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  • Hi, I'm John Green.

  • Welcome to Crash Course Big History.

  • Today we're going to 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.

  • The keystone of our story is:

  • So, in the 1830s, a young Charles Darwin traveled

  • around the world on theH.M.S. 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 overbreed

  • 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 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 publishedOn the Origin of Species,

  • and it sent shockwaves 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:

  • 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:

  • 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:

  • While all genetic mutations are generated

  • by a random copying error or a 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:

  • 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:

  • Whether it be the discovery of a new species

  • that recently moved into a different environment,

  • the development of newly adapted bacteria into superbugs,

  • 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-celled 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 cooperative 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 in oceans.

  • Photosynthesizers may 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:

  • In nature, species compete in niches.

  • It's also calledniches, depending on where you're from,

  • but I call them niches, as:

  • 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 in 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 resembled today's worms, corals, mollusks,

  • various underwater plants.

  • But then in the Cambrian era,

  • adaptive radiation really got underway

  • and multicellular life filled thousands upon thousands

  • of niches unlocked by their new traits.

  • A lot more is just possible for multi-celled 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 that 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

  • thanHomo sapienshave 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 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 is going to be a thing, Stan.

  • Is there anyway 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 of 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 onto 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 continue 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 wingspan 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 tetropods gain

  • 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

  • in disappearances from the fossil record shows