Chapter 10: Imperfection of the Geological Record

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Welcome back to the Deep Dive.

Today we are, well, we're taking on a massive intellectual challenge.

We really are.

We're opening up the defense brief for one of the most revolutionary theories in history.

We're diving into chapter 10 of Charles Darwin's On the Origin of Species.

And this chapter is all about tackling what Darwin himself called the most obvious and serious objection to natural selection.

Which is?

It's the problem of the missing links, basically.

If his theory says species change gradually over these huge spans of time, blending one into the next,

well, why does nature look so distinct today?

Right.

Why isn't it all just a blur?

Exactly.

And crucially for the historical side of it, why isn't the geological record just full of all these intermediate forms?

Where are they?

That's the question, isn't it?

It's the central attack critics use right away.

And Darwin knew, he knew that if the fossil record really didn't show any of these transitional forms that his theory needs,

the whole thing would just fall apart.

It would.

So our mission today is to follow his argument, step by step, for why these gaps aren't flaw in the theory.

But a flaw in the record itself, an extreme imperfection, as he puts it.

We have to show that, you know, the time was there, but that the process of geology is just so destructive, so intermittent, that finding a fossil is, it's like winning the lottery.

It's the exception, not the rule.

So this chapter is basically the defense argument, proving the main witness, the earth itself, is unreliable.

Not that the crime, so to speak, of evolution didn't happen.

That's a perfect way to put it.

Okay, so let's start where he starts with a really fundamental piece of logic.

Darwin says when people look for a missing link, they almost always make a huge mistake in what they're picturing.

Yeah, they have this, this wholly false view, as he calls it.

What is that view?

People instinctively imagine a form that's directly connecting two species we see alive today.

Like something that's halfway between a horse and a and species B today,

and expecting a blend of the two.

Which would mean a horse came directly from an unchanged taper or something, which makes no sense.

Right.

The real logic is, you have to look for forms that are intermediate between each of those modern species and a common, probably unknown ancestor.

A progenitor, right.

A progenitor that's probably extinct and might look very different from both of its descendants.

Yeah.

You're looking for the fork in the road, not trying to blend two cars that took different paths from that fork.

And this is where his example with domestic pigeons is so, so good.

It just makes that abstract idea totally clear.

Oh, it's brilliant.

He takes two really out there breeds, the fantail pigeon with its crazy sand -like tail, 30 or 40 feathers.

That's a powder pigeon.

Right, with that huge, almost cartoonishly large crop.

And he says, look, we know for a fact that both of these came from the common wild rock pigeon.

Okay.

So if we had a perfect fossil record of pigeon breeding, what would we actually find?

You'd find a smooth chain of fossils connecting the modern fantail back through slightly less fanned out tails, all the way to the original rock pigeon.

And a separate chain for the powder, a totally separate chain, connecting it back through slightly less puffy crops, all the way to that same rock pigeon ancestor.

But, and this is the key, you would not expect to find a pigeon with a slightly fan tail and a slightly puffy crop.

Never, because that wasn't the path of descent.

The lines split first, then they were modified independently.

So applying that to the horse and taper.

Yeah.

We shouldn't be looking for a blend.

We should be looking for two separate lines going back in time until they meet at some ancient parent form.

And Darwin adds, that parent form was probably wiped out a long, long time ago.

Which brings us to the next point about why these links are so rare.

It's the mechanism of natural selection itself.

Yes.

The theory requires competition, and that competition is fiercest between the new improved version and the old one.

The child and the parent, as he puts it.

Exactly.

A new improved variety will almost always supplant the older, unimproved parent form.

It's just better adapted, more successful.

It wins.

And in winning, it drives the parent and all the little steps in between to extinction.

So the survival of the fittest is also the act of erasure of the links.

The very success of the descendants means the parents get wiped out.

Wow.

He says the extermination has been total.

The number of intermediate links that must have existed, if the theory is true,

is inconceivably great.

They lived, but then they were just annihilated.

So the gaps exist because successful species naturally destroy their own historical records.

That's the argument.

Okay.

So that leads us right into the second huge objection Darwin had to deal with.

Time.

Right.

Critics would look at the rocks and say, look, even if evolution is slow, there just hasn't been enough time for all this change to happen.

It's a really important point to tackle.

And Darwin basically says, okay, before we go any further, you have to accept one huge idea.

Sir Charles Lyell's idea.

Yes.

The concept of uniformitarianism from his geology.

Darwin pretty much tells the reader, if you can't get on board with this idea of vast, slow cumulative change, you should at once close this volume.

And that idea simply is that the slow geological forces we see today, a bit of erosion here, a bit of sediment there are the same forces that shape the entire planet just acting over, well, astronomical time scales.

That's it.

And Darwin says the best way to get a sense of that time is to look at denudation, the wearing away of rock.

Because the thickness of all the sedimentary rock we see, that's just a measure of how much rock was worn away from somewhere else.

Exactly.

And to show how slow it is, he starts with coastal erosion, you know, waves crashing against cliffs.

We think of that as this incredibly powerful, fast force.

But he points out it's actually pretty ineffective a lot of the time.

Very ineffective.

The waves only hit the base of the cliff for a couple of hours a day at high tide, and they only really do damage if they're full of sand and pebbles.

And he has that amazing little observation about the boulders at the bottom of cliffs.

Oh, that's brilliant.

He says you see these huge rounded boulders just stickly closed by marine productions covered in sea life.

Which means they're not getting rolled around very often at all.

Right.

It proves how slow and sporadic that kind of erosion really is.

But then he pulls the rug out and says that's not even the main engine of change.

The real power is in subaerial degradation.

The slow invisible stuff, air, rainwater with a bit of carbonic acid, frost, the stuff that's constantly silently acting on entire continents.

And the evidence for this is just the sheer scale of the landscape.

He uses the Wieldon District in England as a case study.

He does.

Geologists had shown that these huge cliffs, these escarpments that look just like ancient coastlines, they're actually made of a single type of rock.

So they couldn't have been carved by waves.

No.

They were formed because

softer rock all around them just slowly wore away over millions of years, leaving the harder rock sticking out.

And when you see that, this massive result from an invisible cause, it forces your mind to grapple with the vast duration of time.

That scale is just driven home when he talks about faults.

These are just huge cracks in the crust where rock layers have shifted, sometimes thousands of feet.

He mentions the Craven Fault with the displacement of up to 20 ,000 feet.

And another one in Wales that was maybe

12 ,000 feet.

12 ,000 feet.

That's over two miles of vertical shift.

It's insane.

It is.

But here's the kicker.

Despite these gigantic movements deep underground, the surface of the land above is often so completely planed down by that slow sub -aerial weathering that you can't even see it.

It's externally invisible.

The surface is just flat, which means the two miles of solid rock was just stripped away, atom by atom,

by rain and frost.

The amount of missing rock is the proof of the time that's passed.

And then if you look at the rock that is there, the stuff that was deposited, the numbers are just as mind -boggling.

Professor Ramsey's estimate for Great Britain was 72 ,584 feet.

Almost 14 miles of rock all stacked up.

But even that's not the whole story.

Not even close.

Darwin stresses that between each of those formations, there are these huge blank periods of gaps where nothing was deposited or where deposits were made and then destroyed.

The time we can't see is way, way longer than the time we can.

So how do you even start to imagine that kind of timescale?

He brings in a calculation from a Mr.

Kroll, right?

Yeah, Kroll tried to quantify it based on how much sediment rivers carry to the sea each year.

And his calculation was that it would take about six million years to wear away just 1 ,000 feet of solid rock from an area.

Six million years for 1 ,000 feet.

And Darwin says, fine, you can have that or quarter it.

It doesn't matter.

You're still talking millions and millions of years.

And to make that number feel real, Kroll came up with that paper strip analogy.

I love this.

It's so good.

Okay, so you take a strip of paper that's 83 feet and four inches long.

Right, a really long strip.

That whole length represents one million years.

Now on that 83 -foot strip,

a section just one -tenth of an inch long, barely visible,

that represents 100 years.

A century.

So basically all of recorded human history is just this tiny sliver at the end of an enormous ribbon.

Exactly.

And then he contrasts that.

In that 100 -year span, that tenth of an inch, a couple of good breeders using methodical selection can create a whole new sub -breed of sheep or cattle.

So human action can create huge change on a timescale that is geologically meaningless.

Precisely.

And natural selection is so much slower, even than our unconscious selection.

It needs new niches in nature to open up.

It takes immense amounts of time.

But, Darwin argues, the evidence of denudation proves that geology provides that time in abundance.

Okay, so the time is there.

Now we have to get back to why the record of that time is so, so gappy.

Why don't we find these transitional forms?

Right.

And he starts by just pointing out that even the paleontologists of his day agreed that the imperfect.

Many species were known from just a single broken fossil.

So what are the basic reasons for that?

Well, the first things are obvious.

We've only explored a tiny fraction of the planet geologically.

And the biology of the creature matters.

A huge amount.

Anything totally soft just can't be preserved.

It rots.

You need hard parts, shells, or bones, and they have to be buried fast.

And Darwin says we have this wrong idea that the ocean floor is just constantly being covered in sediment, ready to bury things.

A completely erroneous view.

He says large parts of the deep ocean are crystal clear.

The sea bottom can just sit there unchanged for ages.

Nothing gets buried.

So huge amounts of time can pass with just no record being made at all.

None.

And even if something does get buried, say in sand, if that rock layer is later pushed up and exposed to rain,

the acidic water can just dissolve the fossil completely.

So a fossil can be destroyed millions of years after it was formed.

Absolutely.

And then there's habitat bias.

This is a huge one.

Think about animals that live right on the shoreline.

Like barnacles.

His favorite example is serapedes.

They are everywhere, in infinite numbers.

But they live in that zone between high and low tide, which is constantly being smashed by waves.

It's a destructive environment.

So even though they're incredibly abundant, they almost never end up as fossils.

Exactly.

Their lifestyle works against their preservation.

And on the flip side, you can have these incredibly thick rock formations that took eons to form, but they're just empty.

The flish formation.

Yeah.

Up to 6 ,000 feet thick, 300 miles long, and almost completely destitute of organic remains.

So you can have immense time passing with a perfect record of the sediment, but just no life being preserved in it.

And for land animals, it's even worse.

So much worse.

Mammals, land shells, they rarely get washed out to see where most of the preservation happens.

You mostly find them in caves or old lake beds, and those are super rare in the really old rock layers.

The whole system is just fundamentally biased toward preserving marine animals with hard shells.

Okay.

So patchy preservation is one thing, but he says the bigger issue is the intermittence of the record.

That the formations aren't consecutive chapters in a book.

No, they're separated by these wide intervals of time.

Even his critics admitted this was true.

It's just that when you see them all laid out in a chart, Silurian, Devonian, Carboniferous, it creates this illusion of a continuous story.

An illusion he shatters by looking at the geology of different continents.

Right.

He says,

look at Russia.

You'll find these massive blank spots in their geological record.

And during the time represented by those gaps,

huge piles of rock full of new life were being deposited in North America.

So a local geologist would have no idea how much time was missing between the layers in their own backyard.

None at all.

And this leads to the absolute core of his argument here.

The paradox of subsidence versus elevation.

This is a really key concept.

It's crucial.

He looks at what happens on a coastline that's slowly rising, like the West coast of South America.

As it elevates, the waves are constantly wearing away the shoreline deposits.

They're destroyed as fast as they're made.

Exactly.

So periods of elevation are destructive.

They create a blank in the geological record.

But, and here's paradox, those are the best times for new species to form, right?

More land, new coastlines, new opportunities.

That's the tragic irony.

The time that is most favorable for evolution results in a blank geological record.

So when do we get a good record?

Only during the opposite movement.

Subsidence, when the seafloor is sinking.

Why then?

Because as the bottom slowly sinks, sediment can pile up continuously, keeping the water shallow.

And that's perfect for lots of to thrive and get buried quickly before they can decay.

Subsidence is the key to preservation.

But subsidence means less land, more crowded conditions.

And therefore more extinction.

The very conditions you need to create a rich fossil record are the ones that are bad for creating a rich diversity of life.

It's an incredible paradox.

It is.

It means the record we have is fundamentally biased.

We're only seeing snapshots from these periods of sinking and preservation, separated by vast unrecorded ages of uplift and rapid evolution that left no trace.

We should pause on that idea of destruction because Darwin really wants us to feel the sheer scale of it.

He says all geologists know that huge amounts of rock have been stripped away.

Enormous amounts.

And the best way to see this is to look at the parts of the continents that are made of granite or nice, these deep metamorphic basement rocks.

By definition, those rocks had to form deep underground under incredible heat and pressure.

Right.

So if we see them at the surface today over huge areas,

it means that whatever was on top of them,

miles and miles of rock potentially is just gone.

Completely erased.

And the scale is just

continental.

He talks about a granite region in South America that was 19 times the size of Switzerland,

another area equal to Spain, France, Italy, and Britain combined.

A whole continent's worth of history just scrubbed clean.

Exactly.

He says it proves whole formations have completely been denuded, with not a wreck left behind.

The destruction isn't just a missing page hero there.

It's the complete annihilation of entire volumes of the book.

That's it.

It's those slow sub -aerial forces we talked about grinding away not just mountains but entire geological eras.

This is maybe his most powerful argument for why the links are missing.

They were ground into dust and washed out to sea.

Okay, let's zoom in on a more specific problem critics had.

They'd say fine, the global record is a mess, but why don't we see gradual change within one thick rock formation?

It's a fair question.

Surely if a layer took millions of years to form,

you should see a species slowly changing from bottom to top.

And he does admit there are a few rare examples where you do see that.

Very few.

Like some fossil snails in a Swiss lakebed.

But they're the exception that proves the rule.

So what's the first big reason for why we don't usually see it?

It's about time again.

The time it takes for one species to morph into another is probably much, much longer than the time it takes for a single formation to be deposited.

And there's another complication.

Migration.

A big one.

Just because a fossil suddenly appears at the bottom of a formation, that doesn't mean it evolved there.

It probably just moved into the area.

Ah, so a species might appear, stick around for a while, and then disappear from the local record when the climate changes.

Exactly.

It gives you a false impression of how long that species actually existed on the planet.

The slow gradual change was happening somewhere else entirely.

And to get a perfect record of that change, you'd need the species to stay in one place while sediment was being laid down continuously for a huge amount of time.

Which brings us back to that balance problem.

It's just a really rare thing for the rate of subsidence and the rate of sediment supply to be perfectly balanced for that long.

So even a thick solid looking formation is probably not continuous.

Almost certainly not.

He uses that incredible example from the coal beds in Nova Scotia.

The fossil trees.

The fossil trees.

They found a formation 1 ,400 feet thick.

But inside it, there were fossilized trees standing upright at 68 different levels.

68 different forests.

Which means that bit of land had to be dryland 68 times and then subside and be buried 68 times.

It proves there were huge hidden time gaps that you'd never see otherwise.

Wow.

So even the most complete looking records are full of holes.

They are.

And the final reason we don't see these links is, well, it's about human bias.

The way naturalists classify things.

How does that work?

Well, there's no golden rule for telling a species from a variety.

So if a paleontologist finds form A in an old rock layer and then slightly different forms B and C in a younger layer, they'll call them three different species.

They will.

Unless you can them a perfect unbroken chain of tiny intermediate steps connecting them.

And since we've already established that those chains are almost never preserved.

The original parent and its descendants get classified as separate distinct species.

The gaps in the record create the illusion of sudden jumps.

That's it.

Exactly.

He asks, you know, could a future geologist finding fossils of our modern dogs and sheep ever prove they came from a single stock without our written records?

It'd be impossible.

Improbable in the highest degree, he says.

The failure to find the links isn't a failure of the theory.

It's a direct consequence of how the world works.

All right.

So let's scale up.

What about the sudden appearance of whole groups of species,

whole families or genera that seem to just pop into existence?

This was a huge objection from people like Agassiz.

They said it was fatal to the theory because developing a whole new body plan should be incredibly slow.

It can't just happen overnight.

Right.

But Darwin's counter here is simple and powerful.

The worthlessness of negative evidence.

Meaning just because you haven't found it doesn't mean it didn't exist.

Yes.

Just because we haven't found, say, fish fossils below a certain layer in Europe doesn't mean fish weren't thriving for millions of years somewhere else on the planet that we haven't looked.

We just overrate how complete our little European and American records are.

Constantly.

A group could have evolved and diversified for ages in some isolated part of the world.

And then when conditions were right, spread rapidly across the globe.

To us in our limited record, it looks like they were suddenly created.

And he connects this to big evolutionary innovations like flight.

Right.

The transition to flight might take a really long time.

And all those weird intermediate forms might be confined to one small region.

But once flight is perfected.

Then the new group can explode,

diversify into many forms, and spread everywhere really quickly.

And we only see them after that explosion.

This is where he pushes back against the idea that a transitional limb, something that's not quite an arm and not quite a wing, would be useless.

Yeah, that was Professor Pick -Ted's objection.

And Darwin just points to the penguin.

A living, breathing, incredibly successful example.

A perfect example.

The penguin's flipper is in that precise intermediate state.

It can't fly, but it uses it to swim with the enormous power.

It's holding its own, victoriously, in the battle for life.

So the intermediate form isn't useless.

It's just adapted for something different.

And it provides the raw material for future change.

Darwin asks, what's so hard about imagining a descendant of a penguin learning to flap on the water like a logger -headed duck does and eventually taking to the air?

The pathway is there.

And to really hammer home this point about negative evidence, he just rattles off a list of recent discoveries that changed everything.

It's a great list.

Mammals, for instance.

They were thought to have appeared suddenly in the tertiary.

But then they started finding them deeper in the secondary period.

The goalposts just kept moving back.

Same with birds.

Same with birds.

First they were Eocene, then they were found in the Cretaceous green sand, and then came the bombshell.

Archaeopteryx.

Archaeopteryx.

This incredible fossil from Germany with a lizard -like tail and claws on its wings.

It was a perfect intermediate form that completely shattered the idea that birds appeared suddenly.

And he even includes a personal story of his own mistake.

I love that he does this.

It's so honest.

With the barnacles, his own area of expertise, he couldn't find any in the secondary rocks and concluded they must have suddenly evolved in the tertiary.

He said it was a sore trouble for his theory.

And then what happened?

Almost as soon as his book was published, someone found a perfect fossil of a modern barnacle genus deep in the chalk of Belgium,

squarely in the secondary period.

The ultimate proof that you can't trust the absence of evidence.

The next fossil discovery can rewrite everything.

Everything.

He says it's just rash to dogmatize on the succession of organic forms when we know so little about the world outside of Europe and the US.

Okay, now we come to it.

What Darwin himself calls the most serious difficulty.

The big one.

The great void.

The fact that in the lowest known fossil -bearing rocks, the Cambrian strata, you suddenly get this explosion of complex life.

Trilobites, nautilus.

All these highly organized creatures seemingly appearing from nowhere.

And the logic is inescapable.

If you have a complex trilobite, it must have evolved from a simpler crustacean ancestor.

Which must have evolved from something even simpler.

Right, so it implies there's a period of time before the Cambrian, or maybe even longer than the entire time from the Cambrian to today, when the world must have been swarming with living creatures.

A whole missing prequel to the book of life.

But where are the fossils?

Where are the rocks from that time?

This was a huge problem.

And the early time estimates made it even worse.

Darwin mentions Kroll's estimate of maybe 60 million years since the Cambrian.

And Darwin says that's way too short.

He says it's a very short time.

For all the evolution we know has happened since then.

So the missing pre -Cambrian history has to be absolutely colossal.

But there were clues even then that life was much older.

Oh yes.

The idea that life started in the Silurian was already being disproven.

People were finding trilobites in lower and lower Cambrian beds.

And then there was the big discovery of Eosun in Canada.

And this was found in rocks that were much, much older.

The Laurentian formation.

And the geologist, Sir William Logan, estimated that this formation alone might be thicker than all the rocks that came after it combined.

Thicker than everything from the Cambrian to the present day?

So the time was there.

The proof of life, however primitive, was there.

The question got sharper.

Why are these vast ancient piles of rock almost completely empty of fossils?

It can't all have just been destroyed because the Cambrian rocks right on top of them are preserved just fine.

Exactly.

So Darwin offers this grand, almost epic hypothesis.

It's about shifting continents and oceans.

That's the core of it.

He suggests that where we have continents now, Europe, North America, we've mostly had land since the Cambrian.

Those were areas of uplift, erosion, and patchy records.

So where was all the pre -Cambrian life?

Maybe on continents that are now at the bottom of our oceans.

The Atlantic, the Pacific.

Those ancient continents would have had their own ecosystems, their own sediments.

And that record is now either inaccessible or has been completely destroyed.

And how would it have been destroyed?

By being buried so deep.

He suggests that strata that sank for miles toward the Earth's center under the immense pressure of the ocean above them would have undergone far more metamorphic action.

So heat and pressure?

Extreme heat and pressure that would just cook the rock and obliterate any fossil it contained.

So those vast, naked areas of granite we talked about earlier?

He suggests they might be the completely metamorphosed, scrubbed, clean remnants of those pre -Cambrian continents.

They are the pages that were heated, pressed, and erased until not a single word was left.

It's an incredible idea.

He admits it's inexplicable with the evidence they had, but he provides a logical, geological mechanism for how the greatest library in history could have been burned to the ground.

It's an amazing piece of reasoning.

So this whole deep dive into Chapter 10, it really shows Darwin as more than just a biologist.

He's a master strategist.

He's using geology, the field of his biggest critics, as the foundation for his defense.

He completely reframes the problem.

He takes their three biggest objections,

no transitional forms, sudden appearance of groups, the Cambrian void, and shows how they are the expected results of the Earth's history, not flaws in his theory.

He changes how we even look at the fossil record.

The links are missing because natural selection itself gets rid of them.

And geological processes uplift, erosion, subsidence, destroy the evidence of what's left.

Apparent suddenness is just a trick of our incomplete local perspective.

And it all builds to that final, incredible analogy, one of the most famous passages in all of science.

The geological record, he says, should be seen as A history of the world imperfectly kept and written in a changing dialect.

Of this history, we possess the last volume alone, relating only to two or three countries.

Of this volume, only here and there a short chapter has been preserved.

And of each page, only here and there a few lines.

And through that lens, all those massive difficulties, they are greatly diminish or even disappear.

I think for us, the biggest takeaway is just that monumental scale of what we don't know.

The history of life isn't written in the miles of rock we can see.

It's written in the silence of the gaps, in the time represented by a flat, eroded plane of granite.

The history of life is defined by his absences.

A truly profound and humbling thought to end on.

Thank you so much for joining us on this deep dive into Darwin's brilliant defense.