The Evolutionary Epic: Crash Course Big History #5

Summary

In this episode of Crash Course Big History, John Green explores the evolutionary journey of life on Earth, highlighting natural selection as the driving force. From Darwin's voyage on the HMS Beagle to the discovery of finches' beaks adaptation, the video delves into how species evolve to fit their environments. It discusses mass extinctions and adaptive radiations, showcasing the resilience and diversity of life. The script also touches on the scientific evidence supporting evolution, including fossils, DNA sequencing, and radiometric dating, emphasizing its status as a well-established scientific theory.

Takeaways

• 🌿 **Evolutionary Epic**: The script narrates the vast evolutionary history of life on Earth, spanning 3.8 billion years with numerous species and extinctions.
• 🐦 **Natural Selection**: Charles Darwin's theory of evolution by natural selection is central, explaining how species adapt to their environments over generations.
• 🌍 **Darwin's Voyage**: Darwin's travels on the HMS Beagle contributed significantly to his understanding of evolution, particularly his observations of finches in the Galapagos Islands.
• 📚 **Scientific Method**: Darwin's meticulous approach to gathering evidence over two decades before publishing 'On the Origin of Species' exemplifies the scientific method.
• 🧬 **Genetic Mutations**: The script explains that while mutations occur randomly, the selection of these traits is non-random and determined by environmental pressures.
• 🦕 **Extinctions and Adaptive Radiation**: Mass extinctions have periodically cleared ecological niches, allowing for new species to evolve rapidly in what's known as adaptive radiation.
• 🌱 **Plant Colonization**: The move of plants from aquatic to terrestrial environments and the subsequent increase in atmospheric oxygen levels had profound effects on the evolution of life.
• 🦋 **Arthropod Evolution**: Arthropods, taking advantage of the new oxygen-rich environment, evolved into larger forms during the Silurian period.
• 🐊 **Reptiles and Mammals**: The script outlines the rise of reptiles during the Carboniferous period and the evolution of synapsids, the ancestors of mammals, during the Permian.
• 🌋 **Great Dying**: The most significant extinction event, the Great Dying at the end of the Permian, led to a massive adaptive radiation of reptiles, including the dinosaurs.
• 🦖 **Dinosaur Dominance**: Dinosaurs ruled the Earth for 135 million years, a period much longer than the existence of humans, until their extinction event大约65 million years ago.

Q & A

What is the main theme of the Crash Course Big History video?

The main theme of the video is the evolutionary epic, which encompasses the story of various species, their adaptations, extinctions, and the process of natural selection over approximately 3.8 billion years.

Who is the key figure in the development of the theory of evolution by natural selection discussed in the video?

Charles Darwin is the key figure in the development of the theory of evolution by natural selection. His observations and studies, particularly during his voyage on the HMS Beagle, led to the formulation of this theory.

What is the significance of the finches' beaks that Darwin collected from the Galapagos Islands?

The finches' beaks that Darwin collected from the Galapagos Islands were subtly adapted to their different environments and food sources, which was a pivotal observation that contributed to his theory of natural selection.

What does the term 'natural selection' mean in the context of the video?

In the context of the video, 'natural selection' refers to the process by which organisms with traits that are advantageous for their environment are more likely to survive and reproduce, passing those traits on to their offspring.

Why did Darwin's theory of evolution by natural selection cause such a significant reaction when published?

Darwin's theory caused a significant reaction because it offered a scientific explanation for the origin of species that seemed perfectly adapted to their environment, challenging prevailing beliefs and providing a new understanding of life's diversity.

What is the difference between 'survival of the fittest' and 'natural selection' as discussed in the video?

The term 'survival of the fittest' was coined by Herbert Spencer and is often associated with Social Darwinism, whereas 'natural selection' is Darwin's original phrase that more accurately describes the non-random process by which advantageous traits are selected for in the context of an organism's environment.

How does the video explain the misconception that evolution is 'just a theory'?

The video clarifies that in scientific terms, a theory is a well-substantiated explanation of some aspect of the natural world that is backed by extensive evidence and has been repeatedly tested, unlike the everyday meaning of a guess or speculation.

What evidence supports the theory of evolution as presented in the video?

The video mentions several types of evidence that support evolution: fossil records showing shared traits and lines of descent, DNA sequencing revealing genetic relationships, radiometric dating of fossils, and the geographical distribution of species.

What is the significance of the 'Snowball Earth' event mentioned in the video?

The 'Snowball Earth' event refers to a period when the Earth was almost entirely frozen over, which likely led to increased symbiosis among life forms as they struggled to survive in harsh conditions, setting the stage for the development of complex multicellular life.

What is an adaptive radiation and how is it related to mass extinctions as discussed in the video?

Adaptive radiation is a process where surviving species rapidly evolve new traits to fill empty ecological niches after a mass extinction event. It allows for the diversification of life as species adapt to new environments and resources.

How does the video describe the role of symbiosis in the evolution of complex life forms?

The video describes symbiosis as a relationship where single-celled eukaryotes began to work together, with each cell performing a function that aided the survival of both, eventually leading to the development of more complex, multicellular organisms.

Outlines

00:00

🌿 Evolution and Natural Selection

This paragraph introduces the concept of evolution by natural selection, the central theme of the video. It discusses Charles Darwin's voyage on the HMS Beagle, his observations of finches in the Galapagos Islands, and how these experiences led to his theory of evolution. The paragraph also explains how overbreeding and resource competition drive natural selection, allowing species with advantageous traits to survive and reproduce. The importance of Darwin's work, his publication of 'On the Origin of Species,' and the concept of 'survival of the fittest' are highlighted. The paragraph emphasizes that evolution is a well-supported scientific theory, backed by evidence such as fossil records, DNA sequencing, and radiometric dating.

05:01

🧬 Symbiosis and Multicellular Life

This section delves into the origins of complex life forms, starting with the symbiosis of single-celled eukaryotes. It describes how these cells cooperated and became interdependent, which was particularly beneficial during harsh conditions like the 'Snowball Earth' event. The paragraph outlines the five mass extinction events and the concept of niches in nature, explaining how species competition and extinction events lead to adaptive radiations. It provides examples of such events, including the rise of mammals after the dinosaurs' extinction. The Ediacaran era's first multicellular organisms, the Cambrian explosion of life, and the Ordovician period's diversification into new niches are also discussed, along with the impact of environmental changes on species evolution.

10:02

🐉 The Rise and Fall of Reptiles

This paragraph covers the evolutionary journey from the first land plants to the dominance of reptiles. It discusses how plants adapted to land environments and how arthropods took advantage of the increased oxygen levels. The Devonian period's rise of forests and the evolution of vertebrates, including amphibians and reptiles, are highlighted. The paragraph also details the Devonian extinction and the subsequent Carboniferous period's increased forestation. It explains how reptiles adapted to the drier Permian environment and how the 'Great Dying' at the end of the Permian era led to an adaptive radiation of giant reptiles. The Triassic period's dry climate favored reptiles, and the extinction event at the end of this period allowed dinosaurs to dominate. The paragraph concludes with the Jurassic and Cretaceous periods, where dinosaurs ruled until their extinction, which paved the way for mammals to evolve and diversify.

Keywords

💡Evolution

Evolution refers to the process by which species of organisms change over time through genetic variation and natural selection. In the video, evolution is central to the narrative of life's development on Earth, with Charles Darwin's theory of evolution by natural selection being a key concept. The video illustrates how species adapt to their environments, such as the different beak shapes of finches on the Galapagos Islands, which is a classic example of natural selection in action.

💡Natural Selection

Natural selection is the mechanism by which organisms with traits that are better suited to their environment tend to survive and produce more offspring, thereby passing on those advantageous traits. The video explains natural selection through Darwin's observations of finches and how their beaks were adapted to different food sources. This concept is fundamental to understanding how species evolve to fit their ecological niches.

💡Extinction

Extinction is the disappearance of a species or group of species. The video script discusses several mass extinction events throughout Earth's history, which have dramatically altered the course of evolution by clearing ecological niches and allowing for new species to emerge and diversify. These events are pivotal in the evolutionary epic as they create opportunities for adaptive radiation.

💡Adaptive Radiation

Adaptive radiation is the rapid diversification of species into new ecological niches following a significant evolutionary event, often after a mass extinction. The video uses the example of mammals after the extinction of dinosaurs to illustrate how adaptive radiation can lead to a burst of new species filling vacant niches.

💡Fossil Evidence

Fossil evidence is the preserved remains or traces of ancient organisms, which provide a record of life on Earth. In the video, fossils are highlighted as crucial evidence supporting the theory of evolution, showing shared traits with extinct species and helping to map lines of descent.

💡DNA Sequencing

DNA sequencing is the process of determining the precise order of nucleotides within a DNA molecule. The video mentions DNA sequencing as a method to understand evolutionary relationships by comparing the genetic material of different species, which can reveal how closely related they are and when they may have diverged from a common ancestor.

💡Symbiosis

Symbiosis is a close, long-term interaction between organisms of different species, where at least one species benefits. The video describes how some eukaryotes began to work together in symbiosis, leading to the development of complex multicellular life. This concept is important for understanding the cooperation and interdependence that can drive evolutionary innovation.

💡Eukaryotes

Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, including plants, animals, and fungi. The video discusses the evolution of eukaryotes and their transition from single-celled to multicellular forms, which was a significant step in the complexity of life on Earth.

💡Prokaryotes

Prokaryotes are single-celled organisms without a nucleus, such as bacteria. The video contrasts prokaryotes with eukaryotes, highlighting the differences in cell structure and function. Understanding prokaryotes is essential for grasping the origins of cellular life and the evolution of more complex organisms.

💡Cambrian Explosion

The Cambrian Explosion refers to a period of rapid evolutionary development when a wide variety of multicellular life forms began to appear in the fossil record. The video script mentions the Cambrian era as a time of significant adaptive radiation, where many new species and body plans emerged, such as the trilobites and Anomalocaris.

💡Niche

A niche is an organism's specific role and position in its environment, which includes how it obtains food, reproduces, and interacts with other species. The video explains how species compete for niches and how mass extinctions can clear niches, leading to adaptive radiations where surviving species evolve to fill the vacant roles.

Highlights

Evolution by natural selection is the keystone of the evolutionary epic.

Charles Darwin's voyage on the HMS Beagle led to his discovery of finches with beaks adapted to their environments.

Darwin's theory of natural selection was published in 'On the Origin of Species' in 1859.

Darwin's theory suggests that species are shaped by a blind but elegant law of nature.

The phrase 'survival of the fittest' was actually coined by Herbert Spencer, not Darwin.

Evolution is a scientific theory backed by extensive evidence, not a mere guess.

Fossil evidence, DNA sequencing, and radiometric dating provide strong support for evolution.

Evolution can be observed in real-time, such as the development of antibiotic-resistant bacteria.

Symbiosis played a crucial role in the evolution of complex multicellular life.

The Earth's history includes five mass extinction events that have shaped the course of evolution.

Adaptive radiation occurs when surviving species rapidly evolve to fill empty niches after mass extinctions.

The Ediacaran era provides the first extensive fossil evidence for multicellular organisms.

The Cambrian era saw an explosion of multicellular life and the rise of predators like Anomalocaris.

In the Ordovician period, the first plants began to colonize land, leading to a rise in atmospheric oxygen.

The Devonian period saw the rise of trees and the arrival of vertebrate ancestors on land.

The Permian era was marked by the Great Dying, the largest extinction event in the past half billion years.

The Triassic period saw the rise of giant reptiles, including the dinosaurs.

The reign of the dinosaurs ended with a massive extinction event caused by an asteroid impact.

The extinction of the dinosaurs allowed mammals to evolve into a vast array of new forms.

Transcripts

Hi, I'm John Green. Welcome to Crash Course Big History.

Today we're gonna be traversing the evolutionary epic - the great story of magnificent beasts,

terrifying predators, quite a lot of extinctions, and countless varieties of evolutionary forms.

It's the ultimate epic - millions upon millions of species playing out a drama that has so

far lasted 3.8 billion years, with 99% of the actors having already left the stage forever.

And you thought finding employment in this job market was tough - you've already won the lottery my friend!

[Theme Music]

The keystone of our story is evolution by natural selection. So, in the 1830's a young

Charles Darwin traveled around the world on the HMS Beagle - inarguably, by the way, the

most important beagle of all time - I apologize Snoopy, but it's true.

Darwin had the rare and amazing opportunity to study a great variety of the world's wildlife,

and upon returning to England he discovered that a variety of finches he had collected on the Galapagos

Islands had beaks that were subtly adapted to their different environments and food sources.

Darwin later combined this idea with the observation of how populations tend to over-breed and

strain their resources - I mean if there's competition for resources in an environment,

then animals with useful traits would survive and pass those traits on to their offspring.

Those who didn't survive long enough to reproduce would have their traits wiped out from the

evolutionary tree - natural selection.

We've talked some on Crash Course Big History about good science, and Darwin was a good scientist.

He worked on his ideas for two decades, systematically finding new evidence to support his case

And then finally in 1859 he published "On the Origin of Species" and it sent shock waves around the world.

The book offered an explanation for why so many species that seemed perfectly adapted

to their environment could have been formed by a blind but elegant law of nature. Darwin's

theory was so elegant yet so effective that his colleague Thomas Huxley exclaimed "How

extremely stupid not to have thought of that!" Side note: if you ever read "On the Origin

of Species" try to get a first edition, because in later editions Darwin made a bunch of revisions

in answer to some critics, but he got it actually more right the first time.

Speaking of which, one of the phrases only included in the later editions and commonly

attributed to Darwin was "survival of the fittest" but that phrase was actually coined

by Herbert Spencer, father of the more troubling Social Darwinism which tried to apply nature's

rather harsh laws to human social orders.

I prefer Darwin's original phrase, "natural selection". Everything from cuckoo birds that

lay their eggs in the nests of other birds to giraffes whose long necks are good for reaching

food in high trees, to humans, whose brains make up for their fragile bodies, are selected for, naturally.

An even better phrase though, would be "non-random selection" or maybe even "non-random elimination."

While all genetic mutations are generated by a random copying error, or random variation

completely beyond the animal's control, the selection of those traits is not random. Successful

variations that allow you to survive and reproduce are determined by the very specific circumstances

of your environment, where elimination (death) might not be far away.

So, the selection of your traits is done by a very specific, and sometimes brutal, list

of criteria. This is why people who say that they don't understand how all animals could

have "evolved by chance" don't really understand how evolution works.

Here's another phrase that doesn't get it right: "evolution is just a theory." In everyday

speech, theory means guess. But in science, a theory is something that was tested time

and time again, explains many different observations, and is backed up by a mountain of evidence.

Evolution is a theory, like gravity is a theory... And you don't go jumping out your window because

gravity is "just a theory." Why are we so certain? Emily knows.

Evolution is one of the most tested, most utilized, and widely-accepted theories in science.

It's backed up by literal tons of fossil evidence, which can show us shared

traits with species that no longer exist, and help us map out lines of descent for creatures around today.

DNA sequencing further tells us about lines of descent, and you can measure the commonality

of the DNA possessed by two animals to tell how closely related they are and when they

may have split off from a common ancestor.

Radiometric dating allows us to assign dates to various fossils, further helping us map out lines of descent.

Then there's the simple fact that extinct species are always found in the same rock

layers you'd expect to find them, which is why you don't see a bunny skeleton in Cambrian

rock layers from half a billion years ago. That's also how we know that Dimetrodon is not a dinosaur.

Closely related species are often geographically distributed near one another. That's not to

mention that we can see evolution happening before our very eyes - whether it be the discovery

of a new species that recently moved into a different environment, the development of

newly adapted bacteria into super bugs, the evolution of new breeds of rapidly reproducing

insects, or the almost constant changes in gene distribution in animal populations all over the world.

So, remember the prokaryotes and the eukaryotes? Gradually, some single cell eukaryotes began

to work together in a thing called symbiosis, where one cell did something in exchange for

another cell doing something else, thus aiding the survival of both. Some eukaryotes became

so co-operative and even interwoven that one cell could not possibly live without the other.

Symbiosis was particularly handy in times of disaster. Around 650 million years ago,

the earth was completely frozen over. Snowball earth was not a great place for life. Many

underwater bacteria survived under the ice and oceans, photosynthesizers may have have

survived in small hot spots where there was liquid water... In such constrained conditions

it's likely that individual cells started to work together more and more.

Now is where we start to blaze through the evolutionary epic of complex multi-cellular life.

Between the start of complex multicellular life and today, there have been 5 mass extinction events.

In nature, species compete in niches, it's also called niches depending on where

you're from, but I call them niches! It's an area of the environment that requires a

special set of skills and traits to extract food and reproduce. When niches are full,

competition is heavy, traits become finely-tuned and evolution generally slows down a little.

But! When a disastrous extinction event wipes out the majority of the animals living in

a niche, the surviving species have room and a lack of competitors to evolve new traits

very fast to fill the niche again in what we call an adaptive radiation. The evolutionary

epic is dotted with periods of niches filling up, being swept clean by disaster and filling

again by new rapidly-evolving species.

Example: for the longest time, dinosaurs ruled the earth and mammals were a puny, timid race

of small shrew-like creatures that stayed out of their way. Sometimes we burrowed in

the ground or only came out at night, or confined our diet to tiny bugs. We could not compete

with dinosaurs in their niches. Then, the dinosaurs were wiped out and mammals were

able to rapidly fill all the empty niches, creating apes and elephants and horses and even whales.

So after snowball earth, the Ediacaran era gives us the first extensive fossil evidence

for multicellular organisms. There were various ancient forms that resemble today's worms,

corals, molluscs, various underwater plants. But then in the Cambrian era, adaptive radiation

really got under way and multicellular life filled thousands upon thousands of niches

unlocked by their new traits. A lot more is just possible for multicelled organisms than

for single-celled ones. Like, not to brag or to bring up my astonishing strength again,

but I can bench much more than a eukaryote!

Some of the most famous creatures the got their start in the Cambrian were trilobites,

these bug-like creatures with exoskeletons that existed in a variety of species and forms,

occasionally in swarms of thousands. And they didn't go extinct for nearly 300 million years

- that's over a thousand times longer than Homo-sapiens have been on the planet.

Also, as my four-year-old son can tell you, the Cambrian era had predators like Anomalocaris,

which reached sizes of nearly a meter long, with razor-sharp teeth and grasping limbs.

By the time of the Ordovician period, photosynthesizers were making their first tentative steps out

of the sea into a new niche - the land. Plants colonized coastlines first, and then gradually,

over millions and millions of years moved further and further inland. In the oceans,

life continued to be abundant, with fish and sharks multiplying into a variety of forms.

And there were all kinds of crazy life-forms, like underwater scorpions that were 2½ meters long!

I mean, for the first 100 million years of complex evolution, a mind-boggling diversity

of creatures was emerging. But that also meant all the niches on the planet were getting

very full, and many competitors in the same niche made it difficult for a new species

to enter it with ease! And then came extinction! I feel like extinction's gonna be a thing,

Stan. Is there any way we can make a thing for extinction?

Yes!

Ordovician Earth went through first a major freezing period, killing off many warm-water

species, and then a radical heating period, killing off many cold-water species. Many

ecological niches were swept clean, and this removal of competition meant that new species

could enter empty niches and evolve rapidly in one of those adaptive radiations. There

was also incentive to move out of the seas and on to the land.

In the Silurian period, one of those groups that evolved rapidly by filling terrestrial

niches, was the arthropods, those exoskeleton species, and the ancestors of many of today's

bugs. Since plants continued to colonize the land and more and more of the Earth's surface

was becoming forested, that increase in the number of photosynthesizers increased the

percentage of oxygen in the atmosphere, to between 30 and 35%. Today, it's approximately 21%.

Arthropods came out of the sea, started filling niches on land, and their metabolism took

advantage of this all-you-can-respire oxygen buffet, growing to enormous sizes like a dragonfly

with a meter-long wing span or a scorpion 1.8 meters long. Again with the scorpions!

In the early Devonian period, the forests of the earth were composed of mosses, ferns,

and short shrubs. Some plants eventually evolved a woody covering which provided some back

support and allowed them to grow taller and taller and compete with others in their niche

by grasping higher and higher for sunlight. In the first episode,

we did promise to explain the existence of trees. Bingo!

Also, by the Devonian, our vertebrate ancestors had arrived on land. Unlike Arthropods, vertebrate

skeletons are on the inside and our skin is more porous, making it easier for water to

escape. This limited our ability to fill land-based niches at first, we were amphibious. From

this amphibious ancestor, all tetrapods gained their characteristic skeletal structure: Four limbs, five digits.

And then once again, extinction.

Scientists debate about what caused the Devonian extinction, but once again, a couple of sharp

rises and disappearances from the fossil record shows that the niches were being swept clean.

Again, the number of species on the earth drastically declined. But only temporarily.

The carboniferous intensified the forestation of the planet even more. Meanwhile, amphibians

were filling up coastal niches with competition. So to escape into new niches, some evolved

less porous skin to venture further inland without drying out, and they also laid eggs

with a protective shell, meaning that they didn't have to return to water to hatch their young.

These were reptiles. They were able to fill up the inland world, where real estate

was cheap. And come to think of it, real estate still is cheap.

Next up was the Permian. Many of the forests dried out, creating deserts. Reptiles thrived

in this transformed environment with less competition from the forest and river dwellers.

Also during this time, the ancestor of mammals evolved. I'm talking of course, about the

synapsids. So considering that they were the ancestors of everything from you to your dog

to elephants and whales, it gives you an idea of how radically things can change in just

250 million years of evolution. Because then, once again, at the end of the Permian era,

we have extinction. Often referred to as the Great Dying. It was the single largest extinction

event in the past half billion years. Its cause is still debated, but the most dominant

theory is an environmental disaster caused by volcanoes in Siberia. All told, over 90%

of marine life and 70% of terrestrial life - maybe more - died out. Synapsids were hard

hit, leaving space for a huge adaptive radiation of giant reptiles.

Now we are finally closing in on my son Henry's favorite period of history: The giant reptile

period. In the subsequent Triassic period, the earth's climate was ludicrously dry with

many deserts, and then near the north and south poles, it was warm and wet. Again, dry

climates were a big win for reptiles and our mammalian ancestors got a bit of the short

end of the stick because there was so much reptilian competition in many niches, so we hid on the fringes.

Meanwhile, there were many kinds of giant reptiles in the Triassic of which the dinosaurs

were just one kind until, extinction. The Triassic extinction, possibly due to volcanic

super-eruptions or an asteroid impact emptied a lot of niches of competition and allowed

one particular group of giant reptiles, the dinosaurs, to reign supreme.

And that, finally, led to those periods that are what most people think of when they hear

the word paleontology or the word fossil: The Jurassic and Cretaceous periods.

I'll spare you the snarky commentary about how the T-Rex in Jurassic Park actually lived

in the Cretaceous. Dinosaurs were the dominant animals on this planet for a whopping 135

million years. That's 540 times longer than our species has even existed. To help you

understand this, consider two of the most iconic dinosaurs: Stegosaurus and T-Rex.

Stegosaurus was around in the late Jurassic, T-Rex was around in the late Cretaceous. They

are separated by roughly 88 million years. Humans and T-Rex are separated by less time

than T-Rex and stegosaurus. Approximately 65 million years ago, the reign of the dinosaurs ends,

you guessed it, in extinction. A rock roughly 10 kilometers across crashed into the present

day Yucatan Peninsula with a million times more force than all the nuclear arsenals of the world combined.

Bad for the dinosaurs, but it opened up a lot of niches previously occupied by them.

Many small mammals were able to survive by burrowing or simply requiring less food.

They were then in a position for another adaptive radiation. Small mammals quickly evolved into an

immense variety of larger forms. And so was the story of complex life on earth during the evolutionary epoch.

Next week we will explore the nascent beginnings of a new phase of complexity: the accumulation

of more knowledge, generation after generation. And the intensification of a newfangled evolutionary

invention: Culture. See ya next time.