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In which John Green, Hank Green, and Emily Graslie teach you about evolution. So, in the last 3.8 billion years, life on Earth has evolved from single-celled prokaryotes to the dizzying array of life we have today. So how did all this happen? We'll talk about Darwin, evolution, natural selection, and how we got from there to here, and from then to now.

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John: 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!


John: The keystone of our story is evolution by natural selection. So, in the 1830s 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.

 Hank: (2:38)

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.

 Emily (3:58)

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.

 Hank (5:06)

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.

 John (7:06)

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 multicellular 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 million 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 two and a half 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.

 Hank (9:19)

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 wooded 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. Now 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. 

 John (10:56)

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 enter further inland without drying out, and they also laid eggs with a protective shell, meaning 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. 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 synapsid. 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 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, 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 most people think of when they use the word paleontology or the word fossil: The Jurassic and Cretaceous periods. 

 Hank (13:09)

I'll spare you the snarky commentary about how the T-Rex of 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 epic. 

Next week we will explore the nascent beginnings of a new phase of complexity: Accumulation of more knowledge, generation after generation. And the intensification of a newfangled evolutionary invention: Culture. See ya next time.