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
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Goodbye.