Crash Course Big History #5: The Evolutionary Epic


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 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 the 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 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 of 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. And 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 the 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 inJurassic
Parkactually 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. I hope 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:
the accumulation of more knowledge generation
after generation and the intensification of a newfangled evolutionary
invention– culture. See you next time.

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