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Video transcript

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 they 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 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 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 and there are those that do the opposite making life possible by fixing nitrogen from the atmosphere helping animals digest food members of these groups have names like sailors 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 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 artists 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 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 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 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 or just chillaxing Earth's climate has calmed down since then so today archaea are found and some of the world's most extreme environments in underwater hydrothermal vents in oil wells and 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 perform more moderate 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 that would melt your face off I mean serious viral Obus forme REI a species of archaea discovered in the late 1990s 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 screaming hot temperatures there are also halophiles or salt lovers which live in places like the Dead Sea or the Great Salt Lake probably Daniel Tosh's mouth most halophiles breathe oxygen and our 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 is 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 showed 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 thinkyou strep throat your staph infection anything you've ever taken an antibiotic for the 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 immune system and drug makers on their toes like archaea bacteria don't 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 in there 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 ion a bacteria meanwhile are the only prokaryotes that use photosynthesis to make their food and there's 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 chlamydia is 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 and 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 freaking 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 kind of animal-like algae which are kind of plant like and fungus like ones including the tastefully named slime molds one thing all these have in common is they need to live somewhere wet in a bog or in your body or in a snowbank wherever protozoa are actually really cool because they're like tiny animals like us they're heterotroph 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 with 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 I 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 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 sailors eyeball thought to be the biggest single-celled organism on the planet also known as bubble algae it lives on the seafloor and tropical oceans and can grow up to five 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 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 400 five million years ago they're the most abundant and diverse and they have chloroplasts very much like lamb 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 FICO arethe Rijn 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
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