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  • In the past 40 wonderful weeks together,

  • we've explored all of the fundamentals of biology,

  • from the tiny little machines that make us work, to the systems that

  • power the most awe-inspiring and most complex organisms on Earth.

  • Now I want to take our learning to the next level

  • simply by broadening our perspective.

  • With a solid understanding of biology at the small scale

  • under our belts, it's time for the long view.

  • For the next 12 weeks, we'll be learning how the living things

  • that we've studied interact with

  • and influence each other, and their environments.

  • It's the science of ecology.

  • The idea of an organism interacting with its environment may sound

  • simple, but it's actually pretty marvelously complicated.

  • Nothing escapes the long arm of ecology.

  • Life influences the chemical makeup of the atmosphere,

  • the geology of the planet, the climate.

  • Nothing impacts life on earth today more than humanity does.

  • And so we're going to be exploring that relationship,

  • between mankind and our environment,

  • in a lot of detail, in the coming weeks.

  • Life is powerful.

  • And in order to understand how living systems work,

  • you first have to understand how they originated, and developed,

  • and diversified over the past 4.5 billion years of Earth's history.

  • The principles that you hear about here today

  • will form the framework of our next 12 weeks together.

  • Get ready for the epic drama that is the history of life on Earth.

  • Earth is like a cheap rental house in a college town:

  • there's always somebody living there, but tenants are always moving

  • in and out, new tenants moving into old ones' rooms, and sometimes

  • the electricity gets turned off because nobody paid the power bill.

  • It's pretty much just chaos.

  • And like a rental house, the Earth isn't invincible.

  • I mean, a good semester of back-to-back frat parties can

  • tear a house up pretty fast.

  • And then, you know, everybody gets evicted,

  • and some new tenants move in.

  • Which gives you a little bit of perspective on humanity's position

  • as the current dominant species on the planet:

  • There have been others before us,

  • and there will probably be others when we're gone.

  • That's just how the planet rolls.

  • But it's taken eons for the earth to get the way that it is today.

  • Earth formed about 4.6 billion years ago, and for a while,

  • it was just a chunk of rock, circling the sun, suffering collisions

  • with other chunks of rock generating unreal amounts of heat.

  • Eventually the constant smashing slowed down a little bit, and the

  • outer layer of Earth cooled, but the core of the planet remained hot.

  • Like, really hot. As hot as the surface of the sun.

  • It gives you some idea as to how much energy and radiation

  • contributed to the earth's formation when you consider that

  • four and a half billion years later,

  • there's still molten stuff in the middle of our planet.

  • And it's that inner heat that makes the earth's surface so dynamic.

  • The planet's insides have been continually bubbling up,

  • destroying what's there, renovating and rearranging furniture.

  • It's this constant renewal that's part of what makes life here possible.

  • And in the scheme of things, it didn't actually take life

  • very long to show up on that hot rock.

  • About 4.4 billion years ago, the planet was still getting pelted

  • with giant chunks of rock and ice left over from the formation

  • of the solar system, there was no oxygen in the atmosphere,

  • and volcanoes were firing off all over the place.

  • But it was finally cool enough for some of the water

  • in the atmosphere to turn from vapor into liquid.

  • The first seas formed, and in them,

  • and in the atmosphere, a soup of chemicals.

  • Nitrogen, nitrogen oxides, carbon dioxide,

  • methane, ammonia, hydrogen, hydrogen sulfide.

  • All those things that with the help of heat and lightning and who

  • knows what else eventually created small organic molecules.

  • And some way, some how, life happened.

  • We don't know how it happened, I wish we did, but we don't.

  • The first life on Earth probably wasn't even life as we think of it:

  • It was just a collection of chemicals surrounded by a membrane,

  • because phospholipids, as you may remember,

  • spontaneously form bilayer membranes in water.

  • Over time some of the chemicals inside these membranes developed

  • into amino acids and eventually RNA, the nucleic acid

  • that was probably the first genetic material.

  • These collections of chemicals trapped within membranes,

  • called protobionts, most likely began to grow and split

  • and replicate themselves until some crazy copying error

  • gave way to DNA nucleotides, which is a way more stable

  • repository for genetic information,

  • because it's double-stranded and not single-stranded like RNA.

  • And once these little guys had DNA, they were on their way.

  • They might have only been one cell,

  • and they were probably living in the equivalent of

  • a hydrothermal vent, but they were making the most of it.

  • So the first living things were prokaryotes,

  • single-celled organisms with no nuclei, that were probably

  • pretty similar to the archaea that we find today living

  • in hydrothermal vents, sulphur hot springs, and oil wells.

  • And I apologize for pronouncing archaea wrong...

  • for the entire Biology series. My bad.

  • These prokaryotes probably made their grand entrance

  • between 3.9 and 3.5 billion years ago,

  • during what's known as the Archaean Eon.

  • This is the first of what scientists would identify as 3 eons

  • in Earth history: the Archaean, the Proterozoic and the Phanerozoic

  • The Phanerozoic being what we've been experiencing for

  • the last half billion years or so.

  • An eon, as you might expect, takes freaking forever.

  • The first one went on for 2.5 billion years.

  • So, we break up the eons into Eras, Periods, and Epochs, which are

  • organized by the big-deal events that we can see in the fossil record.

  • So, from 3.5 billion years ago until about 2.1 billion years ago,

  • these prokaryotes were all alone on Earth.

  • But then something weird happened: The amount of oxygen

  • in the atmosphere suddenly shot up basically from none

  • to 10% in a very short period of time, geologically speaking.

  • This oxygen was most likely produced by a brand new prokaryote

  • called cyanobacteria, which had figured out how to make

  • its own food through photosynthesis.

  • The more cyanobacteria were out there,

  • the higher the atmospheric concentration of oxygen became.

  • This "oxygen revolution," as it's called, probably spelled D-O-O-M

  • for a lot of prokaryotes out there that had evolved without oxygen.

  • And this was also one of the first real game-changers for life on Earth

  • because it was the first major instance of living things

  • bringing massive change to their own environment.

  • In fact, it may be the earliest example of Ecology at Work:

  • Cyanobacteria changed the atmosphere, judo-chopped the competition,

  • and made way for the evolution of

  • living things to take a new, specific direction.

  • Then about 2.1 billion years ago, a new kind of organism

  • made its big debut: eukaryotes.

  • And these, as you know, are a big deal

  • because they include all plants and animals.

  • Eukaryotes probably evolved by a process called endosymbiosis,

  • where one prokaryote parasitized another prokaryote,

  • or maybe just ate it but didn't digest it,

  • and the result was actually awesome

  • for both the host and the parasite slash undigested prey.

  • It formed a single-celled organism with organelles,

  • specifically mitochondria and plastids, which probably evolved

  • from those eaten or parasitic prokaryotes.

  • This allowed for much more complexity,

  • and by 1.5 billion years ago,

  • we start seeing multicellular eukaryotic organisms in

  • the fossil record, the very first of them probably being algae.

  • But it wasn't until around 535 million years ago

  • that the eukaryotes went berserk.

  • And that's known as the Cambrian explosion, a super-major biological

  • Golden Age when the diversity of all animal life on Earth exploded.

  • Nobody's entirely sure what started it, but suddenly life created

  • innovations that the planet had never seen: Creatures used minerals

  • in the seawater to build skeletons and shells, some acquired weapons

  • like claws, while others developed defensive plates.

  • The evolutionary arms race between predators and prey was underway.

  • This heralded the dawn of the Phanerozoic Eon,

  • the one we're in right now.

  • That's right, the Earth spent the better part

  • of two eons under the rule of a bunch of archaea and bacteria

  • and some, like, soft-bodied worms.

  • Until the Cambrian exploded and we started to see a lot of

  • animal phyla that we actually are hanging out with today.

  • After the Cambrian, the party got so hot in the oceans

  • that by the Ordovician Period around 500 million years ago,

  • plants, animals and fungi started colonizing the land,

  • probably as a strategy for escaping predation.

  • Now there were whole new ecosystems to explore and adapt to and create.

  • During the Devonian Period about 365 million years ago,

  • tetrapods, 4-legged vertebrates that probably evolved from

  • lobe-finned fishes, showed up on land,

  • and so did arthropods, like insects and spiders.

  • From here, we begin to see ecological systems that we

  • recognize today because organisms were changing their environments

  • by consuming oxygen in the atmosphere and releasing carbon dioxide.

  • And you know who likes carbon dioxide? The plants.

  • The Carboniferous Period that extended from 359-299 million years

  • ago was when the plants entirely went nuts.

  • The forests were so dense and so widespread that they made

  • all our fossil fuels, all the coal and oil that we now use

  • to power all the things with, were made over the course of

  • about 60 million years.

  • This time, it was plants that had changed both

  • the climate and the geology of Earth.

  • These forests cranked out so much oxygen that the atmosphere

  • contained around 35% oxygen rather than today's 21%.

  • All this oxygen started cooling the planet because there wasn't

  • enough carbon dioxide to maintain the balmy temperatures

  • that the vast Carboniferous jungles needed to survive.

  • So the whole system crashed, and all that carbon from

  • these forests sunk into swamps and eventually got locked in rocks.

  • Of course, now we're releasing all that carbon by burning fossil

  • fuels, which is certainly helping to keep the planet toasty now.

  • That right there: some good Ecology.

  • In the Permian Period, 299-251 million years ago, all the landmasses

  • of the world joined to form one giant continent that we call Pangea,

  • altering global climate and ocean currents,

  • and animals and plants evolved in response.

  • We start seeing gymnosperms, the first plants with seeds

  • like modern pines and spruces and firs, and archosaurs,

  • the granddaddies of dinosaurs and modern birds showed up.

  • But you've probably picked up enough of a pattern here

  • to predict that this party didn't last forever.

  • About 252 million years ago, something happened,

  • or maybe a lot of things happened in quick succession,

  • but whatever they were, movie executives take note:

  • the Permian-Triassic extinction event would make the most

  • awesome disaster film of all time.

  • Because up to 96% of all marine species

  • and 70% of terrestrial vertebrate species bought the farm,

  • and it's the only known mass extinction of insects.

  • About 57% of all taxonomic families

  • and 83% of all genera became extinct.

  • It was the most significant extinction event on the planet, ever.

  • It's been hard to pinpoint the reason for this extinction event

  • because most of the evidence has been wiped out, of course.

  • It may have been kicked off by an asteroid that released the energy

  • equivalent to the detonation of a few million

  • nuclear weapons all at once.

  • And then insult added to injury when a whole bunch

  • of volcanoes erupted, methane was released from the sea floor,

  • there were probably some gas explosions in what's now Siberia,

  • and then a whole bunch of climate changes, sea-level change

  • and changes in ocean salinity probably occurred.

  • Nobody's sure exactly what happened, but we do know it took

  • a long time for life on earth to make a comeback.

  • But look on the bright side: as a result of

  • the Permian-Triassic event, we got dinosaurs!

  • They were able to evolve during the Triassic because

  • there wasn't much competition for resources, so they evolved

  • to fill an available niche, that is, a combination of

  • the living and nonliving resources

  • that they could use to survive.

  • Remember that word, because a lot of ecology

  • comes down to who's exploiting, leaving,

  • or getting kicked out of, or altering their niches.

  • And during the Triassic Period, there were tons of niches,

  • the sky was the limit, because hey!

  • There weren't very many animals or plants to compete with.

  • So by the Jurassic Period, about 199 million years ago

  • to 145 million years ago, huge, herbivorous dinosaurs

  • were roaming the earth, smaller, mean-as-crap carnivorous

  • dinos were stalking the herbivores, the oceans were full

  • of giant squid, ichthyosaurs and long-necked plesiosaurs,

  • the air was full of pterosaurs and the first birds.

  • And there were mammals, small ones, but they were all over the place.

  • It just wasn't our time to shine.

  • The Jurassic was Dino-time, and the dinos lived it up.

  • They partied down until about 65 million years ago,

  • when they all went extinct, as I'm sure you're aware,

  • except for their surviving descendants, the birds.