We've spent the last 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 you, 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 exceptions of a few protists, they're all unicellular, and they are by far the most abundant and diverse organisms on Earth.

Maybe more importantly, 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.

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. Kind of 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.

(Intro)

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 holds their genetic information. 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 instructions for operating the cell. 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 to 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, in oil wells, in volcanic hot springs and 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 kinda 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 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 which would melt your face off.

I mean serious, Pyrolobus fumarii, a species of archaea discovered in the late 1990s in a hydrothermal vent, live at temperatures around 113º 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 screaming hot temperatures.

There are also halophiles, or salt-lovers, which live in places like the Dead Sea, or the Great Salt Lake or probably Daniel Tosh's mouth. Most halophiles breathe oxygen and are heterotrophic but there are some bizarro outliers, like species which 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 they had some important genetic differences, like the sequence of their ribosomal DNA and the make-up of their RNA, that they were separated into 2 domains.

Bacteria are nearly as ancient as archaea. Fossils show that they were widespread about 1.5 billion years ago, but there's evidence they've been around for nearly 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 your strep throat, your staph infection and 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 combinations as its population multiplies, keeping its host's immune systems 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 which 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 staphylococcus and streptococcus, as well as some colonial bacteria that are responsible for diseases like leprosy and tuberculosis. 

And there are lots of gram negative bacteria too, which have thinner membranes. The biggest group here are the proteobacteria named after Proteus because they take so many forms. These include bacteria that make our lives possible by converting nitrogen in the atmosphere to compounds available to plants, as well as others that cause stuff like food poisoning and legionnaires disease.

Cyanobacteria, meanwhile, are the only prokaryotes that use photosynthesis to make their food and are 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 lime 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 found a way to pass on their DNA without sexual reproduction. But, you know who's a hot fricking mess? Protists.

Evolutionarily, they are the youngest of the three, having evolved from bacteria around 1.7 billion years ago and in a lot of ways they are 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 kind of animal-like, algae which are kind of plant like and fungus-like ones including the tastefully-named slime molds.

The one thing all 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 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 mouth parts, or at least mouth-part sort of things. They can move around by using all kinds of really cool structures. Some have flagella that look like tails to propel them through the water, or cilia, little hair-like structures that work like oars. Some move around with a 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's 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 are not plants because they use different kinds of chlorophyll molecules. Some are unicellular, like tiny diatoms which have a hard shell made of silica.

The amazing thing about single-celled algae is that they can get really honking 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 and tropical oceans and can grow up to 5cm across. How is that thing one cell!?

Anyway, you already know multicellular types of algae, A.K.A. seaweed. They're closely related to land plants, as you can tell by looking at them, and they're generally grouped into 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 freaking 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 that you're a little fuzzy on, table of contents is over there.  

Thanks, of course, to all the people who helped put this episode together and if you have any questions for us, please Facebook, twitter or the comments below, goodbye.