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.

Author Since: Mar 11, 2019

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