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.