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We've spent the past few months talking about animals here on Crash Course, specifically human animals, because 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. 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. 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. 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, cruel 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, and there are those that do the opposite, making life possibly by fixing nitrogen from the atmosphere, helping animals digest food. How many of these groups have names like "Sailor's Eyeballs" and "Dog Vomit" and "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. (theme music) There's no denying it, every multi-cellular organism on this planet, whether it be a mushroom or a vampire bat, evolves from a single-celled organism. While some of these single-celled organisms evolve 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 of Eukarya. Bacteria and Archaea, meanwhile, are their own prokaryotic domains. I hope you haven't forgotten this, the big difference between prokaryotes and eukarya is that eukaryotic organisms, including you and the plants and the fungi and the 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 synthesized 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 Prokaryote actually means "before the nucleus" which is a clue that Prokaryotes are an older form of life. 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 comet 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, even acidic mine drainage. Archaea were probably the earliest living things and they're 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 energy from hydrogen gas and carbon dioxide, which is pretty cool, and they emit methane as their waste product. Methanogen, 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 if these are the Thermophiles which live in temperatures that would melt your face off. [unintelligible], a species of Archaea discovered in the late 1990s in a hydrothermal vent, live at temperatures of 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 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 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 rybosomal DNA and the makeup of their RNA that they were separated into two domains. Bacteria aren't 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 the Prokaryotes on earth and they're super slick when it comes to adapting quickly. Many Bacteria are parasitic; thank you, strep throat, your staph infection and anything you've ever taken an antibiotic for. Bacteria can fend off antibiotics and the ninjas in your immune system and by garbling up their DNA from one generation to another. They can randomly turn genes on and off creating a unique genetic combination as its population multiplies, keeping its host immune system and drug makers on their toes. Like Archaea, Bacteria don't reproduce sexually, but Bacteria has 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. 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 has 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. 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 they're some of the most important members of the aquatic food web providing microscopic forage for all kinds of fresh water 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 the 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 [hipponimous] infection of the urethra. This kind of makes me 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 has developed ways to pass their DNA without sexual reproduction, BUT ... You know who's a hot freaking mess? PROTISTS! Evolutionarily, they're the youngest of the three having evolved from Bacteria around 1.7 billion years ago. In a lot of ways, they're more sophisticated. For starters; they're eukaryotic, but also, some are multi-cellular 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. 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 of these have in common is they need to live somewhere wet; in a bog, in your body, in snowbank, 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 like mouth parts, or at least mouth parts sorts of things, and they can move around by using all kinds of really cool structures. Some have Flagella, the whip-like tail to propel them through the water or Cilia, little hair-like structures that look like oars, and some move around with a kind of blobby, amoeba-like motion. I say amoeba-like because the Protozoans that moved its 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 Protist 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 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 in tropical oceans and can grow up to 5 centimeters across. How is that thing one cell? Anyway, you already know multi-cellular types of algae; a.k.a. "seaweed", they're closely related to land plants as you can tell by looking at them. 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 chloroplast 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. 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 multi-cellular algae. Finally we have our fungus-like Protist which includes 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.