the tale of life on earth has been unfolding for about four billion years and we humans are just the last word on the last page of that story at least so far and the vast stretches of time that are covered by the history of life can be hard for us to fathom we're at our brains trying to imagine what a few hundred years looks like let alone billions of years and like speaking for myself I can't even remember what I had for breakfast this morning so to help us really appreciate the full expanse of time scientists have turned to the rocks by looking at the layers beneath our feet geologists have been able to identify and describe crucial episodes in life's history from bursts of evolutionary diversity to disastrous extinction events these key events of new life and sudden death frame the chapters in the story of life on Earth and the system that we use to bind all these chapters together is the geologic time scale first let's talk about the history of geologic time itself because figuring out how to read history in rocks was not easy for much of human history of course we had no idea how old the earth was or what actually happened in deep time or what happened in what order but in 1669 danish scientist Nicholas steno published the first laws of stratigraphy the science of interpreting the strata or layers of rock and Earth's outer surface steno argued that the layers closer to the surface must be younger than the layers below them so the farther down you dig he thought the older the fossils are that you find there sounds legit right but instead owes day when some people thought that fossils had literally fallen from the sky for some reason this was a pretty revolutionary idea building on stenos ideas italian geologist giovanni arduino went a step further and began naming the layers of rock in the 1760s arduino study the italian alps organizing their layers based on their depth and composition the lowest layers of metamorphic and volcanic rocks he called their primary layer above those were hard sedimentary rocks which he called secondary and the top layers of softer alluvial deposits he named tertiary and quaternary but because rock layers don't appear in this same order all over the world there was no way for geologists to compare rocks from one location to another without a way to compare strata there could be no Universal timescale finally in 1819 English geologist William Smith figured out the solution to this problem fossils by comparing the remains of ancient organisms from different rock formations Smith could match their ages regardless of how far apart they were for example Smith realized that fossils of many early species of trilobite are found below ammonite fossils which are in turn below certain kinds of shellfish so any place in the world where you find these first trilobite s' you know that you're looking at rock that's older than when ammonites live and even in the most ancient rocks that have little or no evidence of life scientists can still look for signs of the very earliest major geologic events like when continents first formed and even when the earth itself cooled and solidified thanks to the work of early geologists like steno Arduino and Smith modern scientists have used these and other clues to create what we now call the geologic time scale or GTS the GTS has been reworked many times to reflect the latest knowledge of Earth's history and today it's organized into five sub groups eons eras periods epochs and ages organizing time and increments like this allows us to ask questions about history on different scales in the largest increments like eons and eras we can ask the biggest of big-picture questions like was there life on Earth at this time if there was what it looked like that live in the water or on land is the kind of top-level view we're going to take today but the smaller increments of time like periods in epochs help us take a tighter focus and ask more specific questions like what was a climate life during this window of a few million years and how did life around the world adapt to it we'll be talking about those in more detail in future episodes when we talk about each era period by period ok so let's get the biggest of big picture views of Earth's history right now by taking a tour of all the eons and eras in the GTS eons are the largest slices of time ranging from 1/2 billion to nearly 2 billion years long and the earliest eon is known as the Hadean it begins with a very formation of the earth itself around 4.6 billion years ago and ends 4 billion years ago and this is the only Eon that doesn't have any fossils because back then the world was just hell named after the Greek underworld Hades the Hadean lived up to its name the planet was wracked by volcanic activity cosmic bombardments raging storms and temperatures that were at times hot enough to melt rock but even in this searing wasteland life may have been able to form while no fossils have been found from this Eon small amounts of organic carbon have been discovered in Haiti and rocks that some experts think is evidence of the earliest life these first organisms were tiny and single-celled but they were eventually able to shape the future of the entire planet so their appearance is the one major benchmark of this Eon the Hadean is brought to an end by the cooling of the crust of the earth setting the stage for the continents to eventually form and this cooling marked the beginning of the next phase the Archaean Eon which ran from 4 billion to 2.5 billion years ago named for the Greek word for origin the Archaean was once thought to be when the first signs of life appeared but at the very least it's fair to say is the first time that life flourished forming mats of microbes in the primordial seas the fossils that these microbes left behind are called stromatolites or sometimes demanda lits and they're very oldest of them like those found in western australia date from the Archaean during this time the atmosphere was mostly carbon dioxide but the appearance of cyanobacteria was about to change all that then 2.5 billion years ago the Archaean gave way to the proterozoic eon meaning earlier life and around this time put a synthetic bacteria along with some multicellular forms of life spewed tons of oxygen into the atmosphere this probably wiped out most of the anaerobic life on earth but it cleared the path for crucial new organisms including the ancestral eukaryotes and cells each headed nucleus and organelles wrapped up in membrane eukaryotes developed into the first really big complex and sometimes kinda weird forms of life such as the front like carne in the plate-shaped ik and sonia these new larger organisms quickly diversified and by 541 million years ago we were at the doorstep of the next and current young the phanerozoic its name means visible life and the phanerozoic was when life really became obvious this is the eon that's home to trees dinosaurs newts aardvarks us humans basically life as we know it who how you holdin up you doing okay we've covered about three and a half billion years already she's got another half billion ago and then we're home free okay now from here it's best to explore the phanerozoic eon through its eras the next level down in the divisions of time this will let us explore more recent history in greater detail the first era of our current eon is a Paleozoic era which began 541 million years ago this chapter was defined by the diversification of visible life and it started with a bang actually an explosion the Cambrian explosion this rapid growth and diversity and complexity in the world's oceans is such a huge deal in history of life that all the eons that came before it the Hadean the Archaean the Proterozoic they're all collectively known as the Precambrian at the start of the Paleozoic over about 25 million years the fossil record suddenly reveals the appearance of complex animals with mineralized remains you know hard part's shells exoskeletons that kind of thing and the very first of these new animals to become truly widespread were the trial of mites they were so common all over the world that they've been used as index fossils for the Paleozoic era for centuries ever since the days of William Smith but the trilobite soon add competition fish developed teeth and jaws and came to dominate the seas including the first sharks and armored giants known as placa derms meanwhile the land which have been barren since the formation of continents back in the Archaean was finally being populated first by plants and then my arthropods by 370 million years ago entire ecosystems had developed on primeval continents soon after the earliest afib ian's evolved and hauled themselves out all the water leaving the first vertebrate footprints in the mud 299 million years ago the supercontinent of pangaea had formed with an enormous desert at its center this desert was quickly populated by the ancestors of what would eventually become reptiles and mammals which could thrive in dry conditions unlike Nvidia's but this time of incredible growth couldn't last forever and instead the Paleozoic era ended in Cataclysm 252 million years ago 70% of land vertebrates and 96% of marine species disappeared from the record including survivors our previous extinctions like our friends to trilobite see I still miss those guys the event known as the Great dying was the most severe extinction in our planets history but its exact cause is still unclear a possible meteorite impact off the coast of South America might be one clue and in Siberia layers of basalt show that massive volcanic eruptions covered large swathes of Pangaea and lava both of these incidents coincided with the end of the Paleozoic and it seems more than likely that the extinction had many causes in any case the Paleozoic may have begun as a chapter defined by an explosion of life but it ended in nearly absolute death it took millions of years for life to recover but when it did a new world the Mesozoic era had arrived this is often called the age of reptiles and with good reason right from the start of the Mesozoic reptiles were incredibly successful this is when they took some other most famous forms like dinosaurs pterosaurs and a variety of marine species in fact all of the non-avian dinosaurs lived only in the Mesozoic so they remain one of the best index fossils of this era and many modern groups of organisms also evolved in the shadows of the reptiles like mammals frogs fees and flowering plants but the Mesozoic era came to an end 66 million years ago with yet another episode of devastation this time known as the Cretaceous Paleogene or KPG extinction event like all mass die-offs the KPG had many causes but probably the biggest of them was a giant asteroid that struck the earth sending enormous amounts of ash into the atmosphere blocking out sunlight and creating a vicious cold snap across the planet without the sun's energy entire plant communities died and the animals that relied on those plants perished with the evidence of this impact can be found in a layer of iridium and rocks dating to the end of the Mesozoic iridium is an element that's rare on earth but very common in asteroids and comets and a giant impact crater in the Gulf of Mexico whose age matches the date of this extinction has become The Smoking Gun for the asteroid hypothesis the victims of the KPG extinction were some of the biggest reptiles of the land sea and sky including all of what we now call the non-avian dinosaurs birds of course survived the Cataclysm making them the last surviving lineage of the dinosaurs okay we have 66 million years to go and that's the last major extinction event that we have to talk about but you might want to freshen up so I bought these pre-moistened towelettes it's going to you have some iridium on this side on your forehead other side with all the great reptiles gone the smaller animals that remained were able to eke out a living in the next era the Cenozoic this is our era in more ways than one it's the era that we're in today and it also marks the rise of the mammals soon after the KPG extinction the climate warmed and jungles stretched across the planet mammals quickly recovered in this hothouse world and by 40 million years ago most of the mammal groups that we recognized had come about like whales bats rodents and primates but starting around 34 million years ago the climate began to shift again this time ice caps started to grow at the poles taking up much of the planet's water and these drier conditions created a new habitat the grassland where ancestral horses and antelope were hunted by the first cats and dogs it was also in use grassy plains 7 million years ago that a species of ape called Sahel anthropos 2.6 million years ago the ice caps expanded even more and the earth entered a glacial period this is the one you hear referred to as the Ice Age over the course of these last several million years most modern life forms that we know about developed and thrived alongside giants like mammoths ground sloths and saber-toothed cats once again though this era of diversity came to a morbid end starting around 15,000 years ago the climate began to warm up and over the next few thousand years many of the giant finai went extinct by eleven thousand seven hundred years ago the last major glaciation was over and modern humans inhabited nearly all the corners of the globe but how big a role we played in the extinction of the so called Ice Age megafauna is hotly debated regardless there's no escaping the fact that our species has shaped the earth to its will since then like cyanobacteria and the dinosaurs before us we've had a huge impact on habitats other organisms in the biosphere itself and as we have learned today it's the most dominant forms of life that define each phase of deep time so even though our time when this planet amounts to the last word on the last page of the story of life we are the authors of the next chapter one day the epoch of humans may be detected by the marks we make on the land the traces of our cities and farms and our very bodies will be the index fossils of this time no matter how our chapter ends we get to be characters in a truly amazing story thanks for joining me on this epic or epoch journey through geologic time now what do you want to know about the story of life on Earth let us know in the comments and don't forget to go to youtube.com/scishow and check out some of our sister channels from PBS Digital Studios you

Author Since: Mar 11, 2019

  1. I actually like that thought; being an index fossil one day. Makes me feel like my life and death will mean something. I dunno. Just a thought. I gotta find the right conditions to get fossilized, though. 🤔 Haha! Me and my weird thoughts. 🤣

  2. OK everyone .. Here is a question to Bohol our minds !!.. Way are we "human beings" so ubsest with life ? Way when we dig up a lode of old bones and peace it together and find an big reptile from ages ago do we go whhooooww !!..and even the other life on the planet now and looking out into outer space to see of their is other life ?..way is mankind so ubsest with it? It fascinates way we are so fascinated …lol.

  3. Watch a 30 mins video in 3 mins. The BEST extension in google chrome store. https://chrome.google.com/webstore/detail/threelly-ai-for-youtube/dfohlnjmjiipcppekkbhbabjbnikkibo

  4. I wish your show had been available years ago when I was going through grade school and high school. I used to dream of such audio-visually immersive educational film videos.

    Speak slower, dude. I want to absorb everything you are saying in your narrative.

  5. Ah, you skipped CO2’s rapid decline during glaciated ice age events killing plant life at higher altitudes aka the tree line until the gobi desert grasslands die, leading to vast northern hemispheric dust storms darkening and melting snowball earth.

  6. The great dying probably happened by a meteor that killed the ocean animals and in different time but not at the same time and when the volcano exploded it probably killed the land animals. Probably a 100 year gap?

  7. Does anyone know where they got their background music from? I bet it's perfect for studying/walking😍😍😍

  8. "How big a role we played in the extinction is highly debated." The answer is big. Megafauna became extinct because of hunting: as soon as Sapiens got to a new continent, it's megafauna soon collapsed. And this happened in several separate occasions, which kind of wipes away the climate change theory.

  9. You state that Eons will cover the Periods for each Era in future videos. However, the obtuse names chosen for the videos make it very difficult to tell which videos cover which eras or periods. Some can be guessed, but others are more obscure. For example, "The Other Explosion You Should Know About"… when was that? What event or period was this? For a teacher, it makes finding appropriate videos very difficult.

  10. What a great way to describe. Like us who has never been able to have lesson about this but now it became so interesting. Thank goodness, YouTube is not boring anymore. Thanks a lot to all people who work on this from the bottom of my heart.

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