when earth first formed 4.6 billion years ago it wasn't the Cozy oxygen-filled water abundant home we know and love today well stocked with all the ingredients for living breathing and singing in fact you'd barely recognize it the Earth back then was a hot mess literally a cataclysm of dust Rock and ice in that early atmosphere forget about it it was so thick with water vapor and Ash from volcanic eruptions they wouldn't have been able to see a bottle rocket popping off a few feet from your face now after a few hundred million years the volcano simmered down the temperatures cool and the water Vapors settled to form Earth's vast oceans but that all paled in comparison to what happened next life and to understand the signs of life in all its beautiful varied forms we really need to know how it all began hi I'm Dr Sammy your friendly neighborhood entomologist and this is Crash Course biology hey Patty you digging up there same music [Music] this weird thing that we call life has been around for about 3.8 billion years so it has been witness to a lot of Earth's history in a way they're dance Partners as Earth has changed its Rhythm so is life and vice versa in one long Tango and the steps to this dance are in macro Evolution the big patterns of how living things evolve above the species level so instead of talking about the emergence of say a particular species of bear we're talking about creatures with four limbs in general in this way macroevolution helps us trace the evolutionary history of life scientists are able to understand this history by studying things like geology the climate and most importantly fossils now for life to arise in the first place there needs to be tons of energy energy that could create the perfect environment for large organic molecules to form naturally leading to carbon compounds that life is made of there are a few different ideas about how this might have gone down one is that life arose from the power of lightning volcanoes and radiation from space Brewing the oceans into a nutrient-rich soup of organic molecules another possibility is that those first organic molecules formed in deep sea vents with the perfect temperature and chemical balance others think that those first compounds might have been brought by meteorite the point is there's more than one way life's soup can Brew tasty but life needs more than a strange broth of organic molecules it also needs ways of copying genetic material or biological information passed down from parent to offspring biologists think that probably began with strands of RNA which today transfer genetic information in cells but may have originally formed in Tidal pools molecules at the bottom of these pools may have gone through cycles of baking dehydrating and binding together during low tide and then Cooling and partially dissolving with high tide the molecules that could withstand repeated cycles of heating and cooling would have accumulated and stuck around becoming RNA like how gummy bears will sometimes melt together in the bag if you leave them in a car except instead of a glob of gummies this would eventually pave the way for strands of DNA DNA is a molecule that carries genetic information in all organisms on Earth and it can be copied lots of times with pretty amazing but not perfect accuracy it's these slight differences that set the stage for more diverse forms of life to emerge but changing conditions on earth have shaped life over billions of years and I don't know about you but it's hard to wrap my head around all that time so let's scale down those eons to something easier to understand a 24-hour clock no one is around at midnight but the Earth isn't quiet it's a bombardment of asteroids and exploding volcanoes and then around 4 AM things cool down and Earth's first residents emerge single-celled organisms called prokaryotes these early forms of Life float through the Earth's oceans the prokaryotes are replicating their genetic information and dividing to produce more of themselves one makes two two makes four until there are billions then around 5 30 a.m some of these early organisms began photosynthesizing harnessing solar energy from a faint young Sun around 11AM oxygen from photosynthesis starts to show up in the atmosphere and by noon it has transformed Earth's atmosphere into a mix of gases that allow larger more complex life to develop so by 2PM the first eukaryotes burst onto the scene more complex cells that will eventually support more complex life it isn't until around 5 PM with three quarters of the day gone that the first multi-cellular forms of life join in and by 9 pm things are really getting rowdy as predatory animals begin to evolve creating a new relationship between predator and prey this drives some less formidable organisms to Extinction up until now all of life's drama has happened in the ocean but around 9 30 PM the first plants set their roots on shore at 10 pm insects my favorites and four-legged creatures follow Dinosaurs show up soon after and they prove real party animals before going extinct around 11 40 PM about an hour after arriving on the scene mammals joined the party around 11 pm but it's only in those last 20 minutes before midnight that they really start diversifying as for humans almost all of this tacular day happens without us it's not until that final second right as midnight approaches that we show up fashionably late as always now the farther back in life's evolutionary history we go the trickier it is to reconstruct its course but scientists have found direct and indirect evidence of life's history like you might think that the oldest signs of life would be in fossils right but actually the most ancient Signs of Life can be found in traces of carbon molecules trapped inside Plano rocks you see around 3.5 billion years ago or about four in the morning on our eons clock some of Earth's very first prokaryote residents Left Behind stromatolites or fossilized Mounds formed from thin layers of sediment the prokaryotes fused these layers together themselves kind of like the micro version of a paper mache project except way older than that bowl that I made in third grade that and probably more structurally sound later when some prokaryotes evolved the handy-dandy trick of photosynthesis is spurred a chemical reaction that produced layers of iron oxide those layers settled at the bottom of the sea and today they're found as very old very instructive rocks now all the oxygen released by that early photosynthesis fundamentally changed the Earth's atmosphere and as much as we literally live and breathe the stuff today it wasn't exactly a welcome change for prokaryotes back then it probably poisoned many of them damaging their cells and causing them to die out but an oxygen-rich atmosphere also pushed the survivors to evolve new ways of harvesting energy that's where fossils come in handy fossils tell us that the leap from single-celled organisms to multicellular ones allowed life to evolve new shapes and bigger sizes at first these organisms stayed squishy and pretty small with the biggest ones measuring only about one meter long but eventually more and more diversity erodes like as Hunters with claws evolved so did armored prey with hard shells and outer skeletons these are examples of adaptations or heritable traits that increase an organism's chances of survival and reproduction for example we know that our backboned ancestors came from the oceans thanks to fossils of fish-like animals that involved legs and lungs and gradually moved from sea to shore but figuring it out has involved tracking down puzzle pieces strewn all over the globe let's learn more over in the theater of life it's hard to say when Chinese paleontologist Dr miman Chung is better at puzzles or persevering Zhang first started puzzling over fish fossils in 1955 and the more she studied ancient fish the more captivated she became by their Mysteries especially how they evolved into the land walking air-breathing beings that led to us unfortunately her studies came to a halt in 1966 when the government-centered academic research including zhangs but she persevered and eventually was able to return to a research at the time scientists thought a group of lobe-finned fish that lived millions of years ago and may have had inner as well as outer nostrils might be the key to solving the puzzle they thought these adaptations might have led to the ability to breathe air in later species humans have this trait as well two nostrils on the outside and two on the inside that pass air from our nose to our lungs but Zhang found that fossils from these fishes didn't have inner nostrils just two outer Pairs and then she found yet another type of fossilized fish this one had a pair of outer nostrils and a pair between his front teeth it was like a snapshot in time nostrils Midway to where they would eventually end up and land walking air-breathing creatures like us it took careful observation and some serious perseverance but Zhang pieced together one of life's big puzzles for Every Breath You Take you've got fishy ancestors and a pair of traveling nostrils to thank the theater of life sure knows how to tell a story right you get it you'll get it later it's a thinker anyway as scientists find more fossils they continue unraveling how life's history fits together of course the fossil record will never be fully complete but scientists are on the lookout for those species that represent major steps forward in evolution and along the way they found evidence of some major drivers of macroevolution for example there's plate tectonics or the sliding and floating of Earth's surface place on the molten hot mantle below as these plates have shifted and collided they've driven macroevolution by forming mountain ranges splitting continents and spurring Life to evolve as a result changes in the atmosphere have also been major drivers of evolution like when that early surge of oxygen pushed some organisms to Extinction and others to adapt new ways of using oxygen for energy similarly when Earth has moved between climate extremes organisms have either adapted or died out and mass extinctions themselves have driven evolution in fact five times in history over three quarters of the world species have disappeared but every time life is taken a hit it has bounced back and diversified like when early mammal-like species dwindled reptiles Rose and when many of those reptiles were wiped out new reptiles emerged called dinosaurs and when the dinosaurs left the scene the mammals came back stronger than ever so whenever some species have gone extinct others have risen to prominence evolving new ways of living moving and getting energy the these cycles of living and dying Extinction and Recovery might seem like they're following some Grand Design to us humans but evolution has no goal or plan it's just following the rhythms of life's dance and while we've covered a few of life's slickest evolutionary moves today you can learn even more if you check out this mini-series from our friends over a slash show the dance between life and the planet has been going on for a long time Earth's changing conditions have triggered game-changing leaps for life life in turn has shaped the Earth right back even altering its atmosphere we wouldn't be the same without each other and as much as we humans like to think that we're the peace de resistance when it comes to Evolution we've been around for less than a geological second we've got a lot to learn from all the life that's come before us in our next episode we're gonna break down the ways that Evolution connects us all and how scientists keep track of Evolution's many moves I'll see you then peace this series was produced in collaboration with hhmi biointeractive if you're an educator visit biointeractive.org crash course for classroom resources and professional development related to the topics covered in this course thanks for watching this episode of Crash Course biology which was filmed in our studio in Indianapolis Indiana and was made with the help of all of these nice people if you want to help keep crash course free for everyone forever you can join our community on patreon [Music]