was one of the first to strongly

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argue for transmutation. Meaning he believed

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species weren't fixed. They changed. Yes. And

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he drew a branching diagram for animals. But,

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and this is a really subtle detail in the research,

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he absolutely refused to call it a tree or a

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genealogy. Why the hesitation? If he believed

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things changed, why not call it a family tree?

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Because he didn't believe in common descent.

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Wait, help me parse that. He thought animals

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changed into new forms, but he didn't think they

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were related. Correct. Lamarck's view was wild

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by modern standards. He believed in constant

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spontaneous generation. Meaning life just popping

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into existence. Yes. He thought simple life forms

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were constantly generating from mud or matter

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in parallel lines and then moving from simple

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to complex over time. So instead of one big tree

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where we all come from one root, it's like a

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thousand parallel escalators. Exactly. Nature

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had this drive toward perfection. So a simple

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worm today is just on the bottom step and given

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enough time its descendants will become complex.

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But that worm isn't related to us. It's just

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riding a newer escalator. That is such a distinct

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way of seeing the world. It's about parallel

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progress, not shared history. Right. It explains

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why a single tree metaphor didn't fit his math.

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Exactly. Which brings us to 1840 and an American

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paleontologist named Edward Hitchcock. He's the

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next major stop in our deep dive. He draws these

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two really fascinating charts, one for plants,

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one for animals. You're looking at the notes

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here. The visual details are spectacular, but

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they seem really well regal. They are. Hitchcock

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wasn't subtle at all. He literally crowned his

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trees graphically. The plant tree has palms right

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at the very top. And the animal tree. Let me

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guess. Humans? You got it. A big crown labeled

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man. It reflects this lingering worldview of

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the time that the tree has a destination. It's

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growing toward something. It's the ego again.

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The universe exists to produce me. It's a very

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hard habit to break. The idea that we are the

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final product is comforting. But then... We hit

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1859. The year everything changes. Yes. Charles

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Darwin publishes on the origin of species. And

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this is a piece of trivia I absolutely love.

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The most famous book in the history of biology

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has exactly one illustration in it. Just one.

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And it's not a picture of animals. It's an abstract

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chart. Yeah. Unlike the lush botanical trees

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we just talked about, this thing looks like a

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subway map. It does. And that abstraction is

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its genius. Darwin labels horizontal lines with

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distinct species. just hypothetical letters,

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A through L. And the vertical lines represent

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time. Specifically, thousands of generations,

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right? Yes. So this is technically the very first

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time tree. Because it combines phylogeny, the

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relationships with time. Exactly. Before this,

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maps were about how things looked. Darwin made

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it a mathematical process showing branching descent

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over time. Walk us through the mechanics of the

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diagram. What happens to species A through L?

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Well, look at species A. Over 10 ,000 generations,

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it produces multiple distinct varieties. The

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lines split and branch out, but then look at

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species E, F, and G. Their lines just stop. Right.

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They go extinct. Previous thinkers saw extinction

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as a mistake. Darwin baked it into the very mechanics

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of life. Some lines branch out and diversify.

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Others stop. That brings up the specific text

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Darwin used, which really contrasts with how

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we usually think of the tree of life. We often

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think of it as this peaceful Kumbaya family tree.

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Right. Very harmonious. But Barwin's metaphor

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is actually incredibly violent. It really is.

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I have the quote right here. He writes, at each

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period of growth, all the growing twigs have

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tried to branch out on all sides and to overtop

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and kill the surrounding twigs. Overtop and kill.

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That is definitely not a family reunion. No,

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it represents a great battle for life. He says

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the crust of the earth is filled with dead and

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broken branches, which are the fossils. So it's

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a graveyard of what didn't make it. But Darwin

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kept it abstract with the letters. He didn't

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write human or monkey on that chart. He left

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the labeling to Ernst Hackel. Who coined the

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term phylogeny? He did. And in 1866, Hackel decides

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to take Darwin's abstract lines and turn them

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into literal trees. And these contrast so sharply

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with Darwin. Hackel has these three kingdoms,

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Plantae and Amalia, and Partista. Yes, the 1866

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model. Partista was a big deal, acknowledging

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microscopic life. But the real showstopper is

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his 1879 pedigree of man. Pedigree? That's a

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loaded word. It traces all life. Tumonia at the

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very bottom, and places mention man explicitly

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at the very top of the highest branch. So even

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after Darwin showed that evolution is this sprawling

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competitive bush where every surviving twig is

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technically equal, Haeckel just puts us right

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back on the pinnacle. It's that intense anthropocentrism.

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Putting humans at the top is just so visually

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compelling to people. OK, so when does that image

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finally break? When do we realize we aren't the

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top of the tree? It takes until the 1990s. This

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is the molecular turn. Right, because before

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that, everyone is grouping things based on morphology,

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how they look. Exactly. But in 1990, Carl Woese,

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Otto Kandler, and Mark Willis propose an entirely

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new tree based on molecular phylogenetics. Genetics,

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not just how things look. Because you can't really

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tell bacteria apart just by looking at them.

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No, they just look like tiny rods or dots. So

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these researchers looked at the genetic code,

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and they proposed the three domains of life.

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Bacteria, archaea, and eukarya. Right, and this

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is a big shift. The tree gets divided into those

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three. And recent studies, like a major one in

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2016, show that the tree of life is mainly composed

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of bacteria. Meaning we, the eukaryotes, are

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just a tiny, tiny little branch. A hidden majority

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of life is microscopic. The tools we have now,

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like the time tree database and the open tree

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of life, operate on a massive scale. The 2022

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time tree includes nearly 150 ,000 species. I've

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seen the visuals for these modern trees. They're

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often spiral or circular. They have to be just

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to fit all that genetic data on a screen. So

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we finally mapped the tree using math and genetics.

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We figured it out. This is where the plot twist

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comes in. Right. The tree is broken. It's broken.

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The whole tree metaphor actually fails for prokaryotes,

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which are the bacteria and archaea. Because of

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horizontal gene transfer, HGT. Let's dig into

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that. What exactly is the mechanism here? Think

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about a regular tree. A branch splits, and those

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two new branches never touch again. They just

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grow apart. That's vertical descent, parent to

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child. Right. But organisms like bacteria, and

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even some animals like these tiny creatures called

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boletiloid rotifers, they don't just pass genes

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vertically. They can freely pass genetic info

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horizontally to completely unrelated neighbors.

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So if genes are moving sideways between different

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branches, the lines of descent cross over each

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other. It's no longer a bifurcating tree where

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branches just split. It becomes a web, a network.

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And it gets even wilder when we look at our own

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deep roots. There's this consensus now about

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the fusion at the root of the tree. Yes. Eukaryotes,

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which includes us, plants, animals, we arose

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from the fusion between bacteria and archaea.

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So the very trunk of our part of the tree is

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actually two branches merging together. Exactly.

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It means the tree cannot be fully bifurcating

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at this crucial node. Branches came together.

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Which completely shatters the tree metaphor.

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Trees don't do that. They really don't. Because

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of this complexity, some scientists are now arguing

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that unrooted phylogenetic networks are much

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more accurate than a single tree. Unrooted phylogenetic

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networks. It doesn't have quite the same poetic

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ring to it as the tree of life. No, it doesn't

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look as nice on a t -shirt. But it's closer to

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the reality of the data. It's incredible to look

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back at this journey. We started with Augier's

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God -ordered static chart. Then Darwin's violent

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battle of twigs. Right, and Heckel's pedigree

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putting us right at the top. And now we're looking

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at a messy modern genetic web. It completely

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changes how we see ourselves. We aren't the lonely

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figure standing at the pinnacle of a pine tree.

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We are just a small part of a massive tangled

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web dominated by microscopic life. And it's a

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web that is constantly sharing and swapping information,

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which brings up a really provocative thought

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for you to mull over after listening. If the

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very roots of this structure, the bacteria and

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archaea, are constantly swapping genes sideways,

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and our own branch comes from a fusion of those

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organisms, maybe the Tree of Life is just the

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wrong metaphor entirely. It certainly seems that

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way. Perhaps we need a new image for the history

00:12:32.539 --> 00:12:35.039
of life that allows for branches to fuse back

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together. Something like a massive river delta

00:12:37.679 --> 00:12:40.539
where streams endlessly split off, cross over,

00:12:40.639 --> 00:12:42.720
and merge back into one another. It really makes

00:12:42.720 --> 00:12:45.519
you wonder if life is just an endless tangled

00:12:45.519 --> 00:12:48.480
network of shared code. Is the concept of a totally

00:12:48.480 --> 00:12:51.279
distinct separate species just a convenient human

00:12:51.279 --> 00:12:54.080
illusion? That's a deep one to end on. It really

00:12:54.080 --> 00:12:56.700
is something to think about next time you see

00:12:56.700 --> 00:12:59.580
that classic tree diagram. Thanks for joining

00:12:59.580 --> 00:13:02.000
us on this deep dive. It was a pleasure. We'll

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see you next time