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  • We've spent the past few months talking about animals

  • here on Crash Course, specifically human animals, because...

  • well, because humans... we love talking about ourselves,

  • and also because animals are just really interesting.

  • But it's high time that we talked about the rest of the living world.

  • Because I hate to break it to ya, but most of the alive things on

  • Earth are single-celled organisms.

  • And by "most of the alive things" I mean that these organisms

  • make up two of the three taxonomic domains of all life,

  • plus one of the four kingdoms.

  • I'm talking about archaea, bacteria and protists.

  • With the exception of a few protists, they're all unicellular,

  • and they are, by far, the most abundant and diverse organisms on Earth.

  • More important, they lay claim to the world's oldest

  • and earliest living lineages, dating back to the very first twinkle of life on this planet.

  • So by understanding these three groups,

  • you begin to truly understand life on earth, its origins,

  • and how everything that came after them, including us, came to be.

  • What's more, because their heritage is so ancient,

  • these organisms often take weird, cool forms

  • that don't look like life as we think about it,

  • and they do amazing things.

  • Some not only live but thrive in environments that would kill you, me, and everything we hold dear.

  • And others make their living by invading organisms,

  • including us, and causing disease.

  • Then there are those that do the opposite, making life possible by,

  • fixing nitrogen from the atmosphere and helping animals digest food.

  • Members of these groups have names like Sailor's Eyeballs

  • and Dog Vomit Slime Mold, and they can take the shape of rods, blobs, corkscrews or coils.

  • Kinda like the doddering, eccentric relatives

  • you're forced to spend some holiday with once a year,

  • the archaea, bacteria and protists are our oldest, oddest relatives.

  • And it's about time you got to know them.

  • There's no denying it: Every multicellular organism on this planet,

  • whether it be a mushroom or a vampire bat,

  • evolved from a single celled organism.

  • And while some of these single celled organisms evolved to populate

  • the world as rhinos and strangler figs, others found happiness in the unicellular lifestyle,

  • and they haven't changed much in the past few billion years.

  • Today, nearly all unicellular organisms are either archaea, bacteria or protists.

  • Protists, you'll recall, are eukaryotic organisms

  • that make up the kingdom Protista under the domain Eukarya.

  • Bacteria and Archaea, meanwhile, are their own prokaryotic domains.

  • And I hope you haven't forgotten this, the big difference

  • between prokaryotes and eukaryotes is that eukaryotic organisms,

  • including you and the plants, and fungi and animals that you know,

  • have cells with a nucleus that hold their genetic information,

  • while prokaryotic cells don't have a nucleus or any organelles to speak of.

  • These two groups do have some important things in common,

  • like having plasma membranes that are filled with cytoplasm,

  • and ribosomes that contain RNA and synthesize proteins.

  • And they both have DNA that carries the instructions for operating the cell.

  • But eukaryotic DNA comes in strands in the form of chromosomes,

  • while prokaryotic DNA is found in rings called plasmids.

  • So, again, and this time with feeling:

  • Protists are mostly single-celled eukaryotic organisms.

  • Archaea and bacteria are single-celled prokaryotic organisms.

  • The word "prokaryote" actually means before the nucleus,

  • which is a clue that prokaryotes are an older form of life.

  • And we literally cannot find anything older than Archaea.

  • The first Archaea fossils date back 3.5 billion years ago

  • I'm talking just a billion years after the Earth formed

  • and was still bombarded by comets and meteors,

  • not to mention fried by UV radiation.

  • But in the midst of all that, archaea were just chillaxing.

  • Earth's climate has calmed down since then, so today

  • archaea are found in some of the world's most extreme environments:

  • In underwater hydrothermal vents, oil wells, volcanic hot springs,

  • even acidic mine drainage.

  • Archaea were probably the earliest living things,

  • and their adaptability is probably what allowed them

  • to take root in Earth's early, kind of grody environment.

  • One key group of the archaea are the methanogens.

  • These guys prefer more moderate environments, like mud,

  • swamps and your intestines, but they derive their energy from hydrogen gas and carbon dioxide,

  • which is pretty cool and they emit methane as their waste product.

  • Methanogens, methane generators.

  • We know that waste as swamp gas, and also, other kinds of gas.

  • The other groups are extremophiles, which not only tolerate

  • but prefer really wicked surroundings.

  • The most famous of these are the thermophiles,

  • which live in temperatures that would melt your face off.

  • I mean, serious: Pyrolobus fumarii, a species of archaea

  • discovered in the late 1990's in a hydrothermal vent,

  • live at temperatures around 113 degrees celsius.

  • Not fahrenheit, celsius!

  • Significantly above the boiling point of water!

  • Most organisms can't take heat like that,

  • because it causes their DNA to unwind and their proteins to denature

  • or permanently change shape.

  • But thermophiles have evolved adaptations that keep them stable

  • at these screamin' hot temperatures.

  • There are also halophiles, or salt lovers, which live in places

  • like the Dead Sea or the Great Salt Lake, and probably Daniel Tosh's mouth.

  • Most halophiles breathe oxygen and are heterotrophic,

  • but there are some bizarro outliers, like species that use sunlight

  • to make energy, but not like plants do

  • they have light-harvesting pigments in their membranes

  • that react with light and enable the cell to make ATP for energy.

  • I know, it's crazy!

  • But despite their alien-sounding ways of life,

  • archaea really aren't all that different from bacteria,

  • which are also prokaryotes.

  • In fact, archaea and bacteria were classified together for much of the 20th century.

  • It was only when scientists realized that they had some important genetic differences,

  • like, in the sequence of their ribosomal DNA and the makeup of their RNA,

  • that they were separated into two domains.

  • Bacteria are nearly as ancient as archaea.

  • Fossils show that they were widespread about 1.5 billion years ago,

  • but there's evidence that they've been around for more than 3 billion years.

  • Today, they make up the vast majority of prokaryotes on Earth,

  • and they're super slick when it comes to adapting quickly.

  • Many bacteria are parasitic.

  • Think strep throat, your staph infection,

  • anything you've ever taken an antibiotic for.

  • But bacteria can fend off antibiotics,

  • and the ninjas in your immune system,

  • by garbling up their DNA from one generation to another.

  • They can randomly turn genes on and off,

  • creating unique genetic combination as its population multiplies,

  • keeping its host's immune system, and drug-makers, on their toes.

  • Like archaea, bacteria don't reproduce sexually,

  • but bacteria have devised a way to pass their genetic material to their buddies,

  • a little trick called horizontal gene transfer.

  • For example: you've heard of antibiotic resistance, right?

  • Well, horizontal gene transfer is one reason for it.

  • A strain of bacteria that has genetic resistance to an antibiotic

  • can pass some of its DNA, and that drug resistance,

  • to another strain, which is why we're always in

  • kind of an arms race with the bacteria of the world.

  • And of course bacteria are incredibly diverse,

  • with too many phyla to name, more than two dozen.

  • But one way of classifying them is by their different kinds of cell membranes,

  • which react differently to a staining technique

  • scientists use called Gram staining.

  • Gram positive bacteria have thick cell membranes,

  • and they're a huge group that includes species that live individually like staphlococcus and streptococcus,

  • as well as some colonial bacteria that are responsible for diseases like leprosy and tuberculosis.

  • There are lots of groups of Gram-negative bacteria too,

  • which have thinner membranes.

  • The biggest group here are Proteobacteria,

  • named after Proteus because they take so many forms.

  • These include bacteria that make our lives possible

  • by converting nitrogen in the atmosphere into compounds available to plants,

  • as well as others that cause stuff like food poisoning and Legionnaire's disease.

  • Cyanobacteria, meanwhile, are the only prokaryotes

  • that use photosynthesis to make their food,

  • and they're some of the most important members of aquatic food webs,

  • providing microscopic forage for all kinds of freshwater and marine ecosystems.

  • Spirochetes are the corkscrew-shaped bacteria

  • that you've no doubt heard of most are harmless,

  • but a couple of parasitic species are the culprits behind illnesses like Lyme disease and syphilis.

  • And speaking of sexually transmitted diseases,

  • the last major group of bacteria worth mentioning are Chlamydias,

  • which are strictly parasitic and live only in animal cells.

  • They're scumbags, obviously, and are the leading infectious cause of blindness in the world,

  • as well as that eponymous infection of the urethra

  • that makes me kind of want to, cross my legs, just thinking about it.

  • So, Archaea have managed to make a nice,

  • multi-billion-year living by surviving in weird, out of the way places,

  • and bacteria have developed ways

  • to pass their DNA without sexual reproduction.

  • But you know who's a hot freakin' mess? Protists.

  • Evolutionarily, they're the youngest of the three,

  • having evolved from bacteria around 1.7 billion years ago,

  • and in a lot of ways they're more sophisticated.

  • For starters, they're eukaryotic, but also, some are multicellular,

  • and a few kinds can even reproduce sexually.

  • But their domain is a big crap circus, because some protists

  • seem to be more closely related to plants or animals or fungi than other protists.

  • So scientists tend to talk about them based on what else they resemble.

  • There are Protozoa, which are kinda animal-like,

  • Algae, which are kinda plant-like,

  • and fungus-like ones, including the tastefully-named Slime Molds.

  • The one thing all of these have in common

  • is they need to live somewhere wet:

  • in a bog, or in your body, or in a snow bank, wherever.

  • Protozoa are actually really cool because they're like tiny animals.

  • Like us, they're heterotrophs,

  • so they have to eat other stuff in order to live.

  • And because they need to eat, they've got mouthparts,

  • or at least mouth-part sorts of things,

  • and they can move around by using all kinds of really cool structures.

  • Some have flagella, the whip-like tails, to propel them through the water,

  • or cilia, little hair-like structures

  • that work like oars, and some move around with a kind of blobby amoeba-like motion.

  • I say amoeba-like because the protozoans

  • that move this way are amoebas.

  • And speaking of amoebas, some protozoans are parasitic.

  • You've probably heard of amoebic dysentery: that's caused by amoebas.

  • Malaria is caused by this little guy,

  • a protozoan called Plasmodium vivax.

  • While African Sleeping Sickness is caused by Trypanosoma brucei, this guy here.

  • Moving on to the plant-like protists, which are algae.

  • All algae photosynthesize like plants,

  • even though they're not plants,

  • because they use different kinds of chlorophyll molecules.

  • Some are unicellular, like tiny diatoms,

  • which have a hard shell made of of silica.

  • But the amazing thing about single-celled algae

  • is that they can get really honkin' huge.

  • For example, ladies and gentlemen, cast your gaze

  • upon the Sailor's Eyeball, thought to be the biggest

  • single-celled organism on the planet.

  • Also known as "bubble algae," it lives on the sea floor in tropical oceans

  • and can grow up to 5 centimeters across.

  • How is that thing one cell?

  • Anyway, you already know multicellular types of algae,

  • aka seaweed. They're closely related to land plants,

  • as you can tell by looking at them,

  • and they're generally grouped in to red, green and brown varieties,

  • although these all have their unicellular forms as well.

  • The green algae are probably what gave rise to land plants about 475 million years ago.

  • They're the most abundant and diverse,

  • and they have chloroplasts very much like land plants,

  • so they can only live in shallow water

  • because they need a lot of sunlight.

  • Red algae is able to live at greater depths

  • and has an extra pigment in it called phycoerythrin,

  • which gives its chlorophyll a boost in deeper waters.

  • And brown algae is what most of the seaweed you see in the ocean is.

  • Kelp is an example.

  • They're the largest and most complex of the multicellular algae.

  • Finally, we have our fungus-like protists,

  • which include the delightful slime molds.

  • They absorb nutrients from their environment

  • and produce fruiting bodies like fungi,

  • but even though they look like piles of barf,

  • they can actually move around like an amoeba

  • and eat bacteria by phagocytosis.

  • Slime molds can be pretty easy to spot

  • because they're often brightly-colored,

  • like this charming species which, in all seriousness,

  • is known as Dog Vomit Slime Mold.

  • You heard me. These organisms are so freakin'

  • screwed up that scientists couldn't think of a better name for it

  • than Dog Vomit Slime Mold.

  • Like I said. They're old. They're odd. Get used to it.

  • Thanks for watching this episode of Crash Course Biology.

  • If you want to catch up on anything you're a little fuzzy on:

  • table of contents over there!

  • Thanks, of course, to all the people that helped put this episode together.

  • And if you have any questions for us, please

  • Facebook, Twitter, or the comments below.